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llvm-project/llvm/lib/Transforms/Utils/SimplifyCFG.cpp

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//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/NoFolder.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/PseudoProbe.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <climits>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#include <set>
#include <tuple>
#include <utility>
#include <vector>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "simplifycfg"
cl::opt<bool> llvm::RequireAndPreserveDomTree(
"simplifycfg-require-and-preserve-domtree", cl::Hidden, cl::ZeroOrMore,
cl::init(false),
cl::desc("Temorary development switch used to gradually uplift SimplifyCFG "
"into preserving DomTree,"));
// Chosen as 2 so as to be cheap, but still to have enough power to fold
// a select, so the "clamp" idiom (of a min followed by a max) will be caught.
// To catch this, we need to fold a compare and a select, hence '2' being the
// minimum reasonable default.
static cl::opt<unsigned> PHINodeFoldingThreshold(
"phi-node-folding-threshold", cl::Hidden, cl::init(2),
cl::desc(
"Control the amount of phi node folding to perform (default = 2)"));
static cl::opt<unsigned> TwoEntryPHINodeFoldingThreshold(
"two-entry-phi-node-folding-threshold", cl::Hidden, cl::init(4),
cl::desc("Control the maximal total instruction cost that we are willing "
"to speculatively execute to fold a 2-entry PHI node into a "
"select (default = 4)"));
static cl::opt<bool> DupRet(
"simplifycfg-dup-ret", cl::Hidden, cl::init(false),
cl::desc("Duplicate return instructions into unconditional branches"));
static cl::opt<bool>
HoistCommon("simplifycfg-hoist-common", cl::Hidden, cl::init(true),
cl::desc("Hoist common instructions up to the parent block"));
static cl::opt<bool>
SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true),
cl::desc("Sink common instructions down to the end block"));
static cl::opt<bool> HoistCondStores(
"simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores if an unconditional store precedes"));
static cl::opt<bool> MergeCondStores(
"simplifycfg-merge-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores even if an unconditional store does not "
"precede - hoist multiple conditional stores into a single "
"predicated store"));
static cl::opt<bool> MergeCondStoresAggressively(
"simplifycfg-merge-cond-stores-aggressively", cl::Hidden, cl::init(false),
cl::desc("When merging conditional stores, do so even if the resultant "
"basic blocks are unlikely to be if-converted as a result"));
static cl::opt<bool> SpeculateOneExpensiveInst(
"speculate-one-expensive-inst", cl::Hidden, cl::init(true),
cl::desc("Allow exactly one expensive instruction to be speculatively "
"executed"));
static cl::opt<unsigned> MaxSpeculationDepth(
"max-speculation-depth", cl::Hidden, cl::init(10),
cl::desc("Limit maximum recursion depth when calculating costs of "
"speculatively executed instructions"));
static cl::opt<int>
MaxSmallBlockSize("simplifycfg-max-small-block-size", cl::Hidden, cl::init(10),
cl::desc("Max size of a block which is still considered "
"small enough to thread through"));
// Two is chosen to allow one negation and a logical combine.
static cl::opt<unsigned>
BranchFoldThreshold("simplifycfg-branch-fold-threshold", cl::Hidden,
cl::init(2),
cl::desc("Maximum cost of combining conditions when "
"folding branches"));
STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
STATISTIC(NumLinearMaps,
"Number of switch instructions turned into linear mapping");
STATISTIC(NumLookupTables,
"Number of switch instructions turned into lookup tables");
STATISTIC(
NumLookupTablesHoles,
"Number of switch instructions turned into lookup tables (holes checked)");
STATISTIC(NumTableCmpReuses, "Number of reused switch table lookup compares");
STATISTIC(NumFoldValueComparisonIntoPredecessors,
"Number of value comparisons folded into predecessor basic blocks");
STATISTIC(NumFoldBranchToCommonDest,
"Number of branches folded into predecessor basic block");
STATISTIC(
NumHoistCommonCode,
"Number of common instruction 'blocks' hoisted up to the begin block");
STATISTIC(NumHoistCommonInstrs,
"Number of common instructions hoisted up to the begin block");
STATISTIC(NumSinkCommonCode,
"Number of common instruction 'blocks' sunk down to the end block");
STATISTIC(NumSinkCommonInstrs,
"Number of common instructions sunk down to the end block");
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
STATISTIC(NumInvokes,
"Number of invokes with empty resume blocks simplified into calls");
namespace {
// The first field contains the value that the switch produces when a certain
// case group is selected, and the second field is a vector containing the
// cases composing the case group.
using SwitchCaseResultVectorTy =
SmallVector<std::pair<Constant *, SmallVector<ConstantInt *, 4>>, 2>;
// The first field contains the phi node that generates a result of the switch
// and the second field contains the value generated for a certain case in the
// switch for that PHI.
using SwitchCaseResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
/// ValueEqualityComparisonCase - Represents a case of a switch.
struct ValueEqualityComparisonCase {
ConstantInt *Value;
BasicBlock *Dest;
ValueEqualityComparisonCase(ConstantInt *Value, BasicBlock *Dest)
: Value(Value), Dest(Dest) {}
bool operator<(ValueEqualityComparisonCase RHS) const {
// Comparing pointers is ok as we only rely on the order for uniquing.
return Value < RHS.Value;
}
bool operator==(BasicBlock *RHSDest) const { return Dest == RHSDest; }
};
class SimplifyCFGOpt {
const TargetTransformInfo &TTI;
DomTreeUpdater *DTU;
const DataLayout &DL;
ArrayRef<WeakVH> LoopHeaders;
const SimplifyCFGOptions &Options;
bool Resimplify;
Value *isValueEqualityComparison(Instruction *TI);
BasicBlock *GetValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases);
bool SimplifyEqualityComparisonWithOnlyPredecessor(Instruction *TI,
BasicBlock *Pred,
IRBuilder<> &Builder);
bool PerformValueComparisonIntoPredecessorFolding(Instruction *TI, Value *&CV,
Instruction *PTI,
IRBuilder<> &Builder);
bool FoldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder);
bool simplifyReturn(ReturnInst *RI, IRBuilder<> &Builder);
bool simplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
bool simplifySingleResume(ResumeInst *RI);
bool simplifyCommonResume(ResumeInst *RI);
bool simplifyCleanupReturn(CleanupReturnInst *RI);
bool simplifyUnreachable(UnreachableInst *UI);
bool simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
bool simplifyIndirectBr(IndirectBrInst *IBI);
bool simplifyBranch(BranchInst *Branch, IRBuilder<> &Builder);
bool simplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder);
bool simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder);
bool SimplifyCondBranchToTwoReturns(BranchInst *BI, IRBuilder<> &Builder);
bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
IRBuilder<> &Builder);
bool HoistThenElseCodeToIf(BranchInst *BI, const TargetTransformInfo &TTI);
bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
const TargetTransformInfo &TTI);
bool SimplifyTerminatorOnSelect(Instruction *OldTerm, Value *Cond,
BasicBlock *TrueBB, BasicBlock *FalseBB,
uint32_t TrueWeight, uint32_t FalseWeight);
bool SimplifyBranchOnICmpChain(BranchInst *BI, IRBuilder<> &Builder,
const DataLayout &DL);
bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select);
bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI);
bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder);
public:
SimplifyCFGOpt(const TargetTransformInfo &TTI, DomTreeUpdater *DTU,
const DataLayout &DL, ArrayRef<WeakVH> LoopHeaders,
const SimplifyCFGOptions &Opts)
: TTI(TTI), DTU(DTU), DL(DL), LoopHeaders(LoopHeaders), Options(Opts) {
assert((!DTU || !DTU->hasPostDomTree()) &&
"SimplifyCFG is not yet capable of maintaining validity of a "
"PostDomTree, so don't ask for it.");
}
bool simplifyOnce(BasicBlock *BB);
bool simplifyOnceImpl(BasicBlock *BB);
bool run(BasicBlock *BB);
// Helper to set Resimplify and return change indication.
bool requestResimplify() {
Resimplify = true;
return true;
}
};
} // end anonymous namespace
/// Return true if it is safe to merge these two
/// terminator instructions together.
static bool
SafeToMergeTerminators(Instruction *SI1, Instruction *SI2,
SmallSetVector<BasicBlock *, 4> *FailBlocks = nullptr) {
if (SI1 == SI2)
return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if *that* PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
bool Fail = false;
for (BasicBlock *Succ : successors(SI2BB))
if (SI1Succs.count(Succ))
for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB)) {
if (FailBlocks)
FailBlocks->insert(Succ);
Fail = true;
}
}
return !Fail;
}
/// Update PHI nodes in Succ to indicate that there will now be entries in it
/// from the 'NewPred' block. The values that will be flowing into the PHI nodes
/// will be the same as those coming in from ExistPred, an existing predecessor
/// of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred,
MemorySSAUpdater *MSSAU = nullptr) {
for (PHINode &PN : Succ->phis())
PN.addIncoming(PN.getIncomingValueForBlock(ExistPred), NewPred);
if (MSSAU)
if (auto *MPhi = MSSAU->getMemorySSA()->getMemoryAccess(Succ))
MPhi->addIncoming(MPhi->getIncomingValueForBlock(ExistPred), NewPred);
}
/// Compute an abstract "cost" of speculating the given instruction,
/// which is assumed to be safe to speculate. TCC_Free means cheap,
/// TCC_Basic means less cheap, and TCC_Expensive means prohibitively
/// expensive.
static InstructionCost computeSpeculationCost(const User *I,
const TargetTransformInfo &TTI) {
assert(isSafeToSpeculativelyExecute(I) &&
"Instruction is not safe to speculatively execute!");
return TTI.getUserCost(I, TargetTransformInfo::TCK_SizeAndLatency);
}
/// If we have a merge point of an "if condition" as accepted above,
/// return true if the specified value dominates the block. We
/// don't handle the true generality of domination here, just a special case
/// which works well enough for us.
///
/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
/// see if V (which must be an instruction) and its recursive operands
/// that do not dominate BB have a combined cost lower than Budget and
/// are non-trapping. If both are true, the instruction is inserted into the
/// set and true is returned.
///
/// The cost for most non-trapping instructions is defined as 1 except for
/// Select whose cost is 2.
///
/// After this function returns, Cost is increased by the cost of
/// V plus its non-dominating operands. If that cost is greater than
/// Budget, false is returned and Cost is undefined.
static bool dominatesMergePoint(Value *V, BasicBlock *BB,
SmallPtrSetImpl<Instruction *> &AggressiveInsts,
InstructionCost &Cost,
InstructionCost Budget,
const TargetTransformInfo &TTI,
unsigned Depth = 0) {
// It is possible to hit a zero-cost cycle (phi/gep instructions for example),
// so limit the recursion depth.
// TODO: While this recursion limit does prevent pathological behavior, it
// would be better to track visited instructions to avoid cycles.
if (Depth == MaxSpeculationDepth)
return false;
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
// Non-instructions all dominate instructions, but not all constantexprs
// can be executed unconditionally.
if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
if (C->canTrap())
return false;
return true;
}
BasicBlock *PBB = I->getParent();
// We don't want to allow weird loops that might have the "if condition" in
// the bottom of this block.
if (PBB == BB)
return false;
// If this instruction is defined in a block that contains an unconditional
// branch to BB, then it must be in the 'conditional' part of the "if
// statement". If not, it definitely dominates the region.
BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator());
if (!BI || BI->isConditional() || BI->getSuccessor(0) != BB)
return true;
// If we have seen this instruction before, don't count it again.
if (AggressiveInsts.count(I))
return true;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if it's a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
if (!isSafeToSpeculativelyExecute(I))
return false;
Cost += computeSpeculationCost(I, TTI);
// Allow exactly one instruction to be speculated regardless of its cost
// (as long as it is safe to do so).
// This is intended to flatten the CFG even if the instruction is a division
// or other expensive operation. The speculation of an expensive instruction
// is expected to be undone in CodeGenPrepare if the speculation has not
// enabled further IR optimizations.
if (Cost > Budget &&
(!SpeculateOneExpensiveInst || !AggressiveInsts.empty() || Depth > 0 ||
!Cost.isValid()))
return false;
// Okay, we can only really hoist these out if their operands do
// not take us over the cost threshold.
for (Use &Op : I->operands())
if (!dominatesMergePoint(Op, BB, AggressiveInsts, Cost, Budget, TTI,
Depth + 1))
return false;
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts.insert(I);
return true;
}
/// Extract ConstantInt from value, looking through IntToPtr
/// and PointerNullValue. Return NULL if value is not a constant int.
static ConstantInt *GetConstantInt(Value *V, const DataLayout &DL) {
// Normal constant int.
ConstantInt *CI = dyn_cast<ConstantInt>(V);
if (CI || !isa<Constant>(V) || !V->getType()->isPointerTy())
return CI;
// This is some kind of pointer constant. Turn it into a pointer-sized
// ConstantInt if possible.
IntegerType *PtrTy = cast<IntegerType>(DL.getIntPtrType(V->getType()));
// Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
if (isa<ConstantPointerNull>(V))
return ConstantInt::get(PtrTy, 0);
// IntToPtr const int.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
if (CE->getOpcode() == Instruction::IntToPtr)
if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
// The constant is very likely to have the right type already.
if (CI->getType() == PtrTy)
return CI;
else
return cast<ConstantInt>(
ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
}
return nullptr;
}
namespace {
/// Given a chain of or (||) or and (&&) comparison of a value against a
/// constant, this will try to recover the information required for a switch
/// structure.
/// It will depth-first traverse the chain of comparison, seeking for patterns
/// like %a == 12 or %a < 4 and combine them to produce a set of integer
/// representing the different cases for the switch.
/// Note that if the chain is composed of '||' it will build the set of elements
/// that matches the comparisons (i.e. any of this value validate the chain)
/// while for a chain of '&&' it will build the set elements that make the test
/// fail.
struct ConstantComparesGatherer {
const DataLayout &DL;
/// Value found for the switch comparison
Value *CompValue = nullptr;
/// Extra clause to be checked before the switch
Value *Extra = nullptr;
/// Set of integers to match in switch
SmallVector<ConstantInt *, 8> Vals;
/// Number of comparisons matched in the and/or chain
unsigned UsedICmps = 0;
/// Construct and compute the result for the comparison instruction Cond
ConstantComparesGatherer(Instruction *Cond, const DataLayout &DL) : DL(DL) {
gather(Cond);
}
ConstantComparesGatherer(const ConstantComparesGatherer &) = delete;
ConstantComparesGatherer &
operator=(const ConstantComparesGatherer &) = delete;
private:
/// Try to set the current value used for the comparison, it succeeds only if
/// it wasn't set before or if the new value is the same as the old one
bool setValueOnce(Value *NewVal) {
if (CompValue && CompValue != NewVal)
return false;
CompValue = NewVal;
return (CompValue != nullptr);
}
/// Try to match Instruction "I" as a comparison against a constant and
/// populates the array Vals with the set of values that match (or do not
/// match depending on isEQ).
/// Return false on failure. On success, the Value the comparison matched
/// against is placed in CompValue.
/// If CompValue is already set, the function is expected to fail if a match
/// is found but the value compared to is different.
bool matchInstruction(Instruction *I, bool isEQ) {
// If this is an icmp against a constant, handle this as one of the cases.
ICmpInst *ICI;
ConstantInt *C;
if (!((ICI = dyn_cast<ICmpInst>(I)) &&
(C = GetConstantInt(I->getOperand(1), DL)))) {
return false;
}
Value *RHSVal;
const APInt *RHSC;
// Pattern match a special case
// (x & ~2^z) == y --> x == y || x == y|2^z
// This undoes a transformation done by instcombine to fuse 2 compares.
if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
// It's a little bit hard to see why the following transformations are
// correct. Here is a CVC3 program to verify them for 64-bit values:
/*
ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
x : BITVECTOR(64);
y : BITVECTOR(64);
z : BITVECTOR(64);
mask : BITVECTOR(64) = BVSHL(ONE, z);
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
// Please note that each pattern must be a dual implication (<--> or
// iff). One directional implication can create spurious matches. If the
// implication is only one-way, an unsatisfiable condition on the left
// side can imply a satisfiable condition on the right side. Dual
// implication ensures that satisfiable conditions are transformed to
// other satisfiable conditions and unsatisfiable conditions are
// transformed to other unsatisfiable conditions.
// Here is a concrete example of a unsatisfiable condition on the left
// implying a satisfiable condition on the right:
//
// mask = (1 << z)
// (x & ~mask) == y --> (x == y || x == (y | mask))
//
// Substituting y = 3, z = 0 yields:
// (x & -2) == 3 --> (x == 3 || x == 2)
// Pattern match a special case:
/*
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
*/
if (match(ICI->getOperand(0),
m_And(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = ~*RHSC;
if (Mask.isPowerOf2() && (C->getValue() & ~Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(
ConstantInt::get(C->getContext(),
C->getValue() | Mask));
UsedICmps++;
return true;
}
}
// Pattern match a special case:
/*
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
if (match(ICI->getOperand(0),
m_Or(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = *RHSC;
if (Mask.isPowerOf2() && (C->getValue() | Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(ConstantInt::get(C->getContext(),
C->getValue() & ~Mask));
UsedICmps++;
return true;
}
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(ICI->getOperand(0)))
return false;
UsedICmps++;
Vals.push_back(C);
return ICI->getOperand(0);
}
// If we have "x ult 3", for example, then we can add 0,1,2 to the set.
ConstantRange Span = ConstantRange::makeAllowedICmpRegion(
ICI->getPredicate(), C->getValue());
// Shift the range if the compare is fed by an add. This is the range
// compare idiom as emitted by instcombine.
Value *CandidateVal = I->getOperand(0);
if (match(I->getOperand(0), m_Add(m_Value(RHSVal), m_APInt(RHSC)))) {
Span = Span.subtract(*RHSC);
CandidateVal = RHSVal;
}
// If this is an and/!= check, then we are looking to build the set of
// value that *don't* pass the and chain. I.e. to turn "x ugt 2" into
// x != 0 && x != 1.
if (!isEQ)
Span = Span.inverse();
// If there are a ton of values, we don't want to make a ginormous switch.
if (Span.isSizeLargerThan(8) || Span.isEmptySet()) {
return false;
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(CandidateVal))
return false;
// Add all values from the range to the set
for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
UsedICmps++;
return true;
}
/// Given a potentially 'or'd or 'and'd together collection of icmp
/// eq/ne/lt/gt instructions that compare a value against a constant, extract
/// the value being compared, and stick the list constants into the Vals
/// vector.
/// One "Extra" case is allowed to differ from the other.
void gather(Value *V) {
bool isEQ = match(V, m_LogicalOr(m_Value(), m_Value()));
// Keep a stack (SmallVector for efficiency) for depth-first traversal
SmallVector<Value *, 8> DFT;
SmallPtrSet<Value *, 8> Visited;
// Initialize
Visited.insert(V);
DFT.push_back(V);
while (!DFT.empty()) {
V = DFT.pop_back_val();
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If it is a || (or && depending on isEQ), process the operands.
Value *Op0, *Op1;
if (isEQ ? match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1)))
: match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
if (Visited.insert(Op1).second)
DFT.push_back(Op1);
if (Visited.insert(Op0).second)
DFT.push_back(Op0);
continue;
}
// Try to match the current instruction
if (matchInstruction(I, isEQ))
// Match succeed, continue the loop
continue;
}
// One element of the sequence of || (or &&) could not be match as a
// comparison against the same value as the others.
// We allow only one "Extra" case to be checked before the switch
if (!Extra) {
Extra = V;
continue;
}
// Failed to parse a proper sequence, abort now
CompValue = nullptr;
break;
}
}
};
} // end anonymous namespace
static void EraseTerminatorAndDCECond(Instruction *TI,
MemorySSAUpdater *MSSAU = nullptr) {
Instruction *Cond = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cond = dyn_cast<Instruction>(SI->getCondition());
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
} else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
Cond = dyn_cast<Instruction>(IBI->getAddress());
}
TI->eraseFromParent();
if (Cond)
RecursivelyDeleteTriviallyDeadInstructions(Cond, nullptr, MSSAU);
}
/// Return true if the specified terminator checks
/// to see if a value is equal to constant integer value.
Value *SimplifyCFGOpt::isValueEqualityComparison(Instruction *TI) {
Value *CV = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// Do not permit merging of large switch instructions into their
// predecessors unless there is only one predecessor.
if (!SI->getParent()->hasNPredecessorsOrMore(128 / SI->getNumSuccessors()))
CV = SI->getCondition();
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional() && BI->getCondition()->hasOneUse())
if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
if (ICI->isEquality() && GetConstantInt(ICI->getOperand(1), DL))
CV = ICI->getOperand(0);
}
// Unwrap any lossless ptrtoint cast.
if (CV) {
if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV)) {
Value *Ptr = PTII->getPointerOperand();
if (PTII->getType() == DL.getIntPtrType(Ptr->getType()))
CV = Ptr;
}
}
return CV;
}
/// Given a value comparison instruction,
/// decode all of the 'cases' that it represents and return the 'default' block.
BasicBlock *SimplifyCFGOpt::GetValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cases.reserve(SI->getNumCases());
for (auto Case : SI->cases())
Cases.push_back(ValueEqualityComparisonCase(Case.getCaseValue(),
Case.getCaseSuccessor()));
return SI->getDefaultDest();
}
BranchInst *BI = cast<BranchInst>(TI);
ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
BasicBlock *Succ = BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_NE);
Cases.push_back(ValueEqualityComparisonCase(
GetConstantInt(ICI->getOperand(1), DL), Succ));
return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
}
/// Given a vector of bb/value pairs, remove any entries
/// in the list that match the specified block.
static void
EliminateBlockCases(BasicBlock *BB,
std::vector<ValueEqualityComparisonCase> &Cases) {
llvm::erase_value(Cases, BB);
}
/// Return true if there are any keys in C1 that exist in C2 as well.
static bool ValuesOverlap(std::vector<ValueEqualityComparisonCase> &C1,
std::vector<ValueEqualityComparisonCase> &C2) {
std::vector<ValueEqualityComparisonCase> *V1 = &C1, *V2 = &C2;
// Make V1 be smaller than V2.
if (V1->size() > V2->size())
std::swap(V1, V2);
if (V1->empty())
return false;
if (V1->size() == 1) {
// Just scan V2.
ConstantInt *TheVal = (*V1)[0].Value;
for (unsigned i = 0, e = V2->size(); i != e; ++i)
if (TheVal == (*V2)[i].Value)
return true;
}
// Otherwise, just sort both lists and compare element by element.
array_pod_sort(V1->begin(), V1->end());
array_pod_sort(V2->begin(), V2->end());
unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
while (i1 != e1 && i2 != e2) {
if ((*V1)[i1].Value == (*V2)[i2].Value)
return true;
if ((*V1)[i1].Value < (*V2)[i2].Value)
++i1;
else
++i2;
}
return false;
}
// Set branch weights on SwitchInst. This sets the metadata if there is at
// least one non-zero weight.
static void setBranchWeights(SwitchInst *SI, ArrayRef<uint32_t> Weights) {
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (llvm::any_of(Weights, [](uint32_t W) { return W != 0; }))
N = MDBuilder(SI->getParent()->getContext()).createBranchWeights(Weights);
SI->setMetadata(LLVMContext::MD_prof, N);
}
// Similar to the above, but for branch and select instructions that take
// exactly 2 weights.
static void setBranchWeights(Instruction *I, uint32_t TrueWeight,
uint32_t FalseWeight) {
assert(isa<BranchInst>(I) || isa<SelectInst>(I));
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (TrueWeight || FalseWeight)
N = MDBuilder(I->getParent()->getContext())
.createBranchWeights(TrueWeight, FalseWeight);
I->setMetadata(LLVMContext::MD_prof, N);
}
/// If TI is known to be a terminator instruction and its block is known to
/// only have a single predecessor block, check to see if that predecessor is
/// also a value comparison with the same value, and if that comparison
/// determines the outcome of this comparison. If so, simplify TI. This does a
/// very limited form of jump threading.
bool SimplifyCFGOpt::SimplifyEqualityComparisonWithOnlyPredecessor(
Instruction *TI, BasicBlock *Pred, IRBuilder<> &Builder) {
Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
if (!PredVal)
return false; // Not a value comparison in predecessor.
Value *ThisVal = isValueEqualityComparison(TI);
assert(ThisVal && "This isn't a value comparison!!");
if (ThisVal != PredVal)
return false; // Different predicates.
// TODO: Preserve branch weight metadata, similarly to how
// FoldValueComparisonIntoPredecessors preserves it.
// Find out information about when control will move from Pred to TI's block.
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDef =
GetValueEqualityComparisonCases(Pred->getTerminator(), PredCases);
EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
// Find information about how control leaves this block.
std::vector<ValueEqualityComparisonCase> ThisCases;
BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
// If TI's block is the default block from Pred's comparison, potentially
// simplify TI based on this knowledge.
if (PredDef == TI->getParent()) {
// If we are here, we know that the value is none of those cases listed in
// PredCases. If there are any cases in ThisCases that are in PredCases, we
// can simplify TI.
if (!ValuesOverlap(PredCases, ThisCases))
return false;
if (isa<BranchInst>(TI)) {
// Okay, one of the successors of this condbr is dead. Convert it to a
// uncond br.
assert(ThisCases.size() == 1 && "Branch can only have one case!");
// Insert the new branch.
Instruction *NI = Builder.CreateBr(ThisDef);
(void)NI;
// Remove PHI node entries for the dead edge.
ThisCases[0].Dest->removePredecessor(PredDef);
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
EraseTerminatorAndDCECond(TI);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Delete, PredDef, ThisCases[0].Dest}});
return true;
}
SwitchInstProfUpdateWrapper SI = *cast<SwitchInst>(TI);
// Okay, TI has cases that are statically dead, prune them away.
SmallPtrSet<Constant *, 16> DeadCases;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
DeadCases.insert(PredCases[i].Value);
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI);
SmallMapVector<BasicBlock *, int, 8> NumPerSuccessorCases;
for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
--i;
auto *Successor = i->getCaseSuccessor();
++NumPerSuccessorCases[Successor];
if (DeadCases.count(i->getCaseValue())) {
Successor->removePredecessor(PredDef);
SI.removeCase(i);
--NumPerSuccessorCases[Successor];
}
}
std::vector<DominatorTree::UpdateType> Updates;
for (const std::pair<BasicBlock *, int> &I : NumPerSuccessorCases)
if (I.second == 0)
Updates.push_back({DominatorTree::Delete, PredDef, I.first});
if (DTU)
DTU->applyUpdates(Updates);
LLVM_DEBUG(dbgs() << "Leaving: " << *TI << "\n");
return true;
}
// Otherwise, TI's block must correspond to some matched value. Find out
// which value (or set of values) this is.
ConstantInt *TIV = nullptr;
BasicBlock *TIBB = TI->getParent();
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest == TIBB) {
if (TIV)
return false; // Cannot handle multiple values coming to this block.
TIV = PredCases[i].Value;
}
assert(TIV && "No edge from pred to succ?");
// Okay, we found the one constant that our value can be if we get into TI's
// BB. Find out which successor will unconditionally be branched to.
BasicBlock *TheRealDest = nullptr;
for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
if (ThisCases[i].Value == TIV) {
TheRealDest = ThisCases[i].Dest;
break;
}
// If not handled by any explicit cases, it is handled by the default case.
if (!TheRealDest)
TheRealDest = ThisDef;
SmallSetVector<BasicBlock *, 2> RemovedSuccs;
// Remove PHI node entries for dead edges.
BasicBlock *CheckEdge = TheRealDest;
for (BasicBlock *Succ : successors(TIBB))
if (Succ != CheckEdge) {
if (Succ != TheRealDest)
RemovedSuccs.insert(Succ);
Succ->removePredecessor(TIBB);
} else
CheckEdge = nullptr;
// Insert the new branch.
Instruction *NI = Builder.CreateBr(TheRealDest);
(void)NI;
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
EraseTerminatorAndDCECond(TI);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.reserve(RemovedSuccs.size());
for (auto *RemovedSucc : RemovedSuccs)
Updates.push_back({DominatorTree::Delete, TIBB, RemovedSucc});
DTU->applyUpdates(Updates);
}
return true;
}
namespace {
/// This class implements a stable ordering of constant
/// integers that does not depend on their address. This is important for
/// applications that sort ConstantInt's to ensure uniqueness.
struct ConstantIntOrdering {
bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
return LHS->getValue().ult(RHS->getValue());
}
};
} // end anonymous namespace
static int ConstantIntSortPredicate(ConstantInt *const *P1,
ConstantInt *const *P2) {
const ConstantInt *LHS = *P1;
const ConstantInt *RHS = *P2;
if (LHS == RHS)
return 0;
return LHS->getValue().ult(RHS->getValue()) ? 1 : -1;
}
static inline bool HasBranchWeights(const Instruction *I) {
MDNode *ProfMD = I->getMetadata(LLVMContext::MD_prof);
if (ProfMD && ProfMD->getOperand(0))
if (MDString *MDS = dyn_cast<MDString>(ProfMD->getOperand(0)))
return MDS->getString().equals("branch_weights");
return false;
}
/// Get Weights of a given terminator, the default weight is at the front
/// of the vector. If TI is a conditional eq, we need to swap the branch-weight
/// metadata.
static void GetBranchWeights(Instruction *TI,
SmallVectorImpl<uint64_t> &Weights) {
MDNode *MD = TI->getMetadata(LLVMContext::MD_prof);
assert(MD);
for (unsigned i = 1, e = MD->getNumOperands(); i < e; ++i) {
ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(i));
Weights.push_back(CI->getValue().getZExtValue());
}
// If TI is a conditional eq, the default case is the false case,
// and the corresponding branch-weight data is at index 2. We swap the
// default weight to be the first entry.
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
assert(Weights.size() == 2);
ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
std::swap(Weights.front(), Weights.back());
}
}
/// Keep halving the weights until all can fit in uint32_t.
static void FitWeights(MutableArrayRef<uint64_t> Weights) {
uint64_t Max = *std::max_element(Weights.begin(), Weights.end());
if (Max > UINT_MAX) {
unsigned Offset = 32 - countLeadingZeros(Max);
for (uint64_t &I : Weights)
I >>= Offset;
}
}
static void CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
BasicBlock *BB, BasicBlock *PredBlock, ValueToValueMapTy &VMap) {
Instruction *PTI = PredBlock->getTerminator();
// If we have bonus instructions, clone them into the predecessor block.
// Note that there may be multiple predecessor blocks, so we cannot move
// bonus instructions to a predecessor block.
for (Instruction &BonusInst : *BB) {
if (isa<DbgInfoIntrinsic>(BonusInst) || BonusInst.isTerminator())
continue;
Instruction *NewBonusInst = BonusInst.clone();
if (PTI->getDebugLoc() != NewBonusInst->getDebugLoc()) {
// Unless the instruction has the same !dbg location as the original
// branch, drop it. When we fold the bonus instructions we want to make
// sure we reset their debug locations in order to avoid stepping on
// dead code caused by folding dead branches.
NewBonusInst->setDebugLoc(DebugLoc());
}
RemapInstruction(NewBonusInst, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
VMap[&BonusInst] = NewBonusInst;
// If we moved a load, we cannot any longer claim any knowledge about
// its potential value. The previous information might have been valid
// only given the branch precondition.
// For an analogous reason, we must also drop all the metadata whose
// semantics we don't understand. We *can* preserve !annotation, because
// it is tied to the instruction itself, not the value or position.
NewBonusInst->dropUnknownNonDebugMetadata(LLVMContext::MD_annotation);
PredBlock->getInstList().insert(PTI->getIterator(), NewBonusInst);
NewBonusInst->takeName(&BonusInst);
BonusInst.setName(NewBonusInst->getName() + ".old");
// Update (liveout) uses of bonus instructions,
// now that the bonus instruction has been cloned into predecessor.
SSAUpdater SSAUpdate;
SSAUpdate.Initialize(BonusInst.getType(),
(NewBonusInst->getName() + ".merge").str());
SSAUpdate.AddAvailableValue(BB, &BonusInst);
SSAUpdate.AddAvailableValue(PredBlock, NewBonusInst);
for (Use &U : make_early_inc_range(BonusInst.uses())) {
auto *UI = cast<Instruction>(U.getUser());
if (UI->getParent() != PredBlock)
SSAUpdate.RewriteUseAfterInsertions(U);
else // Use is in the same block as, and comes before, NewBonusInst.
SSAUpdate.RewriteUse(U);
}
}
}
bool SimplifyCFGOpt::PerformValueComparisonIntoPredecessorFolding(
Instruction *TI, Value *&CV, Instruction *PTI, IRBuilder<> &Builder) {
BasicBlock *BB = TI->getParent();
BasicBlock *Pred = PTI->getParent();
std::vector<DominatorTree::UpdateType> Updates;
// Figure out which 'cases' to copy from SI to PSI.
std::vector<ValueEqualityComparisonCase> BBCases;
BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
// Based on whether the default edge from PTI goes to BB or not, fill in
// PredCases and PredDefault with the new switch cases we would like to
// build.
SmallMapVector<BasicBlock *, int, 8> NewSuccessors;
// Update the branch weight metadata along the way
SmallVector<uint64_t, 8> Weights;
bool PredHasWeights = HasBranchWeights(PTI);
bool SuccHasWeights = HasBranchWeights(TI);
if (PredHasWeights) {
GetBranchWeights(PTI, Weights);
// branch-weight metadata is inconsistent here.
if (Weights.size() != 1 + PredCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (SuccHasWeights)
// If there are no predecessor weights but there are successor weights,
// populate Weights with 1, which will later be scaled to the sum of
// successor's weights
Weights.assign(1 + PredCases.size(), 1);
SmallVector<uint64_t, 8> SuccWeights;
if (SuccHasWeights) {
GetBranchWeights(TI, SuccWeights);
// branch-weight metadata is inconsistent here.
if (SuccWeights.size() != 1 + BBCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (PredHasWeights)
SuccWeights.assign(1 + BBCases.size(), 1);
if (PredDefault == BB) {
// If this is the default destination from PTI, only the edges in TI
// that don't occur in PTI, or that branch to BB will be activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest != BB)
PTIHandled.insert(PredCases[i].Value);
else {
// The default destination is BB, we don't need explicit targets.
std::swap(PredCases[i], PredCases.back());
if (PredHasWeights || SuccHasWeights) {
// Increase weight for the default case.
Weights[0] += Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
PredCases.pop_back();
--i;
--e;
}
// Reconstruct the new switch statement we will be building.
if (PredDefault != BBDefault) {
PredDefault->removePredecessor(Pred);
if (PredDefault != BB)
Updates.push_back({DominatorTree::Delete, Pred, PredDefault});
PredDefault = BBDefault;
++NewSuccessors[BBDefault];
}
unsigned CasesFromPred = Weights.size();
uint64_t ValidTotalSuccWeight = 0;
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (!PTIHandled.count(BBCases[i].Value) && BBCases[i].Dest != BBDefault) {
PredCases.push_back(BBCases[i]);
++NewSuccessors[BBCases[i].Dest];
if (SuccHasWeights || PredHasWeights) {
// The default weight is at index 0, so weight for the ith case
// should be at index i+1. Scale the cases from successor by
// PredDefaultWeight (Weights[0]).
Weights.push_back(Weights[0] * SuccWeights[i + 1]);
ValidTotalSuccWeight += SuccWeights[i + 1];
}
}
if (SuccHasWeights || PredHasWeights) {
ValidTotalSuccWeight += SuccWeights[0];
// Scale the cases from predecessor by ValidTotalSuccWeight.
for (unsigned i = 1; i < CasesFromPred; ++i)
Weights[i] *= ValidTotalSuccWeight;
// Scale the default weight by SuccDefaultWeight (SuccWeights[0]).
Weights[0] *= SuccWeights[0];
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
std::map<ConstantInt *, uint64_t> WeightsForHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest == BB) {
PTIHandled.insert(PredCases[i].Value);
if (PredHasWeights || SuccHasWeights) {
WeightsForHandled[PredCases[i].Value] = Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i;
--e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (PTIHandled.count(BBCases[i].Value)) {
// If this is one we are capable of getting...
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[BBCases[i].Value]);
PredCases.push_back(BBCases[i]);
++NewSuccessors[BBCases[i].Dest];
PTIHandled.erase(BBCases[i].Value); // This constant is taken care of
}
// If there are any constants vectored to BB that TI doesn't handle,
// they must go to the default destination of TI.
for (ConstantInt *I : PTIHandled) {
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[I]);
PredCases.push_back(ValueEqualityComparisonCase(I, BBDefault));
++NewSuccessors[BBDefault];
}
}
// Okay, at this point, we know which new successor Pred will get. Make
// sure we update the number of entries in the PHI nodes for these
// successors.
for (const std::pair<BasicBlock *, int /*Num*/> &NewSuccessor :
NewSuccessors) {
for (auto I : seq(0, NewSuccessor.second)) {
(void)I;
AddPredecessorToBlock(NewSuccessor.first, Pred, BB);
}
if (!is_contained(successors(Pred), NewSuccessor.first))
Updates.push_back({DominatorTree::Insert, Pred, NewSuccessor.first});
}
Builder.SetInsertPoint(PTI);
// Convert pointer to int before we switch.
if (CV->getType()->isPointerTy()) {
CV =
Builder.CreatePtrToInt(CV, DL.getIntPtrType(CV->getType()), "magicptr");
}
// Now that the successors are updated, create the new Switch instruction.
SwitchInst *NewSI = Builder.CreateSwitch(CV, PredDefault, PredCases.size());
NewSI->setDebugLoc(PTI->getDebugLoc());
for (ValueEqualityComparisonCase &V : PredCases)
NewSI->addCase(V.Value, V.Dest);
if (PredHasWeights || SuccHasWeights) {
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(Weights);
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
setBranchWeights(NewSI, MDWeights);
}
EraseTerminatorAndDCECond(PTI);
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = nullptr;
for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
if (NewSI->getSuccessor(i) == BB) {
if (!InfLoopBlock) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock =
BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
Updates.push_back({DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
}
NewSI->setSuccessor(i, InfLoopBlock);
}
if (InfLoopBlock)
Updates.push_back({DominatorTree::Insert, Pred, InfLoopBlock});
Updates.push_back({DominatorTree::Delete, Pred, BB});
if (DTU)
DTU->applyUpdates(Updates);
++NumFoldValueComparisonIntoPredecessors;
return true;
}
/// The specified terminator is a value equality comparison instruction
/// (either a switch or a branch on "X == c").
/// See if any of the predecessors of the terminator block are value comparisons
/// on the same value. If so, and if safe to do so, fold them together.
bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder) {
BasicBlock *BB = TI->getParent();
Value *CV = isValueEqualityComparison(TI); // CondVal
assert(CV && "Not a comparison?");
bool Changed = false;
SmallSetVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.pop_back_val();
Instruction *PTI = Pred->getTerminator();
// Don't try to fold into itself.
if (Pred == BB)
continue;
// See if the predecessor is a comparison with the same value.
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV != CV)
continue;
SmallSetVector<BasicBlock *, 4> FailBlocks;
if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
for (auto *Succ : FailBlocks) {
if (!SplitBlockPredecessors(Succ, TI->getParent(), ".fold.split", DTU))
return false;
}
}
PerformValueComparisonIntoPredecessorFolding(TI, CV, PTI, Builder);
Changed = true;
}
return Changed;
}
// If we would need to insert a select that uses the value of this invoke
// (comments in HoistThenElseCodeToIf explain why we would need to do this), we
// can't hoist the invoke, as there is nowhere to put the select in this case.
static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
Instruction *I1, Instruction *I2) {
for (BasicBlock *Succ : successors(BB1)) {
for (const PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V != BB2V && (BB1V == I1 || BB2V == I2)) {
return false;
}
}
}
return true;
}
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified = false);
/// Given a conditional branch that goes to BB1 and BB2, hoist any common code
/// in the two blocks up into the branch block. The caller of this function
/// guarantees that BI's block dominates BB1 and BB2.
bool SimplifyCFGOpt::HoistThenElseCodeToIf(BranchInst *BI,
const TargetTransformInfo &TTI) {
// This does very trivial matching, with limited scanning, to find identical
// instructions in the two blocks. In particular, we don't want to get into
// O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
// such, we currently just scan for obviously identical instructions in an
// identical order.
BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
BasicBlock::iterator BB1_Itr = BB1->begin();
BasicBlock::iterator BB2_Itr = BB2->begin();
Instruction *I1 = &*BB1_Itr++, *I2 = &*BB2_Itr++;
// Skip debug info if it is not identical.
DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
while (isa<DbgInfoIntrinsic>(I1))
I1 = &*BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = &*BB2_Itr++;
}
// FIXME: Can we define a safety predicate for CallBr?
if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
(isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) ||
isa<CallBrInst>(I1))
return false;
BasicBlock *BIParent = BI->getParent();
bool Changed = false;
auto _ = make_scope_exit([&]() {
if (Changed)
++NumHoistCommonCode;
});
do {
// If we are hoisting the terminator instruction, don't move one (making a
// broken BB), instead clone it, and remove BI.
if (I1->isTerminator())
goto HoistTerminator;
// If we're going to hoist a call, make sure that the two instructions we're
// commoning/hoisting are both marked with musttail, or neither of them is
// marked as such. Otherwise, we might end up in a situation where we hoist
// from a block where the terminator is a `ret` to a block where the terminator
// is a `br`, and `musttail` calls expect to be followed by a return.
auto *C1 = dyn_cast<CallInst>(I1);
auto *C2 = dyn_cast<CallInst>(I2);
if (C1 && C2)
if (C1->isMustTailCall() != C2->isMustTailCall())
return Changed;
if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
return Changed;
// If any of the two call sites has nomerge attribute, stop hoisting.
if (const auto *CB1 = dyn_cast<CallBase>(I1))
if (CB1->cannotMerge())
return Changed;
if (const auto *CB2 = dyn_cast<CallBase>(I2))
if (CB2->cannotMerge())
return Changed;
if (isa<DbgInfoIntrinsic>(I1) || isa<DbgInfoIntrinsic>(I2)) {
assert (isa<DbgInfoIntrinsic>(I1) && isa<DbgInfoIntrinsic>(I2));
// The debug location is an integral part of a debug info intrinsic
// and can't be separated from it or replaced. Instead of attempting
// to merge locations, simply hoist both copies of the intrinsic.
BIParent->getInstList().splice(BI->getIterator(),
BB1->getInstList(), I1);
BIParent->getInstList().splice(BI->getIterator(),
BB2->getInstList(), I2);
Changed = true;
} else {
// For a normal instruction, we just move one to right before the branch,
// then replace all uses of the other with the first. Finally, we remove
// the now redundant second instruction.
BIParent->getInstList().splice(BI->getIterator(),
BB1->getInstList(), I1);
if (!I2->use_empty())
I2->replaceAllUsesWith(I1);
I1->andIRFlags(I2);
unsigned KnownIDs[] = {LLVMContext::MD_tbaa,
LLVMContext::MD_range,
LLVMContext::MD_fpmath,
LLVMContext::MD_invariant_load,
LLVMContext::MD_nonnull,
LLVMContext::MD_invariant_group,
LLVMContext::MD_align,
LLVMContext::MD_dereferenceable,
LLVMContext::MD_dereferenceable_or_null,
LLVMContext::MD_mem_parallel_loop_access,
LLVMContext::MD_access_group,
LLVMContext::MD_preserve_access_index};
combineMetadata(I1, I2, KnownIDs, true);
// I1 and I2 are being combined into a single instruction. Its debug
// location is the merged locations of the original instructions.
I1->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
I2->eraseFromParent();
Changed = true;
}
++NumHoistCommonInstrs;
I1 = &*BB1_Itr++;
I2 = &*BB2_Itr++;
// Skip debug info if it is not identical.
DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
while (isa<DbgInfoIntrinsic>(I1))
I1 = &*BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = &*BB2_Itr++;
}
} while (I1->isIdenticalToWhenDefined(I2));
return true;
HoistTerminator:
// It may not be possible to hoist an invoke.
// FIXME: Can we define a safety predicate for CallBr?
if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
return Changed;
// TODO: callbr hoisting currently disabled pending further study.
if (isa<CallBrInst>(I1))
return Changed;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V == BB2V)
continue;
// Check for passingValueIsAlwaysUndefined here because we would rather
// eliminate undefined control flow then converting it to a select.
if (passingValueIsAlwaysUndefined(BB1V, &PN) ||
passingValueIsAlwaysUndefined(BB2V, &PN))
return Changed;
if (isa<ConstantExpr>(BB1V) && !isSafeToSpeculativelyExecute(BB1V))
return Changed;
if (isa<ConstantExpr>(BB2V) && !isSafeToSpeculativelyExecute(BB2V))
return Changed;
}
}
// Okay, it is safe to hoist the terminator.
Instruction *NT = I1->clone();
BIParent->getInstList().insert(BI->getIterator(), NT);
if (!NT->getType()->isVoidTy()) {
I1->replaceAllUsesWith(NT);
I2->replaceAllUsesWith(NT);
NT->takeName(I1);
}
Changed = true;
++NumHoistCommonInstrs;
// Ensure terminator gets a debug location, even an unknown one, in case
// it involves inlinable calls.
NT->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
// PHIs created below will adopt NT's merged DebugLoc.
IRBuilder<NoFolder> Builder(NT);
// Hoisting one of the terminators from our successor is a great thing.
// Unfortunately, the successors of the if/else blocks may have PHI nodes in
// them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
// nodes, so we insert select instruction to compute the final result.
std::map<std::pair<Value *, Value *>, SelectInst *> InsertedSelects;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V == BB2V)
continue;
// These values do not agree. Insert a select instruction before NT
// that determines the right value.
SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
if (!SI) {
// Propagate fast-math-flags from phi node to its replacement select.
IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
if (isa<FPMathOperator>(PN))
Builder.setFastMathFlags(PN.getFastMathFlags());
SI = cast<SelectInst>(
Builder.CreateSelect(BI->getCondition(), BB1V, BB2V,
BB1V->getName() + "." + BB2V->getName(), BI));
}
// Make the PHI node use the select for all incoming values for BB1/BB2
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
if (PN.getIncomingBlock(i) == BB1 || PN.getIncomingBlock(i) == BB2)
PN.setIncomingValue(i, SI);
}
}
SmallVector<DominatorTree::UpdateType, 4> Updates;
// Update any PHI nodes in our new successors.
for (BasicBlock *Succ : successors(BB1)) {
AddPredecessorToBlock(Succ, BIParent, BB1);
Updates.push_back({DominatorTree::Insert, BIParent, Succ});
}
for (BasicBlock *Succ : successors(BI))
Updates.push_back({DominatorTree::Delete, BIParent, Succ});
EraseTerminatorAndDCECond(BI);
if (DTU)
DTU->applyUpdates(Updates);
return Changed;
}
// Check lifetime markers.
static bool isLifeTimeMarker(const Instruction *I) {
if (auto II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default:
break;
case Intrinsic::lifetime_start:
case Intrinsic::lifetime_end:
return true;
}
}
return false;
}
// TODO: Refine this. This should avoid cases like turning constant memcpy sizes
// into variables.
static bool replacingOperandWithVariableIsCheap(const Instruction *I,
int OpIdx) {
return !isa<IntrinsicInst>(I);
}
// All instructions in Insts belong to different blocks that all unconditionally
// branch to a common successor. Analyze each instruction and return true if it
// would be possible to sink them into their successor, creating one common
// instruction instead. For every value that would be required to be provided by
// PHI node (because an operand varies in each input block), add to PHIOperands.
static bool canSinkInstructions(
ArrayRef<Instruction *> Insts,
DenseMap<Instruction *, SmallVector<Value *, 4>> &PHIOperands) {
// Prune out obviously bad instructions to move. Each instruction must have
// exactly zero or one use, and we check later that use is by a single, common
// PHI instruction in the successor.
bool HasUse = !Insts.front()->user_empty();
for (auto *I : Insts) {
// These instructions may change or break semantics if moved.
if (isa<PHINode>(I) || I->isEHPad() || isa<AllocaInst>(I) ||
I->getType()->isTokenTy())
return false;
// Do not try to sink an instruction in an infinite loop - it can cause
// this algorithm to infinite loop.
if (I->getParent()->getSingleSuccessor() == I->getParent())
return false;
// Conservatively return false if I is an inline-asm instruction. Sinking
// and merging inline-asm instructions can potentially create arguments
// that cannot satisfy the inline-asm constraints.
// If the instruction has nomerge attribute, return false.
if (const auto *C = dyn_cast<CallBase>(I))
if (C->isInlineAsm() || C->cannotMerge())
return false;
// Each instruction must have zero or one use.
if (HasUse && !I->hasOneUse())
return false;
if (!HasUse && !I->user_empty())
return false;
}
const Instruction *I0 = Insts.front();
for (auto *I : Insts)
if (!I->isSameOperationAs(I0))
return false;
// All instructions in Insts are known to be the same opcode. If they have a
// use, check that the only user is a PHI or in the same block as the
// instruction, because if a user is in the same block as an instruction we're
// contemplating sinking, it must already be determined to be sinkable.
if (HasUse) {
auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
auto *Succ = I0->getParent()->getTerminator()->getSuccessor(0);
if (!all_of(Insts, [&PNUse,&Succ](const Instruction *I) -> bool {
auto *U = cast<Instruction>(*I->user_begin());
return (PNUse &&
PNUse->getParent() == Succ &&
PNUse->getIncomingValueForBlock(I->getParent()) == I) ||
U->getParent() == I->getParent();
}))
return false;
}
// Because SROA can't handle speculating stores of selects, try not to sink
// loads, stores or lifetime markers of allocas when we'd have to create a
// PHI for the address operand. Also, because it is likely that loads or
// stores of allocas will disappear when Mem2Reg/SROA is run, don't sink
// them.
// This can cause code churn which can have unintended consequences down
// the line - see https://llvm.org/bugs/show_bug.cgi?id=30244.
// FIXME: This is a workaround for a deficiency in SROA - see
// https://llvm.org/bugs/show_bug.cgi?id=30188
if (isa<StoreInst>(I0) && any_of(Insts, [](const Instruction *I) {
return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
}))
return false;
if (isa<LoadInst>(I0) && any_of(Insts, [](const Instruction *I) {
return isa<AllocaInst>(I->getOperand(0)->stripPointerCasts());
}))
return false;
if (isLifeTimeMarker(I0) && any_of(Insts, [](const Instruction *I) {
return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
}))
return false;
for (unsigned OI = 0, OE = I0->getNumOperands(); OI != OE; ++OI) {
Value *Op = I0->getOperand(OI);
if (Op->getType()->isTokenTy())
// Don't touch any operand of token type.
return false;
auto SameAsI0 = [&I0, OI](const Instruction *I) {
assert(I->getNumOperands() == I0->getNumOperands());
return I->getOperand(OI) == I0->getOperand(OI);
};
if (!all_of(Insts, SameAsI0)) {
if ((isa<Constant>(Op) && !replacingOperandWithVariableIsCheap(I0, OI)) ||
!canReplaceOperandWithVariable(I0, OI))
// We can't create a PHI from this GEP.
return false;
// Don't create indirect calls! The called value is the final operand.
if (isa<CallBase>(I0) && OI == OE - 1) {
// FIXME: if the call was *already* indirect, we should do this.
return false;
}
for (auto *I : Insts)
PHIOperands[I].push_back(I->getOperand(OI));
}
}
return true;
}
// Assuming canSinkInstructions(Blocks) has returned true, sink the last
// instruction of every block in Blocks to their common successor, commoning
// into one instruction.
static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
auto *BBEnd = Blocks[0]->getTerminator()->getSuccessor(0);
// canSinkInstructions returning true guarantees that every block has at
// least one non-terminator instruction.
SmallVector<Instruction*,4> Insts;
for (auto *BB : Blocks) {
Instruction *I = BB->getTerminator();
do {
I = I->getPrevNode();
} while (isa<DbgInfoIntrinsic>(I) && I != &BB->front());
if (!isa<DbgInfoIntrinsic>(I))
Insts.push_back(I);
}
// The only checking we need to do now is that all users of all instructions
// are the same PHI node. canSinkInstructions should have checked this but
// it is slightly over-aggressive - it gets confused by commutative
// instructions so double-check it here.
Instruction *I0 = Insts.front();
if (!I0->user_empty()) {
auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
if (!all_of(Insts, [&PNUse](const Instruction *I) -> bool {
auto *U = cast<Instruction>(*I->user_begin());
return U == PNUse;
}))
return false;
}
// We don't need to do any more checking here; canSinkInstructions should
// have done it all for us.
SmallVector<Value*, 4> NewOperands;
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O) {
// This check is different to that in canSinkInstructions. There, we
// cared about the global view once simplifycfg (and instcombine) have
// completed - it takes into account PHIs that become trivially
// simplifiable. However here we need a more local view; if an operand
// differs we create a PHI and rely on instcombine to clean up the very
// small mess we may make.
bool NeedPHI = any_of(Insts, [&I0, O](const Instruction *I) {
return I->getOperand(O) != I0->getOperand(O);
});
if (!NeedPHI) {
NewOperands.push_back(I0->getOperand(O));
continue;
}
// Create a new PHI in the successor block and populate it.
auto *Op = I0->getOperand(O);
assert(!Op->getType()->isTokenTy() && "Can't PHI tokens!");
auto *PN = PHINode::Create(Op->getType(), Insts.size(),
Op->getName() + ".sink", &BBEnd->front());
for (auto *I : Insts)
PN->addIncoming(I->getOperand(O), I->getParent());
NewOperands.push_back(PN);
}
// Arbitrarily use I0 as the new "common" instruction; remap its operands
// and move it to the start of the successor block.
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O)
I0->getOperandUse(O).set(NewOperands[O]);
I0->moveBefore(&*BBEnd->getFirstInsertionPt());
// Update metadata and IR flags, and merge debug locations.
for (auto *I : Insts)
if (I != I0) {
// The debug location for the "common" instruction is the merged locations
// of all the commoned instructions. We start with the original location
// of the "common" instruction and iteratively merge each location in the
// loop below.
// This is an N-way merge, which will be inefficient if I0 is a CallInst.
// However, as N-way merge for CallInst is rare, so we use simplified API
// instead of using complex API for N-way merge.
I0->applyMergedLocation(I0->getDebugLoc(), I->getDebugLoc());
combineMetadataForCSE(I0, I, true);
I0->andIRFlags(I);
}
if (!I0->user_empty()) {
// canSinkLastInstruction checked that all instructions were used by
// one and only one PHI node. Find that now, RAUW it to our common
// instruction and nuke it.
auto *PN = cast<PHINode>(*I0->user_begin());
PN->replaceAllUsesWith(I0);
PN->eraseFromParent();
}
// Finally nuke all instructions apart from the common instruction.
for (auto *I : Insts)
if (I != I0)
I->eraseFromParent();
return true;
}
namespace {
// LockstepReverseIterator - Iterates through instructions
// in a set of blocks in reverse order from the first non-terminator.
// For example (assume all blocks have size n):
// LockstepReverseIterator I([B1, B2, B3]);
// *I-- = [B1[n], B2[n], B3[n]];
// *I-- = [B1[n-1], B2[n-1], B3[n-1]];
// *I-- = [B1[n-2], B2[n-2], B3[n-2]];
// ...
class LockstepReverseIterator {
ArrayRef<BasicBlock*> Blocks;
SmallVector<Instruction*,4> Insts;
bool Fail;
public:
LockstepReverseIterator(ArrayRef<BasicBlock*> Blocks) : Blocks(Blocks) {
reset();
}
void reset() {
Fail = false;
Insts.clear();
for (auto *BB : Blocks) {
Instruction *Inst = BB->getTerminator();
for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
Inst = Inst->getPrevNode();
if (!Inst) {
// Block wasn't big enough.
Fail = true;
return;
}
Insts.push_back(Inst);
}
}
bool isValid() const {
return !Fail;
}
void operator--() {
if (Fail)
return;
for (auto *&Inst : Insts) {
for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
Inst = Inst->getPrevNode();
// Already at beginning of block.
if (!Inst) {
Fail = true;
return;
}
}
}
ArrayRef<Instruction*> operator * () const {
return Insts;
}
};
} // end anonymous namespace
/// Check whether BB's predecessors end with unconditional branches. If it is
/// true, sink any common code from the predecessors to BB.
/// We also allow one predecessor to end with conditional branch (but no more
/// than one).
static bool SinkCommonCodeFromPredecessors(BasicBlock *BB,
DomTreeUpdater *DTU) {
// We support two situations:
// (1) all incoming arcs are unconditional
// (2) one incoming arc is conditional
//
// (2) is very common in switch defaults and
// else-if patterns;
//
// if (a) f(1);
// else if (b) f(2);
//
// produces:
//
// [if]
// / \
// [f(1)] [if]
// | | \
// | | |
// | [f(2)]|
// \ | /
// [ end ]
//
// [end] has two unconditional predecessor arcs and one conditional. The
// conditional refers to the implicit empty 'else' arc. This conditional
// arc can also be caused by an empty default block in a switch.
//
// In this case, we attempt to sink code from all *unconditional* arcs.
// If we can sink instructions from these arcs (determined during the scan
// phase below) we insert a common successor for all unconditional arcs and
// connect that to [end], to enable sinking:
//
// [if]
// / \
// [x(1)] [if]
// | | \
// | | \
// | [x(2)] |
// \ / |
// [sink.split] |
// \ /
// [ end ]
//
SmallVector<BasicBlock*,4> UnconditionalPreds;
Instruction *Cond = nullptr;
for (auto *B : predecessors(BB)) {
auto *T = B->getTerminator();
if (isa<BranchInst>(T) && cast<BranchInst>(T)->isUnconditional())
UnconditionalPreds.push_back(B);
else if ((isa<BranchInst>(T) || isa<SwitchInst>(T)) && !Cond)
Cond = T;
else
return false;
}
if (UnconditionalPreds.size() < 2)
return false;
// We take a two-step approach to tail sinking. First we scan from the end of
// each block upwards in lockstep. If the n'th instruction from the end of each
// block can be sunk, those instructions are added to ValuesToSink and we
// carry on. If we can sink an instruction but need to PHI-merge some operands
// (because they're not identical in each instruction) we add these to
// PHIOperands.
unsigned ScanIdx = 0;
SmallPtrSet<Value*,4> InstructionsToSink;
DenseMap<Instruction*, SmallVector<Value*,4>> PHIOperands;
LockstepReverseIterator LRI(UnconditionalPreds);
while (LRI.isValid() &&
canSinkInstructions(*LRI, PHIOperands)) {
LLVM_DEBUG(dbgs() << "SINK: instruction can be sunk: " << *(*LRI)[0]
<< "\n");
InstructionsToSink.insert((*LRI).begin(), (*LRI).end());
++ScanIdx;
--LRI;
}
// If no instructions can be sunk, early-return.
if (ScanIdx == 0)
return false;
bool Changed = false;
auto ProfitableToSinkInstruction = [&](LockstepReverseIterator &LRI) {
unsigned NumPHIdValues = 0;
for (auto *I : *LRI)
for (auto *V : PHIOperands[I])
if (InstructionsToSink.count(V) == 0)
++NumPHIdValues;
LLVM_DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
unsigned NumPHIInsts = NumPHIdValues / UnconditionalPreds.size();
if ((NumPHIdValues % UnconditionalPreds.size()) != 0)
NumPHIInsts++;
return NumPHIInsts <= 1;
};
if (Cond) {
// Check if we would actually sink anything first! This mutates the CFG and
// adds an extra block. The goal in doing this is to allow instructions that
// couldn't be sunk before to be sunk - obviously, speculatable instructions
// (such as trunc, add) can be sunk and predicated already. So we check that
// we're going to sink at least one non-speculatable instruction.
LRI.reset();
unsigned Idx = 0;
bool Profitable = false;
while (ProfitableToSinkInstruction(LRI) && Idx < ScanIdx) {
if (!isSafeToSpeculativelyExecute((*LRI)[0])) {
Profitable = true;
break;
}
--LRI;
++Idx;
}
if (!Profitable)
return false;
LLVM_DEBUG(dbgs() << "SINK: Splitting edge\n");
// We have a conditional edge and we're going to sink some instructions.
// Insert a new block postdominating all blocks we're going to sink from.
if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split", DTU))
// Edges couldn't be split.
return false;
Changed = true;
}
// Now that we've analyzed all potential sinking candidates, perform the
// actual sink. We iteratively sink the last non-terminator of the source
// blocks into their common successor unless doing so would require too
// many PHI instructions to be generated (currently only one PHI is allowed
// per sunk instruction).
//
// We can use InstructionsToSink to discount values needing PHI-merging that will
// actually be sunk in a later iteration. This allows us to be more
// aggressive in what we sink. This does allow a false positive where we
// sink presuming a later value will also be sunk, but stop half way through
// and never actually sink it which means we produce more PHIs than intended.
// This is unlikely in practice though.
unsigned SinkIdx = 0;
for (; SinkIdx != ScanIdx; ++SinkIdx) {
LLVM_DEBUG(dbgs() << "SINK: Sink: "
<< *UnconditionalPreds[0]->getTerminator()->getPrevNode()
<< "\n");
// Because we've sunk every instruction in turn, the current instruction to
// sink is always at index 0.
LRI.reset();
if (!ProfitableToSinkInstruction(LRI)) {
// Too many PHIs would be created.
LLVM_DEBUG(
dbgs() << "SINK: stopping here, too many PHIs would be created!\n");
break;
}
if (!sinkLastInstruction(UnconditionalPreds)) {
LLVM_DEBUG(
dbgs()
<< "SINK: stopping here, failed to actually sink instruction!\n");
break;
}
NumSinkCommonInstrs++;
Changed = true;
}
if (SinkIdx != 0)
++NumSinkCommonCode;
return Changed;
}
/// Determine if we can hoist sink a sole store instruction out of a
/// conditional block.
///
/// We are looking for code like the following:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... // No other stores or function calls (we could be calling a memory
/// ... // function).
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// store i32 %add5, i32* %arrayidx2
/// br label EndBB
/// EndBB:
/// ...
/// We are going to transform this into:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... //
/// %cmp = icmp ult %x, %y
/// %add.add5 = select i1 %cmp, i32 %add, %add5
/// store i32 %add.add5, i32* %arrayidx2
/// ...
///
/// \return The pointer to the value of the previous store if the store can be
/// hoisted into the predecessor block. 0 otherwise.
static Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB,
BasicBlock *StoreBB, BasicBlock *EndBB) {
StoreInst *StoreToHoist = dyn_cast<StoreInst>(I);
if (!StoreToHoist)
return nullptr;
// Volatile or atomic.
if (!StoreToHoist->isSimple())
return nullptr;
Value *StorePtr = StoreToHoist->getPointerOperand();
// Look for a store to the same pointer in BrBB.
unsigned MaxNumInstToLookAt = 9;
// Skip pseudo probe intrinsic calls which are not really killing any memory
// accesses.
for (Instruction &CurI : reverse(BrBB->instructionsWithoutDebug(true))) {
if (!MaxNumInstToLookAt)
break;
--MaxNumInstToLookAt;
// Could be calling an instruction that affects memory like free().
if (CurI.mayHaveSideEffects() && !isa<StoreInst>(CurI))
return nullptr;
if (auto *SI = dyn_cast<StoreInst>(&CurI)) {
// Found the previous store make sure it stores to the same location.
if (SI->getPointerOperand() == StorePtr)
// Found the previous store, return its value operand.
return SI->getValueOperand();
return nullptr; // Unknown store.
}
}
return nullptr;
}
/// Estimate the cost of the insertion(s) and check that the PHI nodes can be
/// converted to selects.
static bool validateAndCostRequiredSelects(BasicBlock *BB, BasicBlock *ThenBB,
BasicBlock *EndBB,
unsigned &SpeculatedInstructions,
InstructionCost &Cost,
const TargetTransformInfo &TTI) {
TargetTransformInfo::TargetCostKind CostKind =
BB->getParent()->hasMinSize()
? TargetTransformInfo::TCK_CodeSize
: TargetTransformInfo::TCK_SizeAndLatency;
bool HaveRewritablePHIs = false;
for (PHINode &PN : EndBB->phis()) {
Value *OrigV = PN.getIncomingValueForBlock(BB);
Value *ThenV = PN.getIncomingValueForBlock(ThenBB);
// FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf.
// Skip PHIs which are trivial.
if (ThenV == OrigV)
continue;
Cost += TTI.getCmpSelInstrCost(Instruction::Select, PN.getType(), nullptr,
CmpInst::BAD_ICMP_PREDICATE, CostKind);
// Don't convert to selects if we could remove undefined behavior instead.
if (passingValueIsAlwaysUndefined(OrigV, &PN) ||
passingValueIsAlwaysUndefined(ThenV, &PN))
return false;
HaveRewritablePHIs = true;
ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
if (!OrigCE && !ThenCE)
continue; // Known safe and cheap.
if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE)) ||
(OrigCE && !isSafeToSpeculativelyExecute(OrigCE)))
return false;
InstructionCost OrigCost = OrigCE ? computeSpeculationCost(OrigCE, TTI) : 0;
InstructionCost ThenCost = ThenCE ? computeSpeculationCost(ThenCE, TTI) : 0;
InstructionCost MaxCost =
2 * PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
if (OrigCost + ThenCost > MaxCost)
return false;
// Account for the cost of an unfolded ConstantExpr which could end up
// getting expanded into Instructions.
// FIXME: This doesn't account for how many operations are combined in the
// constant expression.
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
}
return HaveRewritablePHIs;
}
/// Speculate a conditional basic block flattening the CFG.
///
/// Note that this is a very risky transform currently. Speculating
/// instructions like this is most often not desirable. Instead, there is an MI
/// pass which can do it with full awareness of the resource constraints.
/// However, some cases are "obvious" and we should do directly. An example of
/// this is speculating a single, reasonably cheap instruction.
///
/// There is only one distinct advantage to flattening the CFG at the IR level:
/// it makes very common but simplistic optimizations such as are common in
/// instcombine and the DAG combiner more powerful by removing CFG edges and
/// modeling their effects with easier to reason about SSA value graphs.
///
///
/// An illustration of this transform is turning this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// %sub = sub %x, %y
/// br label BB2
/// EndBB:
/// %phi = phi [ %sub, %ThenBB ], [ 0, %EndBB ]
/// ...
/// \endcode
///
/// Into this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// %sub = sub %x, %y
/// %cond = select i1 %cmp, 0, %sub
/// ...
/// \endcode
///
/// \returns true if the conditional block is removed.
bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
const TargetTransformInfo &TTI) {
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
if (isa<FCmpInst>(BrCond))
return false;
BasicBlock *BB = BI->getParent();
BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
InstructionCost Budget =
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
// If ThenBB is actually on the false edge of the conditional branch, remember
// to swap the select operands later.
bool Invert = false;
if (ThenBB != BI->getSuccessor(0)) {
assert(ThenBB == BI->getSuccessor(1) && "No edge from 'if' block?");
Invert = true;
}
assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
// Keep a count of how many times instructions are used within ThenBB when
// they are candidates for sinking into ThenBB. Specifically:
// - They are defined in BB, and
// - They have no side effects, and
// - All of their uses are in ThenBB.
SmallDenseMap<Instruction *, unsigned, 4> SinkCandidateUseCounts;
SmallVector<Instruction *, 4> SpeculatedDbgIntrinsics;
unsigned SpeculatedInstructions = 0;
Value *SpeculatedStoreValue = nullptr;
StoreInst *SpeculatedStore = nullptr;
for (BasicBlock::iterator BBI = ThenBB->begin(),
BBE = std::prev(ThenBB->end());
BBI != BBE; ++BBI) {
Instruction *I = &*BBI;
// Skip debug info.
if (isa<DbgInfoIntrinsic>(I)) {
SpeculatedDbgIntrinsics.push_back(I);
continue;
}
// Skip pseudo probes. The consequence is we lose track of the branch
// probability for ThenBB, which is fine since the optimization here takes
// place regardless of the branch probability.
if (isa<PseudoProbeInst>(I)) {
continue;
}
// Only speculatively execute a single instruction (not counting the
// terminator) for now.
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
// Don't hoist the instruction if it's unsafe or expensive.
if (!isSafeToSpeculativelyExecute(I) &&
!(HoistCondStores && (SpeculatedStoreValue = isSafeToSpeculateStore(
I, BB, ThenBB, EndBB))))
return false;
if (!SpeculatedStoreValue &&
computeSpeculationCost(I, TTI) >
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
return false;
// Store the store speculation candidate.
if (SpeculatedStoreValue)
SpeculatedStore = cast<StoreInst>(I);
// Do not hoist the instruction if any of its operands are defined but not
// used in BB. The transformation will prevent the operand from
// being sunk into the use block.
for (Use &Op : I->operands()) {
Instruction *OpI = dyn_cast<Instruction>(Op);
if (!OpI || OpI->getParent() != BB || OpI->mayHaveSideEffects())
continue; // Not a candidate for sinking.
++SinkCandidateUseCounts[OpI];
}
}
// Consider any sink candidates which are only used in ThenBB as costs for
// speculation. Note, while we iterate over a DenseMap here, we are summing
// and so iteration order isn't significant.
for (SmallDenseMap<Instruction *, unsigned, 4>::iterator
I = SinkCandidateUseCounts.begin(),
E = SinkCandidateUseCounts.end();
I != E; ++I)
if (I->first->hasNUses(I->second)) {
++SpeculatedInstructions;
if (SpeculatedInstructions > 1)
return false;
}
// Check that we can insert the selects and that it's not too expensive to do
// so.
bool Convert = SpeculatedStore != nullptr;
InstructionCost Cost = 0;
Convert |= validateAndCostRequiredSelects(BB, ThenBB, EndBB,
SpeculatedInstructions,
Cost, TTI);
if (!Convert || Cost > Budget)
return false;
// If we get here, we can hoist the instruction and if-convert.
LLVM_DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
// Insert a select of the value of the speculated store.
if (SpeculatedStoreValue) {
IRBuilder<NoFolder> Builder(BI);
Value *TrueV = SpeculatedStore->getValueOperand();
Value *FalseV = SpeculatedStoreValue;
if (Invert)
std::swap(TrueV, FalseV);
Value *S = Builder.CreateSelect(
BrCond, TrueV, FalseV, "spec.store.select", BI);
SpeculatedStore->setOperand(0, S);
SpeculatedStore->applyMergedLocation(BI->getDebugLoc(),
SpeculatedStore->getDebugLoc());
}
// Metadata can be dependent on the condition we are hoisting above.
// Conservatively strip all metadata on the instruction. Drop the debug loc
// to avoid making it appear as if the condition is a constant, which would
// be misleading while debugging.
for (auto &I : *ThenBB) {
if (!SpeculatedStoreValue || &I != SpeculatedStore)
I.setDebugLoc(DebugLoc());
I.dropUnknownNonDebugMetadata();
}
// A hoisted conditional probe should be treated as dangling so that it will
// not be over-counted when the samples collected on the non-conditional path
// are counted towards the conditional path. We leave it for the counts
// inference algorithm to figure out a proper count for a danglng probe.
moveAndDanglePseudoProbes(ThenBB, BI);
// Hoist the instructions.
BB->getInstList().splice(BI->getIterator(), ThenBB->getInstList(),
ThenBB->begin(), std::prev(ThenBB->end()));
// Insert selects and rewrite the PHI operands.
IRBuilder<NoFolder> Builder(BI);
for (PHINode &PN : EndBB->phis()) {
unsigned OrigI = PN.getBasicBlockIndex(BB);
unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
Value *OrigV = PN.getIncomingValue(OrigI);
Value *ThenV = PN.getIncomingValue(ThenI);
// Skip PHIs which are trivial.
if (OrigV == ThenV)
continue;
// Create a select whose true value is the speculatively executed value and
// false value is the pre-existing value. Swap them if the branch
// destinations were inverted.
Value *TrueV = ThenV, *FalseV = OrigV;
if (Invert)
std::swap(TrueV, FalseV);
Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV, "spec.select", BI);
PN.setIncomingValue(OrigI, V);
PN.setIncomingValue(ThenI, V);
}
// Remove speculated dbg intrinsics.
// FIXME: Is it possible to do this in a more elegant way? Moving/merging the
// dbg value for the different flows and inserting it after the select.
for (Instruction *I : SpeculatedDbgIntrinsics)
I->eraseFromParent();
++NumSpeculations;
return true;
}
/// Return true if we can thread a branch across this block.
static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
int Size = 0;
for (Instruction &I : BB->instructionsWithoutDebug()) {
if (Size > MaxSmallBlockSize)
return false; // Don't clone large BB's.
// Can't fold blocks that contain noduplicate or convergent calls.
if (CallInst *CI = dyn_cast<CallInst>(&I))
if (CI->cannotDuplicate() || CI->isConvergent())
return false;
// We will delete Phis while threading, so Phis should not be accounted in
// block's size
if (!isa<PHINode>(I))
++Size;
// We can only support instructions that do not define values that are
// live outside of the current basic block.
for (User *U : I.users()) {
Instruction *UI = cast<Instruction>(U);
if (UI->getParent() != BB || isa<PHINode>(UI))
return false;
}
// Looks ok, continue checking.
}
return true;
}
/// If we have a conditional branch on a PHI node value that is defined in the
/// same block as the branch and if any PHI entries are constants, thread edges
/// corresponding to that entry to be branches to their ultimate destination.
static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
const DataLayout &DL, AssumptionCache *AC) {
BasicBlock *BB = BI->getParent();
PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
// NOTE: we currently cannot transform this case if the PHI node is used
// outside of the block.
if (!PN || PN->getParent() != BB || !PN->hasOneUse())
return false;
// Degenerate case of a single entry PHI.
if (PN->getNumIncomingValues() == 1) {
FoldSingleEntryPHINodes(PN->getParent());
return true;
}
// Now we know that this block has multiple preds and two succs.
if (!BlockIsSimpleEnoughToThreadThrough(BB))
return false;
// Okay, this is a simple enough basic block. See if any phi values are
// constants.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
if (!CB || !CB->getType()->isIntegerTy(1))
continue;
// Okay, we now know that all edges from PredBB should be revectored to
// branch to RealDest.
BasicBlock *PredBB = PN->getIncomingBlock(i);
BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
if (RealDest == BB)
continue; // Skip self loops.
// Skip if the predecessor's terminator is an indirect branch.
if (isa<IndirectBrInst>(PredBB->getTerminator()))
continue;
SmallVector<DominatorTree::UpdateType, 3> Updates;
// The dest block might have PHI nodes, other predecessors and other
// difficult cases. Instead of being smart about this, just insert a new
// block that jumps to the destination block, effectively splitting
// the edge we are about to create.
BasicBlock *EdgeBB =
BasicBlock::Create(BB->getContext(), RealDest->getName() + ".critedge",
RealDest->getParent(), RealDest);
BranchInst *CritEdgeBranch = BranchInst::Create(RealDest, EdgeBB);
Updates.push_back({DominatorTree::Insert, EdgeBB, RealDest});
CritEdgeBranch->setDebugLoc(BI->getDebugLoc());
// Update PHI nodes.
AddPredecessorToBlock(RealDest, EdgeBB, BB);
// BB may have instructions that are being threaded over. Clone these
// instructions into EdgeBB. We know that there will be no uses of the
// cloned instructions outside of EdgeBB.
BasicBlock::iterator InsertPt = EdgeBB->begin();
DenseMap<Value *, Value *> TranslateMap; // Track translated values.
for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
continue;
}
// Clone the instruction.
Instruction *N = BBI->clone();
if (BBI->hasName())
N->setName(BBI->getName() + ".c");
// Update operands due to translation.
for (Use &Op : N->operands()) {
DenseMap<Value *, Value *>::iterator PI = TranslateMap.find(Op);
if (PI != TranslateMap.end())
Op = PI->second;
}
// Check for trivial simplification.
if (Value *V = SimplifyInstruction(N, {DL, nullptr, nullptr, AC})) {
if (!BBI->use_empty())
TranslateMap[&*BBI] = V;
if (!N->mayHaveSideEffects()) {
N->deleteValue(); // Instruction folded away, don't need actual inst
N = nullptr;
}
} else {
if (!BBI->use_empty())
TranslateMap[&*BBI] = N;
}
if (N) {
// Insert the new instruction into its new home.
EdgeBB->getInstList().insert(InsertPt, N);
// Register the new instruction with the assumption cache if necessary.
if (AC && match(N, m_Intrinsic<Intrinsic::assume>()))
AC->registerAssumption(cast<IntrinsicInst>(N));
}
}
// Loop over all of the edges from PredBB to BB, changing them to branch
// to EdgeBB instead.
Instruction *PredBBTI = PredBB->getTerminator();
for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
if (PredBBTI->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB);
PredBBTI->setSuccessor(i, EdgeBB);
}
Updates.push_back({DominatorTree::Insert, PredBB, EdgeBB});
Updates.push_back({DominatorTree::Delete, PredBB, BB});
if (DTU)
DTU->applyUpdates(Updates);
// Recurse, simplifying any other constants.
return FoldCondBranchOnPHI(BI, DTU, DL, AC) || true;
}
return false;
}
/// Given a BB that starts with the specified two-entry PHI node,
/// see if we can eliminate it.
static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
DomTreeUpdater *DTU, const DataLayout &DL) {
// Ok, this is a two entry PHI node. Check to see if this is a simple "if
// statement", which has a very simple dominance structure. Basically, we
// are trying to find the condition that is being branched on, which
// subsequently causes this merge to happen. We really want control
// dependence information for this check, but simplifycfg can't keep it up
// to date, and this catches most of the cases we care about anyway.
BasicBlock *BB = PN->getParent();
BasicBlock *IfTrue, *IfFalse;
Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
if (!IfCond ||
// Don't bother if the branch will be constant folded trivially.
isa<ConstantInt>(IfCond))
return false;
// Okay, we found that we can merge this two-entry phi node into a select.
// Doing so would require us to fold *all* two entry phi nodes in this block.
// At some point this becomes non-profitable (particularly if the target
// doesn't support cmov's). Only do this transformation if there are two or
// fewer PHI nodes in this block.
unsigned NumPhis = 0;
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
if (NumPhis > 2)
return false;
// Loop over the PHI's seeing if we can promote them all to select
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
SmallPtrSet<Instruction *, 4> AggressiveInsts;
InstructionCost Cost = 0;
InstructionCost Budget =
TwoEntryPHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
bool Changed = false;
for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
PHINode *PN = cast<PHINode>(II++);
if (Value *V = SimplifyInstruction(PN, {DL, PN})) {
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
Changed = true;
continue;
}
if (!dominatesMergePoint(PN->getIncomingValue(0), BB, AggressiveInsts,
Cost, Budget, TTI) ||
!dominatesMergePoint(PN->getIncomingValue(1), BB, AggressiveInsts,
Cost, Budget, TTI))
return Changed;
}
// If we folded the first phi, PN dangles at this point. Refresh it. If
// we ran out of PHIs then we simplified them all.
PN = dyn_cast<PHINode>(BB->begin());
if (!PN)
return true;
// Return true if at least one of these is a 'not', and another is either
// a 'not' too, or a constant.
auto CanHoistNotFromBothValues = [](Value *V0, Value *V1) {
if (!match(V0, m_Not(m_Value())))
std::swap(V0, V1);
auto Invertible = m_CombineOr(m_Not(m_Value()), m_AnyIntegralConstant());
return match(V0, m_Not(m_Value())) && match(V1, Invertible);
};
// Don't fold i1 branches on PHIs which contain binary operators or
// select form of or/ands, unless one of the incoming values is an 'not' and
// another one is freely invertible.
// These can often be turned into switches and other things.
auto IsBinOpOrAnd = [](Value *V) {
return match(
V, m_CombineOr(m_BinOp(), m_CombineOr(m_LogicalAnd(), m_LogicalOr())));
};
if (PN->getType()->isIntegerTy(1) &&
(IsBinOpOrAnd(PN->getIncomingValue(0)) ||
IsBinOpOrAnd(PN->getIncomingValue(1)) || IsBinOpOrAnd(IfCond)) &&
!CanHoistNotFromBothValues(PN->getIncomingValue(0),
PN->getIncomingValue(1)))
return Changed;
// If all PHI nodes are promotable, check to make sure that all instructions
// in the predecessor blocks can be promoted as well. If not, we won't be able
// to get rid of the control flow, so it's not worth promoting to select
// instructions.
BasicBlock *DomBlock = nullptr;
BasicBlock *IfBlock1 = PN->getIncomingBlock(0);
BasicBlock *IfBlock2 = PN->getIncomingBlock(1);
if (cast<BranchInst>(IfBlock1->getTerminator())->isConditional()) {
IfBlock1 = nullptr;
} else {
DomBlock = *pred_begin(IfBlock1);
for (BasicBlock::iterator I = IfBlock1->begin(); !I->isTerminator(); ++I)
if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I) &&
!isa<PseudoProbeInst>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control flow, so
// the xform is not worth it.
return Changed;
}
}
if (cast<BranchInst>(IfBlock2->getTerminator())->isConditional()) {
IfBlock2 = nullptr;
} else {
DomBlock = *pred_begin(IfBlock2);
for (BasicBlock::iterator I = IfBlock2->begin(); !I->isTerminator(); ++I)
if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I) &&
!isa<PseudoProbeInst>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control flow, so
// the xform is not worth it.
return Changed;
}
}
assert(DomBlock && "Failed to find root DomBlock");
LLVM_DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond
<< " T: " << IfTrue->getName()
<< " F: " << IfFalse->getName() << "\n");
// If we can still promote the PHI nodes after this gauntlet of tests,
// do all of the PHI's now.
Instruction *InsertPt = DomBlock->getTerminator();
IRBuilder<NoFolder> Builder(InsertPt);
// Move all 'aggressive' instructions, which are defined in the
// conditional parts of the if's up to the dominating block.
if (IfBlock1)
hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock1);
if (IfBlock2)
hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock2);
// Propagate fast-math-flags from phi nodes to replacement selects.
IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
if (isa<FPMathOperator>(PN))
Builder.setFastMathFlags(PN->getFastMathFlags());
// Change the PHI node into a select instruction.
Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
Value *Sel = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", InsertPt);
PN->replaceAllUsesWith(Sel);
Sel->takeName(PN);
PN->eraseFromParent();
}
// At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
// has been flattened. Change DomBlock to jump directly to our new block to
// avoid other simplifycfg's kicking in on the diamond.
Instruction *OldTI = DomBlock->getTerminator();
Builder.SetInsertPoint(OldTI);
Builder.CreateBr(BB);
SmallVector<DominatorTree::UpdateType, 3> Updates;
if (DTU) {
Updates.push_back({DominatorTree::Insert, DomBlock, BB});
for (auto *Successor : successors(DomBlock))
Updates.push_back({DominatorTree::Delete, DomBlock, Successor});
}
OldTI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
/// If we found a conditional branch that goes to two returning blocks,
/// try to merge them together into one return,
/// introducing a select if the return values disagree.
bool SimplifyCFGOpt::SimplifyCondBranchToTwoReturns(BranchInst *BI,
IRBuilder<> &Builder) {
auto *BB = BI->getParent();
assert(BI->isConditional() && "Must be a conditional branch");
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
// NOTE: destinations may match, this could be degenerate uncond branch.
ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
// Check to ensure both blocks are empty (just a return) or optionally empty
// with PHI nodes. If there are other instructions, merging would cause extra
// computation on one path or the other.
if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator())
return false;
if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator())
return false;
Builder.SetInsertPoint(BI);
// Okay, we found a branch that is going to two return nodes. If
// there is no return value for this function, just change the
// branch into a return.
if (FalseRet->getNumOperands() == 0) {
TrueSucc->removePredecessor(BB);
FalseSucc->removePredecessor(BB);
Builder.CreateRetVoid();
EraseTerminatorAndDCECond(BI);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.push_back({DominatorTree::Delete, BB, TrueSucc});
if (TrueSucc != FalseSucc)
Updates.push_back({DominatorTree::Delete, BB, FalseSucc});
DTU->applyUpdates(Updates);
}
return true;
}
// Otherwise, figure out what the true and false return values are
// so we can insert a new select instruction.
Value *TrueValue = TrueRet->getReturnValue();
Value *FalseValue = FalseRet->getReturnValue();
// Unwrap any PHI nodes in the return blocks.
if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
if (TVPN->getParent() == TrueSucc)
TrueValue = TVPN->getIncomingValueForBlock(BB);
if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
if (FVPN->getParent() == FalseSucc)
FalseValue = FVPN->getIncomingValueForBlock(BB);
// In order for this transformation to be safe, we must be able to
// unconditionally execute both operands to the return. This is
// normally the case, but we could have a potentially-trapping
// constant expression that prevents this transformation from being
// safe.
if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
if (TCV->canTrap())
return false;
if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
if (FCV->canTrap())
return false;
// Okay, we collected all the mapped values and checked them for sanity, and
// defined to really do this transformation. First, update the CFG.
TrueSucc->removePredecessor(BB);
FalseSucc->removePredecessor(BB);
// Insert select instructions where needed.
Value *BrCond = BI->getCondition();
if (TrueValue) {
// Insert a select if the results differ.
if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
} else if (isa<UndefValue>(TrueValue)) {
TrueValue = FalseValue;
} else {
TrueValue =
Builder.CreateSelect(BrCond, TrueValue, FalseValue, "retval", BI);
}
}
Value *RI =
!TrueValue ? Builder.CreateRetVoid() : Builder.CreateRet(TrueValue);
(void)RI;
LLVM_DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
<< "\n " << *BI << "\nNewRet = " << *RI << "\nTRUEBLOCK: "
<< *TrueSucc << "\nFALSEBLOCK: " << *FalseSucc);
EraseTerminatorAndDCECond(BI);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.push_back({DominatorTree::Delete, BB, TrueSucc});
if (TrueSucc != FalseSucc)
Updates.push_back({DominatorTree::Delete, BB, FalseSucc});
DTU->applyUpdates(Updates);
}
return true;
}
/// Return true if either PBI or BI has branch weight available, and store
/// the weights in {Pred|Succ}{True|False}Weight. If one of PBI and BI does
/// not have branch weight, use 1:1 as its weight.
static bool extractPredSuccWeights(BranchInst *PBI, BranchInst *BI,
uint64_t &PredTrueWeight,
uint64_t &PredFalseWeight,
uint64_t &SuccTrueWeight,
uint64_t &SuccFalseWeight) {
bool PredHasWeights =
PBI->extractProfMetadata(PredTrueWeight, PredFalseWeight);
bool SuccHasWeights =
BI->extractProfMetadata(SuccTrueWeight, SuccFalseWeight);
if (PredHasWeights || SuccHasWeights) {
if (!PredHasWeights)
PredTrueWeight = PredFalseWeight = 1;
if (!SuccHasWeights)
SuccTrueWeight = SuccFalseWeight = 1;
return true;
} else {
return false;
}
}
/// Determine if the two branches share a common destination and deduce a glue
/// that joins the branches' conditions to arrive at the common destination if
/// that would be profitable.
static Optional<std::pair<Instruction::BinaryOps, bool>>
shouldFoldCondBranchesToCommonDestination(BranchInst *BI, BranchInst *PBI,
const TargetTransformInfo *TTI) {
assert(BI && PBI && BI->isConditional() && PBI->isConditional() &&
"Both blocks must end with a conditional branches.");
assert(is_contained(predecessors(BI->getParent()), PBI->getParent()) &&
"PredBB must be a predecessor of BB.");
// We have the potential to fold the conditions together, but if the
// predecessor branch is predictable, we may not want to merge them.
uint64_t PTWeight, PFWeight;
BranchProbability PBITrueProb, Likely;
if (TTI && PBI->extractProfMetadata(PTWeight, PFWeight) &&
(PTWeight + PFWeight) != 0) {
PBITrueProb =
BranchProbability::getBranchProbability(PTWeight, PTWeight + PFWeight);
Likely = TTI->getPredictableBranchThreshold();
}
if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
// Speculate the 2nd condition unless the 1st is probably true.
if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
return {{Instruction::Or, false}};
} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
// Speculate the 2nd condition unless the 1st is probably false.
if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
return {{Instruction::And, false}};
} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
// Speculate the 2nd condition unless the 1st is probably true.
if (PBITrueProb.isUnknown() || PBITrueProb < Likely)
return {{Instruction::And, true}};
} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
// Speculate the 2nd condition unless the 1st is probably false.
if (PBITrueProb.isUnknown() || PBITrueProb.getCompl() < Likely)
return {{Instruction::Or, true}};
}
return None;
}
static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
bool PoisonSafe,
const TargetTransformInfo *TTI) {
BasicBlock *BB = BI->getParent();
BasicBlock *PredBlock = PBI->getParent();
// Determine if the two branches share a common destination.
Instruction::BinaryOps Opc;
bool InvertPredCond;
std::tie(Opc, InvertPredCond) =
*shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI);
LLVM_DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
IRBuilder<> Builder(PBI);
// The builder is used to create instructions to eliminate the branch in BB.
// If BB's terminator has !annotation metadata, add it to the new
// instructions.
Builder.CollectMetadataToCopy(BB->getTerminator(),
{LLVMContext::MD_annotation});
// If we need to invert the condition in the pred block to match, do so now.
if (InvertPredCond) {
Value *NewCond = PBI->getCondition();
if (NewCond->hasOneUse() && isa<CmpInst>(NewCond)) {
CmpInst *CI = cast<CmpInst>(NewCond);
CI->setPredicate(CI->getInversePredicate());
} else {
NewCond =
Builder.CreateNot(NewCond, PBI->getCondition()->getName() + ".not");
}
PBI->setCondition(NewCond);
PBI->swapSuccessors();
}
BasicBlock *UniqueSucc =
PBI->getSuccessor(0) == BB ? BI->getSuccessor(0) : BI->getSuccessor(1);
// Before cloning instructions, notify the successor basic block that it
// is about to have a new predecessor. This will update PHI nodes,
// which will allow us to update live-out uses of bonus instructions.
AddPredecessorToBlock(UniqueSucc, PredBlock, BB, MSSAU);
// Try to update branch weights.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
if (extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight)) {
SmallVector<uint64_t, 8> NewWeights;
if (PBI->getSuccessor(0) == BB) {
// PBI: br i1 %x, BB, FalseDest
// BI: br i1 %y, UniqueSucc, FalseDest
// TrueWeight is TrueWeight for PBI * TrueWeight for BI.
NewWeights.push_back(PredTrueWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * TotalWeight for BI +
// TrueWeight for PBI * FalseWeight for BI.
// We assume that total weights of a BranchInst can fit into 32 bits.
// Therefore, we will not have overflow using 64-bit arithmetic.
NewWeights.push_back(PredFalseWeight *
(SuccFalseWeight + SuccTrueWeight) +
PredTrueWeight * SuccFalseWeight);
} else {
// PBI: br i1 %x, TrueDest, BB
// BI: br i1 %y, TrueDest, UniqueSucc
// TrueWeight is TrueWeight for PBI * TotalWeight for BI +
// FalseWeight for PBI * TrueWeight for BI.
NewWeights.push_back(PredTrueWeight * (SuccFalseWeight + SuccTrueWeight) +
PredFalseWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * FalseWeight for BI.
NewWeights.push_back(PredFalseWeight * SuccFalseWeight);
}
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(NewWeights);
SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(), NewWeights.end());
setBranchWeights(PBI, MDWeights[0], MDWeights[1]);
// TODO: If BB is reachable from all paths through PredBlock, then we
// could replace PBI's branch probabilities with BI's.
} else
PBI->setMetadata(LLVMContext::MD_prof, nullptr);
// Now, update the CFG.
PBI->setSuccessor(PBI->getSuccessor(0) != BB, UniqueSucc);
if (DTU)
DTU->applyUpdates({{DominatorTree::Insert, PredBlock, UniqueSucc},
{DominatorTree::Delete, PredBlock, BB}});
// If BI was a loop latch, it may have had associated loop metadata.
// We need to copy it to the new latch, that is, PBI.
if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
PBI->setMetadata(LLVMContext::MD_loop, LoopMD);
ValueToValueMapTy VMap; // maps original values to cloned values
CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(BB, PredBlock, VMap);
// Now that the Cond was cloned into the predecessor basic block,
// or/and the two conditions together.
Value *NewCond = nullptr;
Value *BICond = VMap[BI->getCondition()];
bool UseBinOp = !PoisonSafe || impliesPoison(BICond, PBI->getCondition());
if (UseBinOp)
NewCond = Builder.CreateBinOp(Opc, PBI->getCondition(), BICond, "or.cond");
else
NewCond =
Opc == Instruction::And
? Builder.CreateLogicalAnd(PBI->getCondition(), BICond, "or.cond")
: Builder.CreateLogicalOr(PBI->getCondition(), BICond, "or.cond");
PBI->setCondition(NewCond);
// Copy any debug value intrinsics into the end of PredBlock.
for (Instruction &I : *BB) {
if (isa<DbgInfoIntrinsic>(I)) {
Instruction *NewI = I.clone();
RemapInstruction(NewI, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
NewI->insertBefore(PBI);
}
}
++NumFoldBranchToCommonDest;
return true;
}
/// If this basic block is simple enough, and if a predecessor branches to us
/// and one of our successors, fold the block into the predecessor and use
/// logical operations to pick the right destination.
bool llvm::FoldBranchToCommonDest(BranchInst *BI, DomTreeUpdater *DTU,
MemorySSAUpdater *MSSAU,
const TargetTransformInfo *TTI,
unsigned BonusInstThreshold,
bool PoisonSafe) {
// If this block ends with an unconditional branch,
// let SpeculativelyExecuteBB() deal with it.
if (!BI->isConditional())
return false;
BasicBlock *BB = BI->getParent();
bool Changed = false;
TargetTransformInfo::TargetCostKind CostKind =
BB->getParent()->hasMinSize() ? TargetTransformInfo::TCK_CodeSize
: TargetTransformInfo::TCK_SizeAndLatency;
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (!Cond || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
Cond->getParent() != BB || !Cond->hasOneUse())
return Changed;
// Cond is known to be a compare or binary operator. Check to make sure that
// neither operand is a potentially-trapping constant expression.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
if (CE->canTrap())
return Changed;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
if (CE->canTrap())
return Changed;
// Finally, don't infinitely unroll conditional loops.
if (is_contained(successors(BB), BB))
return Changed;
// With which predecessors will we want to deal with?
SmallVector<BasicBlock *, 8> Preds;
for (BasicBlock *PredBlock : predecessors(BB)) {
BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
// Check that we have two conditional branches. If there is a PHI node in
// the common successor, verify that the same value flows in from both
// blocks.
if (!PBI || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI))
continue;
// Determine if the two branches share a common destination.
Instruction::BinaryOps Opc;
bool InvertPredCond;
if (auto Recipe = shouldFoldCondBranchesToCommonDestination(BI, PBI, TTI))
std::tie(Opc, InvertPredCond) = *Recipe;
else
continue;
// Check the cost of inserting the necessary logic before performing the
// transformation.
if (TTI) {
Type *Ty = BI->getCondition()->getType();
InstructionCost Cost = TTI->getArithmeticInstrCost(Opc, Ty, CostKind);
if (InvertPredCond && (!PBI->getCondition()->hasOneUse() ||
!isa<CmpInst>(PBI->getCondition())))
Cost += TTI->getArithmeticInstrCost(Instruction::Xor, Ty, CostKind);
if (Cost > BranchFoldThreshold)
continue;
}
// Ok, we do want to deal with this predecessor. Record it.
Preds.emplace_back(PredBlock);
}
const unsigned PredCount = Preds.size();
// Only allow this transformation if computing the condition doesn't involve
// too many instructions and these involved instructions can be executed
// unconditionally. We denote all involved instructions except the condition
// as "bonus instructions", and only allow this transformation when the
// number of the bonus instructions we'll need to create when cloning into
// each predecessor does not exceed a certain threshold.
unsigned NumBonusInsts = 0;
for (Instruction &I : *BB) {
// Don't check the branch condition comparison itself.
if (&I == Cond)
continue;
// Ignore dbg intrinsics, and the terminator.
if (isa<DbgInfoIntrinsic>(I) || isa<BranchInst>(I))
continue;
// I must be safe to execute unconditionally.
if (!isSafeToSpeculativelyExecute(&I))
return Changed;
// Account for the cost of duplicating this instruction into each
// predecessor.
NumBonusInsts += PredCount;
// Early exits once we reach the limit.
if (NumBonusInsts > BonusInstThreshold)
return Changed;
}
// Ok, we have the budget. Perform the transformation.
for (BasicBlock *PredBlock : Preds) {
auto *PBI = cast<BranchInst>(PredBlock->getTerminator());
return performBranchToCommonDestFolding(BI, PBI, DTU, MSSAU, PoisonSafe,
TTI);
}
return Changed;
}
// If there is only one store in BB1 and BB2, return it, otherwise return
// nullptr.
static StoreInst *findUniqueStoreInBlocks(BasicBlock *BB1, BasicBlock *BB2) {
StoreInst *S = nullptr;
for (auto *BB : {BB1, BB2}) {
if (!BB)
continue;
for (auto &I : *BB)
if (auto *SI = dyn_cast<StoreInst>(&I)) {
if (S)
// Multiple stores seen.
return nullptr;
else
S = SI;
}
}
return S;
}
static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
Value *AlternativeV = nullptr) {
// PHI is going to be a PHI node that allows the value V that is defined in
// BB to be referenced in BB's only successor.
//
// If AlternativeV is nullptr, the only value we care about in PHI is V. It
// doesn't matter to us what the other operand is (it'll never get used). We
// could just create a new PHI with an undef incoming value, but that could
// increase register pressure if EarlyCSE/InstCombine can't fold it with some
// other PHI. So here we directly look for some PHI in BB's successor with V
// as an incoming operand. If we find one, we use it, else we create a new
// one.
//
// If AlternativeV is not nullptr, we care about both incoming values in PHI.
// PHI must be exactly: phi <ty> [ %BB, %V ], [ %OtherBB, %AlternativeV]
// where OtherBB is the single other predecessor of BB's only successor.
PHINode *PHI = nullptr;
BasicBlock *Succ = BB->getSingleSuccessor();
for (auto I = Succ->begin(); isa<PHINode>(I); ++I)
if (cast<PHINode>(I)->getIncomingValueForBlock(BB) == V) {
PHI = cast<PHINode>(I);
if (!AlternativeV)
break;
assert(Succ->hasNPredecessors(2));
auto PredI = pred_begin(Succ);
BasicBlock *OtherPredBB = *PredI == BB ? *++PredI : *PredI;
if (PHI->getIncomingValueForBlock(OtherPredBB) == AlternativeV)
break;
PHI = nullptr;
}
if (PHI)
return PHI;
// If V is not an instruction defined in BB, just return it.
if (!AlternativeV &&
(!isa<Instruction>(V) || cast<Instruction>(V)->getParent() != BB))
return V;
PHI = PHINode::Create(V->getType(), 2, "simplifycfg.merge", &Succ->front());
PHI->addIncoming(V, BB);
for (BasicBlock *PredBB : predecessors(Succ))
if (PredBB != BB)
PHI->addIncoming(
AlternativeV ? AlternativeV : UndefValue::get(V->getType()), PredBB);
return PHI;
}
static bool mergeConditionalStoreToAddress(
BasicBlock *PTB, BasicBlock *PFB, BasicBlock *QTB, BasicBlock *QFB,
BasicBlock *PostBB, Value *Address, bool InvertPCond, bool InvertQCond,
DomTreeUpdater *DTU, const DataLayout &DL, const TargetTransformInfo &TTI) {
// For every pointer, there must be exactly two stores, one coming from
// PTB or PFB, and the other from QTB or QFB. We don't support more than one
// store (to any address) in PTB,PFB or QTB,QFB.
// FIXME: We could relax this restriction with a bit more work and performance
// testing.
StoreInst *PStore = findUniqueStoreInBlocks(PTB, PFB);
StoreInst *QStore = findUniqueStoreInBlocks(QTB, QFB);
if (!PStore || !QStore)
return false;
// Now check the stores are compatible.
if (!QStore->isUnordered() || !PStore->isUnordered())
return false;
// Check that sinking the store won't cause program behavior changes. Sinking
// the store out of the Q blocks won't change any behavior as we're sinking
// from a block to its unconditional successor. But we're moving a store from
// the P blocks down through the middle block (QBI) and past both QFB and QTB.
// So we need to check that there are no aliasing loads or stores in
// QBI, QTB and QFB. We also need to check there are no conflicting memory
// operations between PStore and the end of its parent block.
//
// The ideal way to do this is to query AliasAnalysis, but we don't
// preserve AA currently so that is dangerous. Be super safe and just
// check there are no other memory operations at all.
for (auto &I : *QFB->getSinglePredecessor())
if (I.mayReadOrWriteMemory())
return false;
for (auto &I : *QFB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
if (QTB)
for (auto &I : *QTB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
for (auto I = BasicBlock::iterator(PStore), E = PStore->getParent()->end();
I != E; ++I)
if (&*I != PStore && I->mayReadOrWriteMemory())
return false;
// If we're not in aggressive mode, we only optimize if we have some
// confidence that by optimizing we'll allow P and/or Q to be if-converted.
auto IsWorthwhile = [&](BasicBlock *BB, ArrayRef<StoreInst *> FreeStores) {
if (!BB)
return true;
// Heuristic: if the block can be if-converted/phi-folded and the
// instructions inside are all cheap (arithmetic/GEPs), it's worthwhile to
// thread this store.
InstructionCost Cost = 0;
InstructionCost Budget =
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
for (auto &I : BB->instructionsWithoutDebug()) {
// Consider terminator instruction to be free.
if (I.isTerminator())
continue;
// If this is one the stores that we want to speculate out of this BB,
// then don't count it's cost, consider it to be free.
if (auto *S = dyn_cast<StoreInst>(&I))
if (llvm::find(FreeStores, S))
continue;
// Else, we have a white-list of instructions that we are ak speculating.
if (!isa<BinaryOperator>(I) && !isa<GetElementPtrInst>(I))
return false; // Not in white-list - not worthwhile folding.
// And finally, if this is a non-free instruction that we are okay
// speculating, ensure that we consider the speculation budget.
Cost += TTI.getUserCost(&I, TargetTransformInfo::TCK_SizeAndLatency);
if (Cost > Budget)
return false; // Eagerly refuse to fold as soon as we're out of budget.
}
assert(Cost <= Budget &&
"When we run out of budget we will eagerly return from within the "
"per-instruction loop.");
return true;
};
const std::array<StoreInst *, 2> FreeStores = {PStore, QStore};
if (!MergeCondStoresAggressively &&
(!IsWorthwhile(PTB, FreeStores) || !IsWorthwhile(PFB, FreeStores) ||
!IsWorthwhile(QTB, FreeStores) || !IsWorthwhile(QFB, FreeStores)))
return false;
// If PostBB has more than two predecessors, we need to split it so we can
// sink the store.
if (std::next(pred_begin(PostBB), 2) != pred_end(PostBB)) {
// We know that QFB's only successor is PostBB. And QFB has a single
// predecessor. If QTB exists, then its only successor is also PostBB.
// If QTB does not exist, then QFB's only predecessor has a conditional
// branch to QFB and PostBB.
BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
BasicBlock *NewBB =
SplitBlockPredecessors(PostBB, {QFB, TruePred}, "condstore.split", DTU);
if (!NewBB)
return false;
PostBB = NewBB;
}
// OK, we're going to sink the stores to PostBB. The store has to be
// conditional though, so first create the predicate.
Value *PCond = cast<BranchInst>(PFB->getSinglePredecessor()->getTerminator())
->getCondition();
Value *QCond = cast<BranchInst>(QFB->getSinglePredecessor()->getTerminator())
->getCondition();
Value *PPHI = ensureValueAvailableInSuccessor(PStore->getValueOperand(),
PStore->getParent());
Value *QPHI = ensureValueAvailableInSuccessor(QStore->getValueOperand(),
QStore->getParent(), PPHI);
IRBuilder<> QB(&*PostBB->getFirstInsertionPt());
Value *PPred = PStore->getParent() == PTB ? PCond : QB.CreateNot(PCond);
Value *QPred = QStore->getParent() == QTB ? QCond : QB.CreateNot(QCond);
if (InvertPCond)
PPred = QB.CreateNot(PPred);
if (InvertQCond)
QPred = QB.CreateNot(QPred);
Value *CombinedPred = QB.CreateOr(PPred, QPred);
auto *T = SplitBlockAndInsertIfThen(CombinedPred, &*QB.GetInsertPoint(),
/*Unreachable=*/false,
/*BranchWeights=*/nullptr, DTU);
QB.SetInsertPoint(T);
StoreInst *SI = cast<StoreInst>(QB.CreateStore(QPHI, Address));
AAMDNodes AAMD;
PStore->getAAMetadata(AAMD, /*Merge=*/false);
PStore->getAAMetadata(AAMD, /*Merge=*/true);
SI->setAAMetadata(AAMD);
// Choose the minimum alignment. If we could prove both stores execute, we
// could use biggest one. In this case, though, we only know that one of the
// stores executes. And we don't know it's safe to take the alignment from a
// store that doesn't execute.
SI->setAlignment(std::min(PStore->getAlign(), QStore->getAlign()));
QStore->eraseFromParent();
PStore->eraseFromParent();
return true;
}
static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI) {
// The intention here is to find diamonds or triangles (see below) where each
// conditional block contains a store to the same address. Both of these
// stores are conditional, so they can't be unconditionally sunk. But it may
// be profitable to speculatively sink the stores into one merged store at the
// end, and predicate the merged store on the union of the two conditions of
// PBI and QBI.
//
// This can reduce the number of stores executed if both of the conditions are
// true, and can allow the blocks to become small enough to be if-converted.
// This optimization will also chain, so that ladders of test-and-set
// sequences can be if-converted away.
//
// We only deal with simple diamonds or triangles:
//
// PBI or PBI or a combination of the two
// / \ | \
// PTB PFB | PFB
// \ / | /
// QBI QBI
// / \ | \
// QTB QFB | QFB
// \ / | /
// PostBB PostBB
//
// We model triangles as a type of diamond with a nullptr "true" block.
// Triangles are canonicalized so that the fallthrough edge is represented by
// a true condition, as in the diagram above.
BasicBlock *PTB = PBI->getSuccessor(0);
BasicBlock *PFB = PBI->getSuccessor(1);
BasicBlock *QTB = QBI->getSuccessor(0);
BasicBlock *QFB = QBI->getSuccessor(1);
BasicBlock *PostBB = QFB->getSingleSuccessor();
// Make sure we have a good guess for PostBB. If QTB's only successor is
// QFB, then QFB is a better PostBB.
if (QTB->getSingleSuccessor() == QFB)
PostBB = QFB;
// If we couldn't find a good PostBB, stop.
if (!PostBB)
return false;
bool InvertPCond = false, InvertQCond = false;
// Canonicalize fallthroughs to the true branches.
if (PFB == QBI->getParent()) {
std::swap(PFB, PTB);
InvertPCond = true;
}
if (QFB == PostBB) {
std::swap(QFB, QTB);
InvertQCond = true;
}
// From this point on we can assume PTB or QTB may be fallthroughs but PFB
// and QFB may not. Model fallthroughs as a nullptr block.
if (PTB == QBI->getParent())
PTB = nullptr;
if (QTB == PostBB)
QTB = nullptr;
// Legality bailouts. We must have at least the non-fallthrough blocks and
// the post-dominating block, and the non-fallthroughs must only have one
// predecessor.
auto HasOnePredAndOneSucc = [](BasicBlock *BB, BasicBlock *P, BasicBlock *S) {
return BB->getSinglePredecessor() == P && BB->getSingleSuccessor() == S;
};
if (!HasOnePredAndOneSucc(PFB, PBI->getParent(), QBI->getParent()) ||
!HasOnePredAndOneSucc(QFB, QBI->getParent(), PostBB))
return false;
if ((PTB && !HasOnePredAndOneSucc(PTB, PBI->getParent(), QBI->getParent())) ||
(QTB && !HasOnePredAndOneSucc(QTB, QBI->getParent(), PostBB)))
return false;
if (!QBI->getParent()->hasNUses(2))
return false;
// OK, this is a sequence of two diamonds or triangles.
// Check if there are stores in PTB or PFB that are repeated in QTB or QFB.
SmallPtrSet<Value *, 4> PStoreAddresses, QStoreAddresses;
for (auto *BB : {PTB, PFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
PStoreAddresses.insert(SI->getPointerOperand());
}
for (auto *BB : {QTB, QFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
QStoreAddresses.insert(SI->getPointerOperand());
}
set_intersect(PStoreAddresses, QStoreAddresses);
// set_intersect mutates PStoreAddresses in place. Rename it here to make it
// clear what it contains.
auto &CommonAddresses = PStoreAddresses;
bool Changed = false;
for (auto *Address : CommonAddresses)
Changed |=
mergeConditionalStoreToAddress(PTB, PFB, QTB, QFB, PostBB, Address,
InvertPCond, InvertQCond, DTU, DL, TTI);
return Changed;
}
/// If the previous block ended with a widenable branch, determine if reusing
/// the target block is profitable and legal. This will have the effect of
/// "widening" PBI, but doesn't require us to reason about hosting safety.
static bool tryWidenCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
DomTreeUpdater *DTU) {
// TODO: This can be generalized in two important ways:
// 1) We can allow phi nodes in IfFalseBB and simply reuse all the input
// values from the PBI edge.
// 2) We can sink side effecting instructions into BI's fallthrough
// successor provided they doesn't contribute to computation of
// BI's condition.
Value *CondWB, *WC;
BasicBlock *IfTrueBB, *IfFalseBB;
if (!parseWidenableBranch(PBI, CondWB, WC, IfTrueBB, IfFalseBB) ||
IfTrueBB != BI->getParent() || !BI->getParent()->getSinglePredecessor())
return false;
if (!IfFalseBB->phis().empty())
return false; // TODO
// Use lambda to lazily compute expensive condition after cheap ones.
auto NoSideEffects = [](BasicBlock &BB) {
return !llvm::any_of(BB, [](const Instruction &I) {
return I.mayWriteToMemory() || I.mayHaveSideEffects();
});
};
if (BI->getSuccessor(1) != IfFalseBB && // no inf looping
BI->getSuccessor(1)->getTerminatingDeoptimizeCall() && // profitability
NoSideEffects(*BI->getParent())) {
auto *OldSuccessor = BI->getSuccessor(1);
OldSuccessor->removePredecessor(BI->getParent());
BI->setSuccessor(1, IfFalseBB);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Insert, BI->getParent(), IfFalseBB},
{DominatorTree::Delete, BI->getParent(), OldSuccessor}});
return true;
}
if (BI->getSuccessor(0) != IfFalseBB && // no inf looping
BI->getSuccessor(0)->getTerminatingDeoptimizeCall() && // profitability
NoSideEffects(*BI->getParent())) {
auto *OldSuccessor = BI->getSuccessor(0);
OldSuccessor->removePredecessor(BI->getParent());
BI->setSuccessor(0, IfFalseBB);
if (DTU)
DTU->applyUpdates(
{{DominatorTree::Insert, BI->getParent(), IfFalseBB},
{DominatorTree::Delete, BI->getParent(), OldSuccessor}});
return true;
}
return false;
}
/// If we have a conditional branch as a predecessor of another block,
/// this function tries to simplify it. We know
/// that PBI and BI are both conditional branches, and BI is in one of the
/// successor blocks of PBI - PBI branches to BI.
static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
DomTreeUpdater *DTU,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
assert(PBI->isConditional() && BI->isConditional());
BasicBlock *BB = BI->getParent();
// If this block ends with a branch instruction, and if there is a
// predecessor that ends on a branch of the same condition, make
// this conditional branch redundant.
if (PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
// Okay, the outcome of this conditional branch is statically
// knowable. If this block had a single pred, handle specially.
if (BB->getSinglePredecessor()) {
// Turn this into a branch on constant.
bool CondIsTrue = PBI->getSuccessor(0) == BB;
BI->setCondition(
ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue));
return true; // Nuke the branch on constant.
}
// Otherwise, if there are multiple predecessors, insert a PHI that merges
// in the constant and simplify the block result. Subsequent passes of
// simplifycfg will thread the block.
if (BlockIsSimpleEnoughToThreadThrough(BB)) {
pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
PHINode *NewPN = PHINode::Create(
Type::getInt1Ty(BB->getContext()), std::distance(PB, PE),
BI->getCondition()->getName() + ".pr", &BB->front());
// Okay, we're going to insert the PHI node. Since PBI is not the only
// predecessor, compute the PHI'd conditional value for all of the preds.
// Any predecessor where the condition is not computable we keep symbolic.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) && PBI != BI &&
PBI->isConditional() && PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
bool CondIsTrue = PBI->getSuccessor(0) == BB;
NewPN->addIncoming(
ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue),
P);
} else {
NewPN->addIncoming(BI->getCondition(), P);
}
}
BI->setCondition(NewPN);
return true;
}
}
// If the previous block ended with a widenable branch, determine if reusing
// the target block is profitable and legal. This will have the effect of
// "widening" PBI, but doesn't require us to reason about hosting safety.
if (tryWidenCondBranchToCondBranch(PBI, BI, DTU))
return true;
if (auto *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
if (CE->canTrap())
return false;
// If both branches are conditional and both contain stores to the same
// address, remove the stores from the conditionals and create a conditional
// merged store at the end.
if (MergeCondStores && mergeConditionalStores(PBI, BI, DTU, DL, TTI))
return true;
// If this is a conditional branch in an empty block, and if any
// predecessors are a conditional branch to one of our destinations,
// fold the conditions into logical ops and one cond br.
// Ignore dbg intrinsics.
if (&*BB->instructionsWithoutDebug().begin() != BI)
return false;
int PBIOp, BIOp;
if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
PBIOp = 0;
BIOp = 0;
} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
PBIOp = 0;
BIOp = 1;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
PBIOp = 1;
BIOp = 0;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
PBIOp = 1;
BIOp = 1;
} else {
return false;
}
// Check to make sure that the other destination of this branch
// isn't BB itself. If so, this is an infinite loop that will
// keep getting unwound.
if (PBI->getSuccessor(PBIOp) == BB)
return false;
// Do not perform this transformation if it would require
// insertion of a large number of select instructions. For targets
// without predication/cmovs, this is a big pessimization.
// Also do not perform this transformation if any phi node in the common
// destination block can trap when reached by BB or PBB (PR17073). In that
// case, it would be unsafe to hoist the operation into a select instruction.
BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
BasicBlock *RemovedDest = PBI->getSuccessor(PBIOp ^ 1);
unsigned NumPhis = 0;
for (BasicBlock::iterator II = CommonDest->begin(); isa<PHINode>(II);
++II, ++NumPhis) {
if (NumPhis > 2) // Disable this xform.
return false;
PHINode *PN = cast<PHINode>(II);
Value *BIV = PN->getIncomingValueForBlock(BB);
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BIV))
if (CE->canTrap())
return false;
unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN->getIncomingValue(PBBIdx);
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(PBIV))
if (CE->canTrap())
return false;
}
// Finally, if everything is ok, fold the branches to logical ops.
BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
LLVM_DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
<< "AND: " << *BI->getParent());
SmallVector<DominatorTree::UpdateType, 5> Updates;
// If OtherDest *is* BB, then BB is a basic block with a single conditional
// branch in it, where one edge (OtherDest) goes back to itself but the other
// exits. We don't *know* that the program avoids the infinite loop
// (even though that seems likely). If we do this xform naively, we'll end up
// recursively unpeeling the loop. Since we know that (after the xform is
// done) that the block *is* infinite if reached, we just make it an obviously
// infinite loop with no cond branch.
if (OtherDest == BB) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
BasicBlock *InfLoopBlock =
BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
Updates.push_back({DominatorTree::Insert, InfLoopBlock, InfLoopBlock});
OtherDest = InfLoopBlock;
}
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// BI may have other predecessors. Because of this, we leave
// it alone, but modify PBI.
// Make sure we get to CommonDest on True&True directions.
Value *PBICond = PBI->getCondition();
IRBuilder<NoFolder> Builder(PBI);
if (PBIOp)
PBICond = Builder.CreateNot(PBICond, PBICond->getName() + ".not");
Value *BICond = BI->getCondition();
if (BIOp)
BICond = Builder.CreateNot(BICond, BICond->getName() + ".not");
// Merge the conditions.
Value *Cond = Builder.CreateOr(PBICond, BICond, "brmerge");
// Modify PBI to branch on the new condition to the new dests.
PBI->setCondition(Cond);
PBI->setSuccessor(0, CommonDest);
PBI->setSuccessor(1, OtherDest);
Updates.push_back({DominatorTree::Insert, PBI->getParent(), OtherDest});
Updates.push_back({DominatorTree::Delete, PBI->getParent(), RemovedDest});
if (DTU)
DTU->applyUpdates(Updates);
// Update branch weight for PBI.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
uint64_t PredCommon, PredOther, SuccCommon, SuccOther;
bool HasWeights =
extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight);
if (HasWeights) {
PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
// The weight to CommonDest should be PredCommon * SuccTotal +
// PredOther * SuccCommon.
// The weight to OtherDest should be PredOther * SuccOther.
uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther) +
PredOther * SuccCommon,
PredOther * SuccOther};
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(NewWeights);
setBranchWeights(PBI, NewWeights[0], NewWeights[1]);
}
// OtherDest may have phi nodes. If so, add an entry from PBI's
// block that are identical to the entries for BI's block.
AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);
// We know that the CommonDest already had an edge from PBI to
// it. If it has PHIs though, the PHIs may have different
// entries for BB and PBI's BB. If so, insert a select to make
// them agree.
for (PHINode &PN : CommonDest->phis()) {
Value *BIV = PN.getIncomingValueForBlock(BB);
unsigned PBBIdx = PN.getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN.getIncomingValue(PBBIdx);
if (BIV != PBIV) {
// Insert a select in PBI to pick the right value.
SelectInst *NV = cast<SelectInst>(
Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName() + ".mux"));
PN.setIncomingValue(PBBIdx, NV);
// Although the select has the same condition as PBI, the original branch
// weights for PBI do not apply to the new select because the select's
// 'logical' edges are incoming edges of the phi that is eliminated, not
// the outgoing edges of PBI.
if (HasWeights) {
uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
uint64_t PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
// The weight to PredCommonDest should be PredCommon * SuccTotal.
// The weight to PredOtherDest should be PredOther * SuccCommon.
uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther),
PredOther * SuccCommon};
FitWeights(NewWeights);
setBranchWeights(NV, NewWeights[0], NewWeights[1]);
}
}
}
LLVM_DEBUG(dbgs() << "INTO: " << *PBI->getParent());
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// This basic block is probably dead. We know it has at least
// one fewer predecessor.
return true;
}
// Simplifies a terminator by replacing it with a branch to TrueBB if Cond is
// true or to FalseBB if Cond is false.
// Takes care of updating the successors and removing the old terminator.
// Also makes sure not to introduce new successors by assuming that edges to
// non-successor TrueBBs and FalseBBs aren't reachable.
bool SimplifyCFGOpt::SimplifyTerminatorOnSelect(Instruction *OldTerm,
Value *Cond, BasicBlock *TrueBB,
BasicBlock *FalseBB,
uint32_t TrueWeight,
uint32_t FalseWeight) {
auto *BB = OldTerm->getParent();
// Remove any superfluous successor edges from the CFG.
// First, figure out which successors to preserve.
// If TrueBB and FalseBB are equal, only try to preserve one copy of that
// successor.
BasicBlock *KeepEdge1 = TrueBB;
BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : nullptr;
SmallSetVector<BasicBlock *, 2> RemovedSuccessors;
// Then remove the rest.
for (BasicBlock *Succ : successors(OldTerm)) {
// Make sure only to keep exactly one copy of each edge.
if (Succ == KeepEdge1)
KeepEdge1 = nullptr;
else if (Succ == KeepEdge2)
KeepEdge2 = nullptr;
else {
Succ->removePredecessor(BB,
/*KeepOneInputPHIs=*/true);
if (Succ != TrueBB && Succ != FalseBB)
RemovedSuccessors.insert(Succ);
}
}
IRBuilder<> Builder(OldTerm);
Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
// Insert an appropriate new terminator.
if (!KeepEdge1 && !KeepEdge2) {
if (TrueBB == FalseBB) {
// We were only looking for one successor, and it was present.
// Create an unconditional branch to it.
Builder.CreateBr(TrueBB);
} else {
// We found both of the successors we were looking for.
// Create a conditional branch sharing the condition of the select.
BranchInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB);
if (TrueWeight != FalseWeight)
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
} else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
// Neither of the selected blocks were successors, so this
// terminator must be unreachable.
new UnreachableInst(OldTerm->getContext(), OldTerm);
} else {
// One of the selected values was a successor, but the other wasn't.
// Insert an unconditional branch to the one that was found;
// the edge to the one that wasn't must be unreachable.
if (!KeepEdge1) {
// Only TrueBB was found.
Builder.CreateBr(TrueBB);
} else {
// Only FalseBB was found.
Builder.CreateBr(FalseBB);
}
}
EraseTerminatorAndDCECond(OldTerm);
if (DTU) {
SmallVector<DominatorTree::UpdateType, 2> Updates;
Updates.reserve(RemovedSuccessors.size());
for (auto *RemovedSuccessor : RemovedSuccessors)
Updates.push_back({DominatorTree::Delete, BB, RemovedSuccessor});
DTU->applyUpdates(Updates);
}
return true;
}
// Replaces
// (switch (select cond, X, Y)) on constant X, Y
// with a branch - conditional if X and Y lead to distinct BBs,
// unconditional otherwise.
bool SimplifyCFGOpt::SimplifySwitchOnSelect(SwitchInst *SI,
SelectInst *Select) {
// Check for constant integer values in the select.
ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
if (!TrueVal || !FalseVal)
return false;
// Find the relevant condition and destinations.
Value *Condition = Select->getCondition();
BasicBlock *TrueBB = SI->findCaseValue(TrueVal)->getCaseSuccessor();
BasicBlock *FalseBB = SI->findCaseValue(FalseVal)->getCaseSuccessor();
// Get weight for TrueBB and FalseBB.
uint32_t TrueWeight = 0, FalseWeight = 0;
SmallVector<uint64_t, 8> Weights;
bool HasWeights = HasBranchWeights(SI);
if (HasWeights) {
GetBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
TrueWeight =
(uint32_t)Weights[SI->findCaseValue(TrueVal)->getSuccessorIndex()];
FalseWeight =
(uint32_t)Weights[SI->findCaseValue(FalseVal)->getSuccessorIndex()];
}
}
// Perform the actual simplification.
return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB, TrueWeight,
FalseWeight);
}
// Replaces
// (indirectbr (select cond, blockaddress(@fn, BlockA),
// blockaddress(@fn, BlockB)))
// with
// (br cond, BlockA, BlockB).
bool SimplifyCFGOpt::SimplifyIndirectBrOnSelect(IndirectBrInst *IBI,
SelectInst *SI) {
// Check that both operands of the select are block addresses.
BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
if (!TBA || !FBA)
return false;
// Extract the actual blocks.
BasicBlock *TrueBB = TBA->getBasicBlock();
BasicBlock *FalseBB = FBA->getBasicBlock();
// Perform the actual simplification.
return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB, 0,
0);
}
/// This is called when we find an icmp instruction
/// (a seteq/setne with a constant) as the only instruction in a
/// block that ends with an uncond branch. We are looking for a very specific
/// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
/// this case, we merge the first two "or's of icmp" into a switch, but then the
/// default value goes to an uncond block with a seteq in it, we get something
/// like:
///
/// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
/// DEFAULT:
/// %tmp = icmp eq i8 %A, 92
/// br label %end
/// end:
/// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
///
/// We prefer to split the edge to 'end' so that there is a true/false entry to
/// the PHI, merging the third icmp into the switch.
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
BasicBlock *BB = ICI->getParent();
// If the block has any PHIs in it or the icmp has multiple uses, it is too
// complex.
if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse())
return false;
Value *V = ICI->getOperand(0);
ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
// The pattern we're looking for is where our only predecessor is a switch on
// 'V' and this block is the default case for the switch. In this case we can
// fold the compared value into the switch to simplify things.
BasicBlock *Pred = BB->getSinglePredecessor();
if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
return false;
SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
if (SI->getCondition() != V)
return false;
// If BB is reachable on a non-default case, then we simply know the value of
// V in this block. Substitute it and constant fold the icmp instruction
// away.
if (SI->getDefaultDest() != BB) {
ConstantInt *VVal = SI->findCaseDest(BB);
assert(VVal && "Should have a unique destination value");
ICI->setOperand(0, VVal);
if (Value *V = SimplifyInstruction(ICI, {DL, ICI})) {
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
}
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// Ok, the block is reachable from the default dest. If the constant we're
// comparing exists in one of the other edges, then we can constant fold ICI
// and zap it.
if (SI->findCaseValue(Cst) != SI->case_default()) {
Value *V;
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
V = ConstantInt::getFalse(BB->getContext());
else
V = ConstantInt::getTrue(BB->getContext());
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// The use of the icmp has to be in the 'end' block, by the only PHI node in
// the block.
BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
PHINode *PHIUse = dyn_cast<PHINode>(ICI->user_back());
if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
isa<PHINode>(++BasicBlock::iterator(PHIUse)))
return false;
// If the icmp is a SETEQ, then the default dest gets false, the new edge gets
// true in the PHI.
Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
Constant *NewCst = ConstantInt::getFalse(BB->getContext());
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
std::swap(DefaultCst, NewCst);
// Replace ICI (which is used by the PHI for the default value) with true or
// false depending on if it is EQ or NE.
ICI->replaceAllUsesWith(DefaultCst);
ICI->eraseFromParent();
SmallVector<DominatorTree::UpdateType, 2> Updates;
// Okay, the switch goes to this block on a default value. Add an edge from
// the switch to the merge point on the compared value.
BasicBlock *NewBB =
BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
{
SwitchInstProfUpdateWrapper SIW(*SI);
auto W0 = SIW.getSuccessorWeight(0);
SwitchInstProfUpdateWrapper::CaseWeightOpt NewW;
if (W0) {
NewW = ((uint64_t(*W0) + 1) >> 1);
SIW.setSuccessorWeight(0, *NewW);
}
SIW.addCase(Cst, NewBB, NewW);
Updates.push_back({DominatorTree::Insert, Pred, NewBB});
}
// NewBB branches to the phi block, add the uncond branch and the phi entry.
Builder.SetInsertPoint(NewBB);
Builder.SetCurrentDebugLocation(SI->getDebugLoc());
Builder.CreateBr(SuccBlock);
Updates.push_back({DominatorTree::Insert, NewBB, SuccBlock});
PHIUse->addIncoming(NewCst, NewBB);
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
/// The specified branch is a conditional branch.
/// Check to see if it is branching on an or/and chain of icmp instructions, and
/// fold it into a switch instruction if so.
bool SimplifyCFGOpt::SimplifyBranchOnICmpChain(BranchInst *BI,
IRBuilder<> &Builder,
const DataLayout &DL) {
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (!Cond)
return false;
// Change br (X == 0 | X == 1), T, F into a switch instruction.
// If this is a bunch of seteq's or'd together, or if it's a bunch of
// 'setne's and'ed together, collect them.
// Try to gather values from a chain of and/or to be turned into a switch
ConstantComparesGatherer ConstantCompare(Cond, DL);
// Unpack the result
SmallVectorImpl<ConstantInt *> &Values = ConstantCompare.Vals;
Value *CompVal = ConstantCompare.CompValue;
unsigned UsedICmps = ConstantCompare.UsedICmps;
Value *ExtraCase = ConstantCompare.Extra;
// If we didn't have a multiply compared value, fail.
if (!CompVal)
return false;
// Avoid turning single icmps into a switch.
if (UsedICmps <= 1)
return false;
bool TrueWhenEqual = match(Cond, m_LogicalOr(m_Value(), m_Value()));
// There might be duplicate constants in the list, which the switch
// instruction can't handle, remove them now.
array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
// If Extra was used, we require at least two switch values to do the
// transformation. A switch with one value is just a conditional branch.
if (ExtraCase && Values.size() < 2)
return false;
// TODO: Preserve branch weight metadata, similarly to how
// FoldValueComparisonIntoPredecessors preserves it.
// Figure out which block is which destination.
BasicBlock *DefaultBB = BI->getSuccessor(1);
BasicBlock *EdgeBB = BI->getSuccessor(0);
if (!TrueWhenEqual)
std::swap(DefaultBB, EdgeBB);
BasicBlock *BB = BI->getParent();
// MSAN does not like undefs as branch condition which can be introduced
// with "explicit branch".
if (ExtraCase && BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
return false;
LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
<< " cases into SWITCH. BB is:\n"
<< *BB);
SmallVector<DominatorTree::UpdateType, 2> Updates;
// If there are any extra values that couldn't be folded into the switch
// then we evaluate them with an explicit branch first. Split the block
// right before the condbr to handle it.
if (ExtraCase) {
BasicBlock *NewBB = SplitBlock(BB, BI, DTU, /*LI=*/nullptr,
/*MSSAU=*/nullptr, "switch.early.test");
// Remove the uncond branch added to the old block.
Instruction *OldTI = BB->getTerminator();
Builder.SetInsertPoint(OldTI);
if (TrueWhenEqual)
Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB);
else
Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
OldTI->eraseFromParent();
Updates.push_back({DominatorTree::Insert, BB, EdgeBB});
// If there are PHI nodes in EdgeBB, then we need to add a new entry to them
// for the edge we just added.
AddPredecessorToBlock(EdgeBB, BB, NewBB);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
<< "\nEXTRABB = " << *BB);
BB = NewBB;
}
Builder.SetInsertPoint(BI);
// Convert pointer to int before we switch.
if (CompVal->getType()->isPointerTy()) {
CompVal = Builder.CreatePtrToInt(
CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
}
// Create the new switch instruction now.
SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
// Add all of the 'cases' to the switch instruction.
for (unsigned i = 0, e = Values.size(); i != e; ++i)
New->addCase(Values[i], EdgeBB);
// We added edges from PI to the EdgeBB. As such, if there were any
// PHI nodes in EdgeBB, they need entries to be added corresponding to
// the number of edges added.
for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
Value *InVal = PN->getIncomingValueForBlock(BB);
for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
PN->addIncoming(InVal, BB);
}
// Erase the old branch instruction.
EraseTerminatorAndDCECond(BI);
if (DTU)
DTU->applyUpdates(Updates);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
return true;
}
bool SimplifyCFGOpt::simplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
if (isa<PHINode>(RI->getValue()))
return simplifyCommonResume(RI);
else if (isa<LandingPadInst>(RI->getParent()->getFirstNonPHI()) &&
RI->getValue() == RI->getParent()->getFirstNonPHI())
// The resume must unwind the exception that caused control to branch here.
return simplifySingleResume(RI);
return false;
}
// Check if cleanup block is empty
static bool isCleanupBlockEmpty(iterator_range<BasicBlock::iterator> R) {
for (Instruction &I : R) {
auto *II = dyn_cast<IntrinsicInst>(&I);
if (!II)
return false;
Intrinsic::ID IntrinsicID = II->getIntrinsicID();
switch (IntrinsicID) {
case Intrinsic::dbg_declare:
case Intrinsic::dbg_value:
case Intrinsic::dbg_label:
case Intrinsic::lifetime_end:
break;
default:
return false;
}
}
return true;
}
// Simplify resume that is shared by several landing pads (phi of landing pad).
bool SimplifyCFGOpt::simplifyCommonResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
// Check that there are no other instructions except for debug and lifetime
// intrinsics between the phi's and resume instruction.
if (!isCleanupBlockEmpty(
make_range(RI->getParent()->getFirstNonPHI(), BB->getTerminator())))
return false;
SmallSetVector<BasicBlock *, 4> TrivialUnwindBlocks;
auto *PhiLPInst = cast<PHINode>(RI->getValue());
// Check incoming blocks to see if any of them are trivial.
for (unsigned Idx = 0, End = PhiLPInst->getNumIncomingValues(); Idx != End;
Idx++) {
auto *IncomingBB = PhiLPInst->getIncomingBlock(Idx);
auto *IncomingValue = PhiLPInst->getIncomingValue(Idx);
// If the block has other successors, we can not delete it because
// it has other dependents.
if (IncomingBB->getUniqueSuccessor() != BB)
continue;
auto *LandingPad = dyn_cast<LandingPadInst>(IncomingBB->getFirstNonPHI());
// Not the landing pad that caused the control to branch here.
if (IncomingValue != LandingPad)
continue;
if (isCleanupBlockEmpty(
make_range(LandingPad->getNextNode(), IncomingBB->getTerminator())))
TrivialUnwindBlocks.insert(IncomingBB);
}
// If no trivial unwind blocks, don't do any simplifications.
if (TrivialUnwindBlocks.empty())
return false;
// Turn all invokes that unwind here into calls.
for (auto *TrivialBB : TrivialUnwindBlocks) {
// Blocks that will be simplified should be removed from the phi node.
// Note there could be multiple edges to the resume block, and we need
// to remove them all.
while (PhiLPInst->getBasicBlockIndex(TrivialBB) != -1)
BB->removePredecessor(TrivialBB, true);
for (BasicBlock *Pred :
llvm::make_early_inc_range(predecessors(TrivialBB))) {
removeUnwindEdge(Pred, DTU);
++NumInvokes;
}
// In each SimplifyCFG run, only the current processed block can be erased.
// Otherwise, it will break the iteration of SimplifyCFG pass. So instead
// of erasing TrivialBB, we only remove the branch to the common resume
// block so that we can later erase the resume block since it has no
// predecessors.
TrivialBB->getTerminator()->eraseFromParent();
new UnreachableInst(RI->getContext(), TrivialBB);
if (DTU)
DTU->applyUpdates({{DominatorTree::Delete, TrivialBB, BB}});
}
// Delete the resume block if all its predecessors have been removed.
if (pred_empty(BB)) {
if (DTU)
DTU->deleteBB(BB);
else
BB->eraseFromParent();
}
return !TrivialUnwindBlocks.empty();
}
// Simplify resume that is only used by a single (non-phi) landing pad.
bool SimplifyCFGOpt::simplifySingleResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
auto *LPInst = cast<LandingPadInst>(BB->getFirstNonPHI());
assert(RI->getValue() == LPInst &&
"Resume must unwind the exception that caused control to here");
// Check that there are no other instructions except for debug intrinsics.
if (!isCleanupBlockEmpty(
make_range<Instruction *>(LPInst->getNextNode(), RI)))
return false;
// Turn all invokes that unwind here into calls and delete the basic block.
for (BasicBlock *Pred : llvm::make_early_inc_range(predecessors(BB))) {
removeUnwindEdge(Pred, DTU);
++NumInvokes;
}
// The landingpad is now unreachable. Zap it.
if (DTU)
DTU->deleteBB(BB);
else
BB->eraseFromParent();
return true;
}
static bool removeEmptyCleanup(CleanupReturnInst *RI, DomTreeUpdater *DTU) {
// If this is a trivial cleanup pad that executes no instructions, it can be
// eliminated. If the cleanup pad continues to the caller, any predecessor
// that is an EH pad will be updated to continue to the caller and any
// predecessor that terminates with an invoke instruction will have its invoke
// instruction converted to a call instruction. If the cleanup pad being
// simplified does not continue to the caller, each predecessor will be
// updated to continue to the unwind destination of the cleanup pad being
// simplified.
BasicBlock *BB = RI->getParent();
CleanupPadInst *CPInst = RI->getCleanupPad();
if (CPInst->getParent() != BB)
// This isn't an empty cleanup.
return false;
// We cannot kill the pad if it has multiple uses. This typically arises
// from unreachable basic blocks.
if (!CPInst->hasOneUse())
return false;
// Check that there are no other instructions except for benign intrinsics.
if (!isCleanupBlockEmpty(
make_range<Instruction *>(CPInst->getNextNode(), RI)))
return false;
// If the cleanup return we are simplifying unwinds to the caller, this will
// set UnwindDest to nullptr.
BasicBlock *UnwindDest = RI->getUnwindDest();
Instruction *DestEHPad = UnwindDest ? UnwindDest->getFirstNonPHI() : nullptr;
// We're about to remove BB from the control flow. Before we do, sink any
// PHINodes into the unwind destination. Doing this before changing the
// control flow avoids some potentially slow checks, since we can currently
// be certain that UnwindDest and BB have no common predecessors (since they
// are both EH pads).
if (UnwindDest) {
// First, go through the PHI nodes in UnwindDest and update any nodes that
// reference the block we are removing
for (BasicBlock::iterator I = UnwindDest->begin(),
IE = DestEHPad->getIterator();
I != IE; ++I) {
PHINode *DestPN = cast<PHINode>(I);
int Idx = DestPN->getBasicBlockIndex(BB);
// Since BB unwinds to UnwindDest, it has to be in the PHI node.
assert(Idx != -1);
// This PHI node has an incoming value that corresponds to a control
// path through the cleanup pad we are removing. If the incoming
// value is in the cleanup pad, it must be a PHINode (because we
// verified above that the block is otherwise empty). Otherwise, the
// value is either a constant or a value that dominates the cleanup
// pad being removed.
//
// Because BB and UnwindDest are both EH pads, all of their
// predecessors must unwind to these blocks, and since no instruction
// can have multiple unwind destinations, there will be no overlap in
// incoming blocks between SrcPN and DestPN.
Value *SrcVal = DestPN->getIncomingValue(Idx);
PHINode *SrcPN = dyn_cast<PHINode>(SrcVal);
// Remove the entry for the block we are deleting.
DestPN->removeIncomingValue(Idx, false);
if (SrcPN && SrcPN->getParent() == BB) {
// If the incoming value was a PHI node in the cleanup pad we are
// removing, we need to merge that PHI node's incoming values into
// DestPN.
for (unsigned SrcIdx = 0, SrcE = SrcPN->getNumIncomingValues();
SrcIdx != SrcE; ++SrcIdx) {
DestPN->addIncoming(SrcPN->getIncomingValue(SrcIdx),
SrcPN->getIncomingBlock(SrcIdx));
}
} else {
// Otherwise, the incoming value came from above BB and
// so we can just reuse it. We must associate all of BB's
// predecessors with this value.
for (auto *pred : predecessors(BB)) {
DestPN->addIncoming(SrcVal, pred);
}
}
}
// Sink any remaining PHI nodes directly into UnwindDest.
Instruction *InsertPt = DestEHPad;
for (BasicBlock::iterator I = BB->begin(),
IE = BB->getFirstNonPHI()->getIterator();
I != IE;) {
// The iterator must be incremented here because the instructions are
// being moved to another block.
PHINode *PN = cast<PHINode>(I++);
if (PN->use_empty() || !PN->isUsedOutsideOfBlock(BB))
// If the PHI node has no uses or all of its uses are in this basic
// block (meaning they are debug or lifetime intrinsics), just leave
// it. It will be erased when we erase BB below.
continue;
// Otherwise, sink this PHI node into UnwindDest.
// Any predecessors to UnwindDest which are not already represented
// must be back edges which inherit the value from the path through
// BB. In this case, the PHI value must reference itself.
for (auto *pred : predecessors(UnwindDest))
if (pred != BB)
PN->addIncoming(PN, pred);
PN->moveBefore(InsertPt);
}
}
std::vector<DominatorTree::UpdateType> Updates;
// We use make_early_inc_range here because we may remove some predecessors.
for (BasicBlock *PredBB : llvm::make_early_inc_range(predecessors(BB))) {
if (UnwindDest == nullptr) {
if (DTU)
DTU->applyUpdates(Updates);
Updates.clear();
removeUnwindEdge(PredBB, DTU);
++NumInvokes;
} else {
Instruction *TI = PredBB->getTerminator();
TI->replaceUsesOfWith(BB, UnwindDest);
Updates.push_back({DominatorTree::Insert, PredBB, UnwindDest});
Updates.push_back({DominatorTree::Delete, PredBB, BB});
}
}
if (DTU) {
DTU->applyUpdates(Updates);
DTU->deleteBB(BB);
} else
// The cleanup pad is now unreachable. Zap it.
BB->eraseFromParent();
return true;
}
// Try to merge two cleanuppads together.
static bool mergeCleanupPad(CleanupReturnInst *RI) {
// Skip any cleanuprets which unwind to caller, there is nothing to merge
// with.
BasicBlock *UnwindDest = RI->getUnwindDest();
if (!UnwindDest)
return false;
// This cleanupret isn't the only predecessor of this cleanuppad, it wouldn't
// be safe to merge without code duplication.
if (UnwindDest->getSinglePredecessor() != RI->getParent())
return false;
// Verify that our cleanuppad's unwind destination is another cleanuppad.
auto *SuccessorCleanupPad = dyn_cast<CleanupPadInst>(&UnwindDest->front());
if (!SuccessorCleanupPad)
return false;
CleanupPadInst *PredecessorCleanupPad = RI->getCleanupPad();
// Replace any uses of the successor cleanupad with the predecessor pad
// The only cleanuppad uses should be this cleanupret, it's cleanupret and
// funclet bundle operands.
SuccessorCleanupPad->replaceAllUsesWith(PredecessorCleanupPad);
// Remove the old cleanuppad.
SuccessorCleanupPad->eraseFromParent();
// Now, we simply replace the cleanupret with a branch to the unwind
// destination.
BranchInst::Create(UnwindDest, RI->getParent());
RI->eraseFromParent();
return true;
}
bool SimplifyCFGOpt::simplifyCleanupReturn(CleanupReturnInst *RI) {
// It is possible to transiantly have an undef cleanuppad operand because we
// have deleted some, but not all, dead blocks.
// Eventually, this block will be deleted.
if (isa<UndefValue>(RI->getOperand(0)))
return false;
if (mergeCleanupPad(RI))
return true;
if (removeEmptyCleanup(RI, DTU))
return true;
return false;
}
bool SimplifyCFGOpt::simplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
BasicBlock *BB = RI->getParent();
if (!BB->getFirstNonPHIOrDbg()->isTerminator())
return false;
// Find predecessors that end with branches.
SmallVector<BasicBlock *, 8> UncondBranchPreds;
SmallVector<BranchInst *, 8> CondBranchPreds;
for (BasicBlock *P : predecessors(BB)) {
Instruction *PTI = P->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
if (BI->isUnconditional())
UncondBranchPreds.push_back(P);
else
CondBranchPreds.push_back(BI);
}
}
// If we found some, do the transformation!
if (!UncondBranchPreds.empty() && DupRet) {
while (!UncondBranchPreds.empty()) {
BasicBlock *Pred = UncondBranchPreds.pop_back_val();
LLVM_DEBUG(dbgs() << "FOLDING: " << *BB
<< "INTO UNCOND BRANCH PRED: " << *Pred);
(void)FoldReturnIntoUncondBranch(RI, BB, Pred, DTU);
}
// If we eliminated all predecessors of the block, delete the block now.
if (pred_empty(BB)) {
// We know there are no successors, so just nuke the block.
if (DTU)
DTU->deleteBB(BB);
else
BB->eraseFromParent();
}
return true;
}
// Check out all of the conditional branches going to this return
// instruction. If any of them just select between returns, change the
// branch itself into a select/return pair.
while (!CondBranchPreds.empty()) {
BranchInst *BI = CondBranchPreds.pop_back_val();
// Check to see if the non-BB successor is also a return block.
if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
SimplifyCondBranchToTwoReturns(BI, Builder))
return true;
}
return false;
}
bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
BasicBlock *BB = UI->getParent();
bool Changed = false;
// If there are any instructions immediately before the unreachable that can
// be removed, do so.
while (UI->getIterator() != BB->begin()) {
BasicBlock::iterator BBI = UI->getIterator();
--BBI;
// Do not delete instructions that can have side effects which might cause
// the unreachable to not be reachable; specifically, calls and volatile
// operations may have this effect.
if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI))
break;
if (BBI->mayHaveSideEffects()) {
if (auto *SI = dyn_cast<StoreInst>(BBI)) {
if (SI->isVolatile())
break;
} else if (auto *LI = dyn_cast<LoadInst>(BBI)) {
if (LI->isVolatile())
break;
} else if (auto *RMWI = dyn_cast<AtomicRMWInst>(BBI)) {
if (RMWI->isVolatile())
break;
} else if (auto *CXI = dyn_cast<AtomicCmpXchgInst>(BBI)) {
if (CXI->isVolatile())
break;
} else if (isa<CatchPadInst>(BBI)) {
// A catchpad may invoke exception object constructors and such, which
// in some languages can be arbitrary code, so be conservative by
// default.
// For CoreCLR, it just involves a type test, so can be removed.
if (classifyEHPersonality(BB->getParent()->getPersonalityFn()) !=
EHPersonality::CoreCLR)
break;
} else if (!isa<FenceInst>(BBI) && !isa<VAArgInst>(BBI) &&
!isa<LandingPadInst>(BBI)) {
break;
}
// Note that deleting LandingPad's here is in fact okay, although it
// involves a bit of subtle reasoning. If this inst is a LandingPad,
// all the predecessors of this block will be the unwind edges of Invokes,
// and we can therefore guarantee this block will be erased.
}
// Delete this instruction (any uses are guaranteed to be dead)
if (!BBI->use_empty())
BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
BBI->eraseFromParent();
Changed = true;
}
// If the unreachable instruction is the first in the block, take a gander
// at all of the predecessors of this instruction, and simplify them.
if (&BB->front() != UI)
return Changed;
std::vector<DominatorTree::UpdateType> Updates;
SmallSetVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
auto *Predecessor = Preds[i];
Instruction *TI = Predecessor->getTerminator();
IRBuilder<> Builder(TI);
if (auto *BI = dyn_cast<BranchInst>(TI)) {
// We could either have a proper unconditional branch,
// or a degenerate conditional branch with matching destinations.
if (all_of(BI->successors(),
[BB](auto *Successor) { return Successor == BB; })) {
new UnreachableInst(TI->getContext(), TI);
TI->eraseFromParent();
Changed = true;
} else {
assert(BI->isConditional() && "Can't get here with an uncond branch.");
Value* Cond = BI->getCondition();
assert(BI->getSuccessor(0) != BI->getSuccessor(1) &&
"The destinations are guaranteed to be different here.");
if (BI->getSuccessor(0) == BB) {
Builder.CreateAssumption(Builder.CreateNot(Cond));
Builder.CreateBr(BI->getSuccessor(1));
} else {
assert(BI->getSuccessor(1) == BB && "Incorrect CFG");
Builder.CreateAssumption(Cond);
Builder.CreateBr(BI->getSuccessor(0));
}
EraseTerminatorAndDCECond(BI);
Changed = true;
}
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
} else if (auto *SI = dyn_cast<SwitchInst>(TI)) {
SwitchInstProfUpdateWrapper SU(*SI);
for (auto i = SU->case_begin(), e = SU->case_end(); i != e;) {
if (i->getCaseSuccessor() != BB) {
++i;
continue;
}
BB->removePredecessor(SU->getParent());
i = SU.removeCase(i);
e = SU->case_end();
Changed = true;
}
// Note that the default destination can't be removed!
if (SI->getDefaultDest() != BB)
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
} else if (auto *II = dyn_cast<InvokeInst>(TI)) {
if (II->getUnwindDest() == BB) {
if (DTU)
DTU->applyUpdates(Updates);
Updates.clear();
removeUnwindEdge(TI->getParent(), DTU);
Changed = true;
}
} else if (auto *CSI = dyn_cast<CatchSwitchInst>(TI)) {
if (CSI->getUnwindDest() == BB) {
if (DTU)
DTU->applyUpdates(Updates);
Updates.clear();
removeUnwindEdge(TI->getParent(), DTU);
Changed = true;
continue;
}
for (CatchSwitchInst::handler_iterator I = CSI->handler_begin(),
E = CSI->handler_end();
I != E; ++I) {
if (*I == BB) {
CSI->removeHandler(I);
--I;
--E;
Changed = true;
}
}
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
if (CSI->getNumHandlers() == 0) {
if (CSI->hasUnwindDest()) {
// Redirect all predecessors of the block containing CatchSwitchInst
// to instead branch to the CatchSwitchInst's unwind destination.
for (auto *PredecessorOfPredecessor : predecessors(Predecessor)) {
Updates.push_back({DominatorTree::Insert, PredecessorOfPredecessor,
CSI->getUnwindDest()});
Updates.push_back(
{DominatorTree::Delete, PredecessorOfPredecessor, Predecessor});
}
Predecessor->replaceAllUsesWith(CSI->getUnwindDest());
} else {
// Rewrite all preds to unwind to caller (or from invoke to call).
if (DTU)
DTU->applyUpdates(Updates);
Updates.clear();
SmallVector<BasicBlock *, 8> EHPreds(predecessors(Predecessor));
for (BasicBlock *EHPred : EHPreds)
removeUnwindEdge(EHPred, DTU);
}
// The catchswitch is no longer reachable.
new UnreachableInst(CSI->getContext(), CSI);
CSI->eraseFromParent();
Changed = true;
}
} else if (auto *CRI = dyn_cast<CleanupReturnInst>(TI)) {
(void)CRI;
assert(CRI->hasUnwindDest() && CRI->getUnwindDest() == BB &&
"Expected to always have an unwind to BB.");
Updates.push_back({DominatorTree::Delete, Predecessor, BB});
new UnreachableInst(TI->getContext(), TI);
TI->eraseFromParent();
Changed = true;
}
}
if (DTU)
DTU->applyUpdates(Updates);
// If this block is now dead, remove it.
if (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) {
// We know there are no successors, so just nuke the block.
if (DTU)
DTU->deleteBB(BB);
else
BB->eraseFromParent();
return true;
}
return Changed;
}
static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
assert(Cases.size() >= 1);
array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate);
for (size_t I = 1, E = Cases.size(); I != E; ++I) {
if (Cases[I - 1]->getValue() != Cases[I]->getValue() + 1)
return false;
}
return true;
}
static void createUnreachableSwitchDefault(SwitchInst *Switch,
DomTreeUpdater *DTU) {
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
auto *BB = Switch->getParent();
BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
Switch->getDefaultDest(), Switch->getParent(), "", DTU);
auto *OrigDefaultBlock = Switch->getDefaultDest();
Switch->setDefaultDest(&*NewDefaultBlock);
if (DTU)
DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
{DominatorTree::Delete, BB, OrigDefaultBlock}});
SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
SmallVector<DominatorTree::UpdateType, 2> Updates;
for (auto *Successor : successors(NewDefaultBlock))
Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
auto *NewTerminator = NewDefaultBlock->getTerminator();
new UnreachableInst(Switch->getContext(), NewTerminator);
EraseTerminatorAndDCECond(NewTerminator);
if (DTU)
DTU->applyUpdates(Updates);
}
/// Turn a switch with two reachable destinations into an integer range
/// comparison and branch.
bool SimplifyCFGOpt::TurnSwitchRangeIntoICmp(SwitchInst *SI,
IRBuilder<> &Builder) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
bool HasDefault =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
auto *BB = SI->getParent();
// Partition the cases into two sets with different destinations.
BasicBlock *DestA = HasDefault ? SI->getDefaultDest() : nullptr;
BasicBlock *DestB = nullptr;
SmallVector<ConstantInt *, 16> CasesA;
SmallVector<ConstantInt *, 16> CasesB;
for (auto Case : SI->cases()) {
BasicBlock *Dest = Case.getCaseSuccessor();
if (!DestA)
DestA = Dest;
if (Dest == DestA) {
CasesA.push_back(Case.getCaseValue());
continue;
}
if (!DestB)
DestB = Dest;
if (Dest == DestB) {
CasesB.push_back(Case.getCaseValue());
continue;
}
return false; // More than two destinations.
}
assert(DestA && DestB &&
"Single-destination switch should have been folded.");
assert(DestA != DestB);
assert(DestB != SI->getDefaultDest());
assert(!CasesB.empty() && "There must be non-default cases.");
assert(!CasesA.empty() || HasDefault);
// Figure out if one of the sets of cases form a contiguous range.
SmallVectorImpl<ConstantInt *> *ContiguousCases = nullptr;
BasicBlock *ContiguousDest = nullptr;
BasicBlock *OtherDest = nullptr;
if (!CasesA.empty() && CasesAreContiguous(CasesA)) {
ContiguousCases = &CasesA;
ContiguousDest = DestA;
OtherDest = DestB;
} else if (CasesAreContiguous(CasesB)) {
ContiguousCases = &CasesB;
ContiguousDest = DestB;
OtherDest = DestA;
} else
return false;
// Start building the compare and branch.
Constant *Offset = ConstantExpr::getNeg(ContiguousCases->back());
Constant *NumCases =
ConstantInt::get(Offset->getType(), ContiguousCases->size());
Value *Sub = SI->getCondition();
if (!Offset->isNullValue())
Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
Value *Cmp;
// If NumCases overflowed, then all possible values jump to the successor.
if (NumCases->isNullValue() && !ContiguousCases->empty())
Cmp = ConstantInt::getTrue(SI->getContext());
else
Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
BranchInst *NewBI = Builder.CreateCondBr(Cmp, ContiguousDest, OtherDest);
// Update weight for the newly-created conditional branch.
if (HasBranchWeights(SI)) {
SmallVector<uint64_t, 8> Weights;
GetBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
uint64_t TrueWeight = 0;
uint64_t FalseWeight = 0;
for (size_t I = 0, E = Weights.size(); I != E; ++I) {
if (SI->getSuccessor(I) == ContiguousDest)
TrueWeight += Weights[I];
else
FalseWeight += Weights[I];
}
while (TrueWeight > UINT32_MAX || FalseWeight > UINT32_MAX) {
TrueWeight /= 2;
FalseWeight /= 2;
}
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
}
// Prune obsolete incoming values off the successors' PHI nodes.
for (auto BBI = ContiguousDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = ContiguousCases->size();
if (ContiguousDest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
}
for (auto BBI = OtherDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = SI->getNumCases() - ContiguousCases->size();
if (OtherDest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
}
// Clean up the default block - it may have phis or other instructions before
// the unreachable terminator.
if (!HasDefault)
createUnreachableSwitchDefault(SI, DTU);
auto *UnreachableDefault = SI->getDefaultDest();
// Drop the switch.
SI->eraseFromParent();
if (!HasDefault && DTU)
DTU->applyUpdates({{DominatorTree::Delete, BB, UnreachableDefault}});
return true;
}
/// Compute masked bits for the condition of a switch
/// and use it to remove dead cases.
static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
AssumptionCache *AC,
const DataLayout &DL) {
Value *Cond = SI->getCondition();
unsigned Bits = Cond->getType()->getIntegerBitWidth();
KnownBits Known = computeKnownBits(Cond, DL, 0, AC, SI);
// We can also eliminate cases by determining that their values are outside of
// the limited range of the condition based on how many significant (non-sign)
// bits are in the condition value.
unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
unsigned MaxSignificantBitsInCond = Bits - ExtraSignBits;
// Gather dead cases.
SmallVector<ConstantInt *, 8> DeadCases;
SmallMapVector<BasicBlock *, int, 8> NumPerSuccessorCases;
for (auto &Case : SI->cases()) {
auto *Successor = Case.getCaseSuccessor();
++NumPerSuccessorCases[Successor];
const APInt &CaseVal = Case.getCaseValue()->getValue();
if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
(CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
DeadCases.push_back(Case.getCaseValue());
--NumPerSuccessorCases[Successor];
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
<< " is dead.\n");
}
}
// If we can prove that the cases must cover all possible values, the
// default destination becomes dead and we can remove it. If we know some
// of the bits in the value, we can use that to more precisely compute the
// number of possible unique case values.
bool HasDefault =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
const unsigned NumUnknownBits =
Bits - (Known.Zero | Known.One).countPopulation();
assert(NumUnknownBits <= Bits);
if (HasDefault && DeadCases.empty() &&
NumUnknownBits < 64 /* avoid overflow */ &&
SI->getNumCases() == (1ULL << NumUnknownBits)) {
createUnreachableSwitchDefault(SI, DTU);
return true;
}
if (DeadCases.empty())
return false;
SwitchInstProfUpdateWrapper SIW(*SI);
for (ConstantInt *DeadCase : DeadCases) {
SwitchInst::CaseIt CaseI = SI->findCaseValue(DeadCase);
assert(CaseI != SI->case_default() &&
"Case was not found. Probably mistake in DeadCases forming.");
// Prune unused values from PHI nodes.
CaseI->getCaseSuccessor()->removePredecessor(SI->getParent());
SIW.removeCase(CaseI);
}
std::vector<DominatorTree::UpdateType> Updates;
for (const std::pair<BasicBlock *, int> &I : NumPerSuccessorCases)
if (I.second == 0)
Updates.push_back({DominatorTree::Delete, SI->getParent(), I.first});
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
/// If BB would be eligible for simplification by
/// TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
/// by an unconditional branch), look at the phi node for BB in the successor
/// block and see if the incoming value is equal to CaseValue. If so, return
/// the phi node, and set PhiIndex to BB's index in the phi node.
static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
BasicBlock *BB, int *PhiIndex) {
if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
return nullptr; // BB must be empty to be a candidate for simplification.
if (!BB->getSinglePredecessor())
return nullptr; // BB must be dominated by the switch.
BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
if (!Branch || !Branch->isUnconditional())
return nullptr; // Terminator must be unconditional branch.
BasicBlock *Succ = Branch->getSuccessor(0);
for (PHINode &PHI : Succ->phis()) {
int Idx = PHI.getBasicBlockIndex(BB);
assert(Idx >= 0 && "PHI has no entry for predecessor?");
Value *InValue = PHI.getIncomingValue(Idx);
if (InValue != CaseValue)
continue;
*PhiIndex = Idx;
return &PHI;
}
return nullptr;
}
/// Try to forward the condition of a switch instruction to a phi node
/// dominated by the switch, if that would mean that some of the destination
/// blocks of the switch can be folded away. Return true if a change is made.
static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
using ForwardingNodesMap = DenseMap<PHINode *, SmallVector<int, 4>>;
ForwardingNodesMap ForwardingNodes;
BasicBlock *SwitchBlock = SI->getParent();
bool Changed = false;
for (auto &Case : SI->cases()) {
ConstantInt *CaseValue = Case.getCaseValue();
BasicBlock *CaseDest = Case.getCaseSuccessor();
// Replace phi operands in successor blocks that are using the constant case
// value rather than the switch condition variable:
// switchbb:
// switch i32 %x, label %default [
// i32 17, label %succ
// ...
// succ:
// %r = phi i32 ... [ 17, %switchbb ] ...
// -->
// %r = phi i32 ... [ %x, %switchbb ] ...
for (PHINode &Phi : CaseDest->phis()) {
// This only works if there is exactly 1 incoming edge from the switch to
// a phi. If there is >1, that means multiple cases of the switch map to 1
// value in the phi, and that phi value is not the switch condition. Thus,
// this transform would not make sense (the phi would be invalid because
// a phi can't have different incoming values from the same block).
int SwitchBBIdx = Phi.getBasicBlockIndex(SwitchBlock);
if (Phi.getIncomingValue(SwitchBBIdx) == CaseValue &&
count(Phi.blocks(), SwitchBlock) == 1) {
Phi.setIncomingValue(SwitchBBIdx, SI->getCondition());
Changed = true;
}
}
// Collect phi nodes that are indirectly using this switch's case constants.
int PhiIdx;
if (auto *Phi = FindPHIForConditionForwarding(CaseValue, CaseDest, &PhiIdx))
ForwardingNodes[Phi].push_back(PhiIdx);
}
for (auto &ForwardingNode : ForwardingNodes) {
PHINode *Phi = ForwardingNode.first;
SmallVectorImpl<int> &Indexes = ForwardingNode.second;
if (Indexes.size() < 2)
continue;
for (int Index : Indexes)
Phi->setIncomingValue(Index, SI->getCondition());
Changed = true;
}
return Changed;
}
/// Return true if the backend will be able to handle
/// initializing an array of constants like C.
static bool ValidLookupTableConstant(Constant *C, const TargetTransformInfo &TTI) {
if (C->isThreadDependent())
return false;
if (C->isDLLImportDependent())
return false;
if (!isa<ConstantFP>(C) && !isa<ConstantInt>(C) &&
!isa<ConstantPointerNull>(C) && !isa<GlobalValue>(C) &&
!isa<UndefValue>(C) && !isa<ConstantExpr>(C))
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
if (!CE->isGEPWithNoNotionalOverIndexing())
return false;
if (!ValidLookupTableConstant(CE->getOperand(0), TTI))
return false;
}
if (!TTI.shouldBuildLookupTablesForConstant(C))
return false;
return true;
}
/// If V is a Constant, return it. Otherwise, try to look up
/// its constant value in ConstantPool, returning 0 if it's not there.
static Constant *
LookupConstant(Value *V,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (Constant *C = dyn_cast<Constant>(V))
return C;
return ConstantPool.lookup(V);
}
/// Try to fold instruction I into a constant. This works for
/// simple instructions such as binary operations where both operands are
/// constant or can be replaced by constants from the ConstantPool. Returns the
/// resulting constant on success, 0 otherwise.
static Constant *
ConstantFold(Instruction *I, const DataLayout &DL,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (SelectInst *Select = dyn_cast<SelectInst>(I)) {
Constant *A = LookupConstant(Select->getCondition(), ConstantPool);
if (!A)
return nullptr;
if (A->isAllOnesValue())
return LookupConstant(Select->getTrueValue(), ConstantPool);
if (A->isNullValue())
return LookupConstant(Select->getFalseValue(), ConstantPool);
return nullptr;
}
SmallVector<Constant *, 4> COps;
for (unsigned N = 0, E = I->getNumOperands(); N != E; ++N) {
if (Constant *A = LookupConstant(I->getOperand(N), ConstantPool))
COps.push_back(A);
else
return nullptr;
}
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
return ConstantFoldCompareInstOperands(Cmp->getPredicate(), COps[0],
COps[1], DL);
}
return ConstantFoldInstOperands(I, COps, DL);
}
/// Try to determine the resulting constant values in phi nodes
/// at the common destination basic block, *CommonDest, for one of the case
/// destionations CaseDest corresponding to value CaseVal (0 for the default
/// case), of a switch instruction SI.
static bool
GetCaseResults(SwitchInst *SI, ConstantInt *CaseVal, BasicBlock *CaseDest,
BasicBlock **CommonDest,
SmallVectorImpl<std::pair<PHINode *, Constant *>> &Res,
const DataLayout &DL, const TargetTransformInfo &TTI) {
// The block from which we enter the common destination.
BasicBlock *Pred = SI->getParent();
// If CaseDest is empty except for some side-effect free instructions through
// which we can constant-propagate the CaseVal, continue to its successor.
SmallDenseMap<Value *, Constant *> ConstantPool;
ConstantPool.insert(std::make_pair(SI->getCondition(), CaseVal));
for (Instruction &I :CaseDest->instructionsWithoutDebug()) {
if (I.isTerminator()) {
// If the terminator is a simple branch, continue to the next block.
if (I.getNumSuccessors() != 1 || I.isExceptionalTerminator())
return false;
Pred = CaseDest;
CaseDest = I.getSuccessor(0);
} else if (Constant *C = ConstantFold(&I, DL, ConstantPool)) {
// Instruction is side-effect free and constant.
// If the instruction has uses outside this block or a phi node slot for
// the block, it is not safe to bypass the instruction since it would then
// no longer dominate all its uses.
for (auto &Use : I.uses()) {
User *User = Use.getUser();
if (Instruction *I = dyn_cast<Instruction>(User))
if (I->getParent() == CaseDest)
continue;
if (PHINode *Phi = dyn_cast<PHINode>(User))
if (Phi->getIncomingBlock(Use) == CaseDest)
continue;
return false;
}
ConstantPool.insert(std::make_pair(&I, C));
} else {
break;
}
}
// If we did not have a CommonDest before, use the current one.
if (!*CommonDest)
*CommonDest = CaseDest;
// If the destination isn't the common one, abort.
if (CaseDest != *CommonDest)
return false;
// Get the values for this case from phi nodes in the destination block.
for (PHINode &PHI : (*CommonDest)->phis()) {
int Idx = PHI.getBasicBlockIndex(Pred);
if (Idx == -1)
continue;
Constant *ConstVal =
LookupConstant(PHI.getIncomingValue(Idx), ConstantPool);
if (!ConstVal)
return false;
// Be conservative about which kinds of constants we support.
if (!ValidLookupTableConstant(ConstVal, TTI))
return false;
Res.push_back(std::make_pair(&PHI, ConstVal));
}
return Res.size() > 0;
}
// Helper function used to add CaseVal to the list of cases that generate
// Result. Returns the updated number of cases that generate this result.
static uintptr_t MapCaseToResult(ConstantInt *CaseVal,
SwitchCaseResultVectorTy &UniqueResults,
Constant *Result) {
for (auto &I : UniqueResults) {
if (I.first == Result) {
I.second.push_back(CaseVal);
return I.second.size();
}
}
UniqueResults.push_back(
std::make_pair(Result, SmallVector<ConstantInt *, 4>(1, CaseVal)));
return 1;
}
// Helper function that initializes a map containing
// results for the PHI node of the common destination block for a switch
// instruction. Returns false if multiple PHI nodes have been found or if
// there is not a common destination block for the switch.
static bool
InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
SwitchCaseResultVectorTy &UniqueResults,
Constant *&DefaultResult, const DataLayout &DL,
const TargetTransformInfo &TTI,
uintptr_t MaxUniqueResults, uintptr_t MaxCasesPerResult) {
for (auto &I : SI->cases()) {
ConstantInt *CaseVal = I.getCaseValue();
// Resulting value at phi nodes for this case value.
SwitchCaseResultsTy Results;
if (!GetCaseResults(SI, CaseVal, I.getCaseSuccessor(), &CommonDest, Results,
DL, TTI))
return false;
// Only one value per case is permitted.
if (Results.size() > 1)
return false;
// Add the case->result mapping to UniqueResults.
const uintptr_t NumCasesForResult =
MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
// Early out if there are too many cases for this result.
if (NumCasesForResult > MaxCasesPerResult)
return false;
// Early out if there are too many unique results.
if (UniqueResults.size() > MaxUniqueResults)
return false;
// Check the PHI consistency.
if (!PHI)
PHI = Results[0].first;
else if (PHI != Results[0].first)
return false;
}
// Find the default result value.
SmallVector<std::pair<PHINode *, Constant *>, 1> DefaultResults;
BasicBlock *DefaultDest = SI->getDefaultDest();
GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResults,
DL, TTI);
// If the default value is not found abort unless the default destination
// is unreachable.
DefaultResult =
DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
if ((!DefaultResult &&
!isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())))
return false;
return true;
}
// Helper function that checks if it is possible to transform a switch with only
// two cases (or two cases + default) that produces a result into a select.
// Example:
// switch (a) {
// case 10: %0 = icmp eq i32 %a, 10
// return 10; %1 = select i1 %0, i32 10, i32 4
// case 20: ----> %2 = icmp eq i32 %a, 20
// return 2; %3 = select i1 %2, i32 2, i32 %1
// default:
// return 4;
// }
static Value *ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy &ResultVector,
Constant *DefaultResult, Value *Condition,
IRBuilder<> &Builder) {
assert(ResultVector.size() == 2 &&
"We should have exactly two unique results at this point");
// If we are selecting between only two cases transform into a simple
// select or a two-way select if default is possible.
if (ResultVector[0].second.size() == 1 &&
ResultVector[1].second.size() == 1) {
ConstantInt *const FirstCase = ResultVector[0].second[0];
ConstantInt *const SecondCase = ResultVector[1].second[0];
bool DefaultCanTrigger = DefaultResult;
Value *SelectValue = ResultVector[1].first;
if (DefaultCanTrigger) {
Value *const ValueCompare =
Builder.CreateICmpEQ(Condition, SecondCase, "switch.selectcmp");
SelectValue = Builder.CreateSelect(ValueCompare, ResultVector[1].first,
DefaultResult, "switch.select");
}
Value *const ValueCompare =
Builder.CreateICmpEQ(Condition, FirstCase, "switch.selectcmp");
return Builder.CreateSelect(ValueCompare, ResultVector[0].first,
SelectValue, "switch.select");
}
return nullptr;
}
// Helper function to cleanup a switch instruction that has been converted into
// a select, fixing up PHI nodes and basic blocks.
static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
Value *SelectValue,
IRBuilder<> &Builder,
DomTreeUpdater *DTU) {
std::vector<DominatorTree::UpdateType> Updates;
BasicBlock *SelectBB = SI->getParent();
BasicBlock *DestBB = PHI->getParent();
if (!is_contained(predecessors(DestBB), SelectBB))
Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
Builder.CreateBr(DestBB);
// Remove the switch.
while (PHI->getBasicBlockIndex(SelectBB) >= 0)
PHI->removeIncomingValue(SelectBB);
PHI->addIncoming(SelectValue, SelectBB);
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == DestBB)
continue;
Succ->removePredecessor(SelectBB);
Updates.push_back({DominatorTree::Delete, SelectBB, Succ});
}
SI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
}
/// If the switch is only used to initialize one or more
/// phi nodes in a common successor block with only two different
/// constant values, replace the switch with select.
static bool switchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI) {
Value *const Cond = SI->getCondition();
PHINode *PHI = nullptr;
BasicBlock *CommonDest = nullptr;
Constant *DefaultResult;
SwitchCaseResultVectorTy UniqueResults;
// Collect all the cases that will deliver the same value from the switch.
if (!InitializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
DL, TTI, 2, 1))
return false;
// Selects choose between maximum two values.
if (UniqueResults.size() != 2)
return false;
assert(PHI != nullptr && "PHI for value select not found");
Builder.SetInsertPoint(SI);
Value *SelectValue =
ConvertTwoCaseSwitch(UniqueResults, DefaultResult, Cond, Builder);
if (SelectValue) {
RemoveSwitchAfterSelectConversion(SI, PHI, SelectValue, Builder, DTU);
return true;
}
// The switch couldn't be converted into a select.
return false;
}
namespace {
/// This class represents a lookup table that can be used to replace a switch.
class SwitchLookupTable {
public:
/// Create a lookup table to use as a switch replacement with the contents
/// of Values, using DefaultValue to fill any holes in the table.
SwitchLookupTable(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName);
/// Build instructions with Builder to retrieve the value at
/// the position given by Index in the lookup table.
Value *BuildLookup(Value *Index, IRBuilder<> &Builder);
/// Return true if a table with TableSize elements of
/// type ElementType would fit in a target-legal register.
static bool WouldFitInRegister(const DataLayout &DL, uint64_t TableSize,
Type *ElementType);
private:
// Depending on the contents of the table, it can be represented in
// different ways.
enum {
// For tables where each element contains the same value, we just have to
// store that single value and return it for each lookup.
SingleValueKind,
// For tables where there is a linear relationship between table index
// and values. We calculate the result with a simple multiplication
// and addition instead of a table lookup.
LinearMapKind,
// For small tables with integer elements, we can pack them into a bitmap
// that fits into a target-legal register. Values are retrieved by
// shift and mask operations.
BitMapKind,
// The table is stored as an array of values. Values are retrieved by load
// instructions from the table.
ArrayKind
} Kind;
// For SingleValueKind, this is the single value.
Constant *SingleValue = nullptr;
// For BitMapKind, this is the bitmap.
ConstantInt *BitMap = nullptr;
IntegerType *BitMapElementTy = nullptr;
// For LinearMapKind, these are the constants used to derive the value.
ConstantInt *LinearOffset = nullptr;
ConstantInt *LinearMultiplier = nullptr;
// For ArrayKind, this is the array.
GlobalVariable *Array = nullptr;
};
} // end anonymous namespace
SwitchLookupTable::SwitchLookupTable(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName) {
assert(Values.size() && "Can't build lookup table without values!");
assert(TableSize >= Values.size() && "Can't fit values in table!");
// If all values in the table are equal, this is that value.
SingleValue = Values.begin()->second;
Type *ValueType = Values.begin()->second->getType();
// Build up the table contents.
SmallVector<Constant *, 64> TableContents(TableSize);
for (size_t I = 0, E = Values.size(); I != E; ++I) {
ConstantInt *CaseVal = Values[I].first;
Constant *CaseRes = Values[I].second;
assert(CaseRes->getType() == ValueType);
uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
TableContents[Idx] = CaseRes;
if (CaseRes != SingleValue)
SingleValue = nullptr;
}
// Fill in any holes in the table with the default result.
if (Values.size() < TableSize) {
assert(DefaultValue &&
"Need a default value to fill the lookup table holes.");
assert(DefaultValue->getType() == ValueType);
for (uint64_t I = 0; I < TableSize; ++I) {
if (!TableContents[I])
TableContents[I] = DefaultValue;
}
if (DefaultValue != SingleValue)
SingleValue = nullptr;
}
// If each element in the table contains the same value, we only need to store
// that single value.
if (SingleValue) {
Kind = SingleValueKind;
return;
}
// Check if we can derive the value with a linear transformation from the
// table index.
if (isa<IntegerType>(ValueType)) {
bool LinearMappingPossible = true;
APInt PrevVal;
APInt DistToPrev;
assert(TableSize >= 2 && "Should be a SingleValue table.");
// Check if there is the same distance between two consecutive values.
for (uint64_t I = 0; I < TableSize; ++I) {
ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
if (!ConstVal) {
// This is an undef. We could deal with it, but undefs in lookup tables
// are very seldom. It's probably not worth the additional complexity.
LinearMappingPossible = false;
break;
}
const APInt &Val = ConstVal->getValue();
if (I != 0) {
APInt Dist = Val - PrevVal;
if (I == 1) {
DistToPrev = Dist;
} else if (Dist != DistToPrev) {
LinearMappingPossible = false;
break;
}
}
PrevVal = Val;
}
if (LinearMappingPossible) {
LinearOffset = cast<ConstantInt>(TableContents[0]);
LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
Kind = LinearMapKind;
++NumLinearMaps;
return;
}
}
// If the type is integer and the table fits in a register, build a bitmap.
if (WouldFitInRegister(DL, TableSize, ValueType)) {
IntegerType *IT = cast<IntegerType>(ValueType);
APInt TableInt(TableSize * IT->getBitWidth(), 0);
for (uint64_t I = TableSize; I > 0; --I) {
TableInt <<= IT->getBitWidth();
// Insert values into the bitmap. Undef values are set to zero.
if (!isa<UndefValue>(TableContents[I - 1])) {
ConstantInt *Val = cast<ConstantInt>(TableContents[I - 1]);
TableInt |= Val->getValue().zext(TableInt.getBitWidth());
}
}
BitMap = ConstantInt::get(M.getContext(), TableInt);
BitMapElementTy = IT;
Kind = BitMapKind;
++NumBitMaps;
return;
}
// Store the table in an array.
ArrayType *ArrayTy = ArrayType::get(ValueType, TableSize);
Constant *Initializer = ConstantArray::get(ArrayTy, TableContents);
Array = new GlobalVariable(M, ArrayTy, /*isConstant=*/true,
GlobalVariable::PrivateLinkage, Initializer,
"switch.table." + FuncName);
Array->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
// Set the alignment to that of an array items. We will be only loading one
// value out of it.
Array->setAlignment(Align(DL.getPrefTypeAlignment(ValueType)));
Kind = ArrayKind;
}
Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) {
switch (Kind) {
case SingleValueKind:
return SingleValue;
case LinearMapKind: {
// Derive the result value from the input value.
Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
false, "switch.idx.cast");
if (!LinearMultiplier->isOne())
Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult");
if (!LinearOffset->isZero())
Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset");
return Result;
}
case BitMapKind: {
// Type of the bitmap (e.g. i59).
IntegerType *MapTy = BitMap->getType();
// Cast Index to the same type as the bitmap.
// Note: The Index is <= the number of elements in the table, so
// truncating it to the width of the bitmask is safe.
Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast");
// Multiply the shift amount by the element width.
ShiftAmt = Builder.CreateMul(
ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
"switch.shiftamt");
// Shift down.
Value *DownShifted =
Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift");
// Mask off.
return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked");
}
case ArrayKind: {
// Make sure the table index will not overflow when treated as signed.
IntegerType *IT = cast<IntegerType>(Index->getType());
uint64_t TableSize =
Array->getInitializer()->getType()->getArrayNumElements();
if (TableSize > (1ULL << (IT->getBitWidth() - 1)))
Index = Builder.CreateZExt(
Index, IntegerType::get(IT->getContext(), IT->getBitWidth() + 1),
"switch.tableidx.zext");
Value *GEPIndices[] = {Builder.getInt32(0), Index};
Value *GEP = Builder.CreateInBoundsGEP(Array->getValueType(), Array,
GEPIndices, "switch.gep");
return Builder.CreateLoad(
cast<ArrayType>(Array->getValueType())->getElementType(), GEP,
"switch.load");
}
}
llvm_unreachable("Unknown lookup table kind!");
}
bool SwitchLookupTable::WouldFitInRegister(const DataLayout &DL,
uint64_t TableSize,
Type *ElementType) {
auto *IT = dyn_cast<IntegerType>(ElementType);
if (!IT)
return false;
// FIXME: If the type is wider than it needs to be, e.g. i8 but all values
// are <= 15, we could try to narrow the type.
// Avoid overflow, fitsInLegalInteger uses unsigned int for the width.
if (TableSize >= UINT_MAX / IT->getBitWidth())
return false;
return DL.fitsInLegalInteger(TableSize * IT->getBitWidth());
}
/// Determine whether a lookup table should be built for this switch, based on
/// the number of cases, size of the table, and the types of the results.
static bool
ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
const TargetTransformInfo &TTI, const DataLayout &DL,
const SmallDenseMap<PHINode *, Type *> &ResultTypes) {
if (SI->getNumCases() > TableSize || TableSize >= UINT64_MAX / 10)
return false; // TableSize overflowed, or mul below might overflow.
bool AllTablesFitInRegister = true;
bool HasIllegalType = false;
for (const auto &I : ResultTypes) {
Type *Ty = I.second;
// Saturate this flag to true.
HasIllegalType = HasIllegalType || !TTI.isTypeLegal(Ty);
// Saturate this flag to false.
AllTablesFitInRegister =
AllTablesFitInRegister &&
SwitchLookupTable::WouldFitInRegister(DL, TableSize, Ty);
// If both flags saturate, we're done. NOTE: This *only* works with
// saturating flags, and all flags have to saturate first due to the
// non-deterministic behavior of iterating over a dense map.
if (HasIllegalType && !AllTablesFitInRegister)
break;
}
// If each table would fit in a register, we should build it anyway.
if (AllTablesFitInRegister)
return true;
// Don't build a table that doesn't fit in-register if it has illegal types.
if (HasIllegalType)
return false;
// The table density should be at least 40%. This is the same criterion as for
// jump tables, see SelectionDAGBuilder::handleJTSwitchCase.
// FIXME: Find the best cut-off.
return SI->getNumCases() * 10 >= TableSize * 4;
}
/// Try to reuse the switch table index compare. Following pattern:
/// \code
/// if (idx < tablesize)
/// r = table[idx]; // table does not contain default_value
/// else
/// r = default_value;
/// if (r != default_value)
/// ...
/// \endcode
/// Is optimized to:
/// \code
/// cond = idx < tablesize;
/// if (cond)
/// r = table[idx];
/// else
/// r = default_value;
/// if (cond)
/// ...
/// \endcode
/// Jump threading will then eliminate the second if(cond).
static void reuseTableCompare(
User *PhiUser, BasicBlock *PhiBlock, BranchInst *RangeCheckBranch,
Constant *DefaultValue,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values) {
ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
if (!CmpInst)
return;
// We require that the compare is in the same block as the phi so that jump
// threading can do its work afterwards.
if (CmpInst->getParent() != PhiBlock)
return;
Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
if (!CmpOp1)
return;
Value *RangeCmp = RangeCheckBranch->getCondition();
Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
// Check if the compare with the default value is constant true or false.
Constant *DefaultConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
DefaultValue, CmpOp1, true);
if (DefaultConst != TrueConst && DefaultConst != FalseConst)
return;
// Check if the compare with the case values is distinct from the default
// compare result.
for (auto ValuePair : Values) {
Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
ValuePair.second, CmpOp1, true);
if (!CaseConst || CaseConst == DefaultConst || isa<UndefValue>(CaseConst))
return;
assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
"Expect true or false as compare result.");
}
// Check if the branch instruction dominates the phi node. It's a simple
// dominance check, but sufficient for our needs.
// Although this check is invariant in the calling loops, it's better to do it
// at this late stage. Practically we do it at most once for a switch.
BasicBlock *BranchBlock = RangeCheckBranch->getParent();
for (BasicBlock *Pred : predecessors(PhiBlock)) {
if (Pred != BranchBlock && Pred->getUniquePredecessor() != BranchBlock)
return;
}
if (DefaultConst == FalseConst) {
// The compare yields the same result. We can replace it.
CmpInst->replaceAllUsesWith(RangeCmp);
++NumTableCmpReuses;
} else {
// The compare yields the same result, just inverted. We can replace it.
Value *InvertedTableCmp = BinaryOperator::CreateXor(
RangeCmp, ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
RangeCheckBranch);
CmpInst->replaceAllUsesWith(InvertedTableCmp);
++NumTableCmpReuses;
}
}
/// If the switch is only used to initialize one or more phi nodes in a common
/// successor block with different constant values, replace the switch with
/// lookup tables.
static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU, const DataLayout &DL,
const TargetTransformInfo &TTI) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
BasicBlock *BB = SI->getParent();
Function *Fn = BB->getParent();
// Only build lookup table when we have a target that supports it or the
// attribute is not set.
if (!TTI.shouldBuildLookupTables() ||
(Fn->getFnAttribute("no-jump-tables").getValueAsString() == "true"))
return false;
// FIXME: If the switch is too sparse for a lookup table, perhaps we could
// split off a dense part and build a lookup table for that.
// FIXME: This creates arrays of GEPs to constant strings, which means each
// GEP needs a runtime relocation in PIC code. We should just build one big
// string and lookup indices into that.
// Ignore switches with less than three cases. Lookup tables will not make
// them faster, so we don't analyze them.
if (SI->getNumCases() < 3)
return false;
// Figure out the corresponding result for each case value and phi node in the
// common destination, as well as the min and max case values.
assert(!SI->cases().empty());
SwitchInst::CaseIt CI = SI->case_begin();
ConstantInt *MinCaseVal = CI->getCaseValue();
ConstantInt *MaxCaseVal = CI->getCaseValue();
BasicBlock *CommonDest = nullptr;
using ResultListTy = SmallVector<std::pair<ConstantInt *, Constant *>, 4>;
SmallDenseMap<PHINode *, ResultListTy> ResultLists;
SmallDenseMap<PHINode *, Constant *> DefaultResults;
SmallDenseMap<PHINode *, Type *> ResultTypes;
SmallVector<PHINode *, 4> PHIs;
for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
ConstantInt *CaseVal = CI->getCaseValue();
if (CaseVal->getValue().slt(MinCaseVal->getValue()))
MinCaseVal = CaseVal;
if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
MaxCaseVal = CaseVal;
// Resulting value at phi nodes for this case value.
using ResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
ResultsTy Results;
if (!GetCaseResults(SI, CaseVal, CI->getCaseSuccessor(), &CommonDest,
Results, DL, TTI))
return false;
// Append the result from this case to the list for each phi.
for (const auto &I : Results) {
PHINode *PHI = I.first;
Constant *Value = I.second;
if (!ResultLists.count(PHI))
PHIs.push_back(PHI);
ResultLists[PHI].push_back(std::make_pair(CaseVal, Value));
}
}
// Keep track of the result types.
for (PHINode *PHI : PHIs) {
ResultTypes[PHI] = ResultLists[PHI][0].second->getType();
}
uint64_t NumResults = ResultLists[PHIs[0]].size();
APInt RangeSpread = MaxCaseVal->getValue() - MinCaseVal->getValue();
uint64_t TableSize = RangeSpread.getLimitedValue() + 1;
bool TableHasHoles = (NumResults < TableSize);
// If the table has holes, we need a constant result for the default case
// or a bitmask that fits in a register.
SmallVector<std::pair<PHINode *, Constant *>, 4> DefaultResultsList;
bool HasDefaultResults =
GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest,
DefaultResultsList, DL, TTI);
bool NeedMask = (TableHasHoles && !HasDefaultResults);
if (NeedMask) {
// As an extra penalty for the validity test we require more cases.
if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark).
return false;
if (!DL.fitsInLegalInteger(TableSize))
return false;
}
for (const auto &I : DefaultResultsList) {
PHINode *PHI = I.first;
Constant *Result = I.second;
DefaultResults[PHI] = Result;
}
if (!ShouldBuildLookupTable(SI, TableSize, TTI, DL, ResultTypes))
return false;
std::vector<DominatorTree::UpdateType> Updates;
// Create the BB that does the lookups.
Module &Mod = *CommonDest->getParent()->getParent();
BasicBlock *LookupBB = BasicBlock::Create(
Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest);
// Compute the table index value.
Builder.SetInsertPoint(SI);
Value *TableIndex;
if (MinCaseVal->isNullValue())
TableIndex = SI->getCondition();
else
TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
"switch.tableidx");
// Compute the maximum table size representable by the integer type we are
// switching upon.
unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
assert(MaxTableSize >= TableSize &&
"It is impossible for a switch to have more entries than the max "
"representable value of its input integer type's size.");
// If the default destination is unreachable, or if the lookup table covers
// all values of the conditional variable, branch directly to the lookup table
// BB. Otherwise, check that the condition is within the case range.
const bool DefaultIsReachable =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
BranchInst *RangeCheckBranch = nullptr;
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
Builder.CreateBr(LookupBB);
Updates.push_back({DominatorTree::Insert, BB, LookupBB});
// Note: We call removeProdecessor later since we need to be able to get the
// PHI value for the default case in case we're using a bit mask.
} else {
Value *Cmp = Builder.CreateICmpULT(
TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
RangeCheckBranch =
Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
Updates.push_back({DominatorTree::Insert, BB, LookupBB});
}
// Populate the BB that does the lookups.
Builder.SetInsertPoint(LookupBB);
if (NeedMask) {
// Before doing the lookup, we do the hole check. The LookupBB is therefore
// re-purposed to do the hole check, and we create a new LookupBB.
BasicBlock *MaskBB = LookupBB;
MaskBB->setName("switch.hole_check");
LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup",
CommonDest->getParent(), CommonDest);
// Make the mask's bitwidth at least 8-bit and a power-of-2 to avoid
// unnecessary illegal types.
uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
APInt MaskInt(TableSizePowOf2, 0);
APInt One(TableSizePowOf2, 1);
// Build bitmask; fill in a 1 bit for every case.
const ResultListTy &ResultList = ResultLists[PHIs[0]];
for (size_t I = 0, E = ResultList.size(); I != E; ++I) {
uint64_t Idx = (ResultList[I].first->getValue() - MinCaseVal->getValue())
.getLimitedValue();
MaskInt |= One << Idx;
}
ConstantInt *TableMask = ConstantInt::get(Mod.getContext(), MaskInt);
// Get the TableIndex'th bit of the bitmask.
// If this bit is 0 (meaning hole) jump to the default destination,
// else continue with table lookup.
IntegerType *MapTy = TableMask->getType();
Value *MaskIndex =
Builder.CreateZExtOrTrunc(TableIndex, MapTy, "switch.maskindex");
Value *Shifted = Builder.CreateLShr(TableMask, MaskIndex, "switch.shifted");
Value *LoBit = Builder.CreateTrunc(
Shifted, Type::getInt1Ty(Mod.getContext()), "switch.lobit");
Builder.CreateCondBr(LoBit, LookupBB, SI->getDefaultDest());
Updates.push_back({DominatorTree::Insert, MaskBB, LookupBB});
Updates.push_back({DominatorTree::Insert, MaskBB, SI->getDefaultDest()});
Builder.SetInsertPoint(LookupBB);
AddPredecessorToBlock(SI->getDefaultDest(), MaskBB, BB);
}
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
// We cached PHINodes in PHIs. To avoid accessing deleted PHINodes later,
// do not delete PHINodes here.
SI->getDefaultDest()->removePredecessor(BB,
/*KeepOneInputPHIs=*/true);
Updates.push_back({DominatorTree::Delete, BB, SI->getDefaultDest()});
}
bool ReturnedEarly = false;
for (PHINode *PHI : PHIs) {
const ResultListTy &ResultList = ResultLists[PHI];
// If using a bitmask, use any value to fill the lookup table holes.
Constant *DV = NeedMask ? ResultLists[PHI][0].second : DefaultResults[PHI];
StringRef FuncName = Fn->getName();
SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultList, DV, DL,
FuncName);
Value *Result = Table.BuildLookup(TableIndex, Builder);
// If the result is used to return immediately from the function, we want to
// do that right here.
if (PHI->hasOneUse() && isa<ReturnInst>(*PHI->user_begin()) &&
PHI->user_back() == CommonDest->getFirstNonPHIOrDbg()) {
Builder.CreateRet(Result);
ReturnedEarly = true;
break;
}
// Do a small peephole optimization: re-use the switch table compare if
// possible.
if (!TableHasHoles && HasDefaultResults && RangeCheckBranch) {
BasicBlock *PhiBlock = PHI->getParent();
// Search for compare instructions which use the phi.
for (auto *User : PHI->users()) {
reuseTableCompare(User, PhiBlock, RangeCheckBranch, DV, ResultList);
}
}
PHI->addIncoming(Result, LookupBB);
}
if (!ReturnedEarly) {
Builder.CreateBr(CommonDest);
Updates.push_back({DominatorTree::Insert, LookupBB, CommonDest});
}
// Remove the switch.
SmallSetVector<BasicBlock *, 8> RemovedSuccessors;
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == SI->getDefaultDest())
continue;
Succ->removePredecessor(BB);
RemovedSuccessors.insert(Succ);
}
SI->eraseFromParent();
if (DTU) {
for (BasicBlock *RemovedSuccessor : RemovedSuccessors)
Updates.push_back({DominatorTree::Delete, BB, RemovedSuccessor});
DTU->applyUpdates(Updates);
}
++NumLookupTables;
if (NeedMask)
++NumLookupTablesHoles;
return true;
}
static bool isSwitchDense(ArrayRef<int64_t> Values) {
// See also SelectionDAGBuilder::isDense(), which this function was based on.
uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
uint64_t Range = Diff + 1;
uint64_t NumCases = Values.size();
// 40% is the default density for building a jump table in optsize/minsize mode.
uint64_t MinDensity = 40;
return NumCases * 100 >= Range * MinDensity;
}
/// Try to transform a switch that has "holes" in it to a contiguous sequence
/// of cases.
///
/// A switch such as: switch(i) {case 5: case 9: case 13: case 17:} can be
/// range-reduced to: switch ((i-5) / 4) {case 0: case 1: case 2: case 3:}.
///
/// This converts a sparse switch into a dense switch which allows better
/// lowering and could also allow transforming into a lookup table.
static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
auto *CondTy = cast<IntegerType>(SI->getCondition()->getType());
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
// Only bother with this optimization if there are more than 3 switch cases;
// SDAG will only bother creating jump tables for 4 or more cases.
if (SI->getNumCases() < 4)
return false;
// This transform is agnostic to the signedness of the input or case values. We
// can treat the case values as signed or unsigned. We can optimize more common
// cases such as a sequence crossing zero {-4,0,4,8} if we interpret case values
// as signed.
SmallVector<int64_t,4> Values;
for (auto &C : SI->cases())
Values.push_back(C.getCaseValue()->getValue().getSExtValue());
llvm::sort(Values);
// If the switch is already dense, there's nothing useful to do here.
if (isSwitchDense(Values))
return false;
// First, transform the values such that they start at zero and ascend.
int64_t Base = Values[0];
for (auto &V : Values)
V -= (uint64_t)(Base);
// Now we have signed numbers that have been shifted so that, given enough
// precision, there are no negative values. Since the rest of the transform
// is bitwise only, we switch now to an unsigned representation.
// This transform can be done speculatively because it is so cheap - it
// results in a single rotate operation being inserted.
// FIXME: It's possible that optimizing a switch on powers of two might also
// be beneficial - flag values are often powers of two and we could use a CLZ
// as the key function.
// countTrailingZeros(0) returns 64. As Values is guaranteed to have more than
// one element and LLVM disallows duplicate cases, Shift is guaranteed to be
// less than 64.
unsigned Shift = 64;
for (auto &V : Values)
Shift = std::min(Shift, countTrailingZeros((uint64_t)V));
assert(Shift < 64);
if (Shift > 0)
for (auto &V : Values)
V = (int64_t)((uint64_t)V >> Shift);
if (!isSwitchDense(Values))
// Transform didn't create a dense switch.
return false;
// The obvious transform is to shift the switch condition right and emit a
// check that the condition actually cleanly divided by GCD, i.e.
// C & (1 << Shift - 1) == 0
// inserting a new CFG edge to handle the case where it didn't divide cleanly.
//
// A cheaper way of doing this is a simple ROTR(C, Shift). This performs the
// shift and puts the shifted-off bits in the uppermost bits. If any of these
// are nonzero then the switch condition will be very large and will hit the
// default case.
auto *Ty = cast<IntegerType>(SI->getCondition()->getType());
Builder.SetInsertPoint(SI);
auto *ShiftC = ConstantInt::get(Ty, Shift);
auto *Sub = Builder.CreateSub(SI->getCondition(), ConstantInt::get(Ty, Base));
auto *LShr = Builder.CreateLShr(Sub, ShiftC);
auto *Shl = Builder.CreateShl(Sub, Ty->getBitWidth() - Shift);
auto *Rot = Builder.CreateOr(LShr, Shl);
SI->replaceUsesOfWith(SI->getCondition(), Rot);
for (auto Case : SI->cases()) {
auto *Orig = Case.getCaseValue();
auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base);
Case.setValue(
cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(ShiftC->getValue()))));
}
return true;
}
bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
BasicBlock *BB = SI->getParent();
if (isValueEqualityComparison(SI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
return requestResimplify();
Value *Cond = SI->getCondition();
if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
if (SimplifySwitchOnSelect(SI, Select))
return requestResimplify();
// If the block only contains the switch, see if we can fold the block
// away into any preds.
if (SI == &*BB->instructionsWithoutDebug().begin())
if (FoldValueComparisonIntoPredecessors(SI, Builder))
return requestResimplify();
}
// Try to transform the switch into an icmp and a branch.
if (TurnSwitchRangeIntoICmp(SI, Builder))
return requestResimplify();
// Remove unreachable cases.
if (eliminateDeadSwitchCases(SI, DTU, Options.AC, DL))
return requestResimplify();
if (switchToSelect(SI, Builder, DTU, DL, TTI))
return requestResimplify();
if (Options.ForwardSwitchCondToPhi && ForwardSwitchConditionToPHI(SI))
return requestResimplify();
// The conversion from switch to lookup tables results in difficult-to-analyze
// code and makes pruning branches much harder. This is a problem if the
// switch expression itself can still be restricted as a result of inlining or
// CVP. Therefore, only apply this transformation during late stages of the
// optimisation pipeline.
if (Options.ConvertSwitchToLookupTable &&
SwitchToLookupTable(SI, Builder, DTU, DL, TTI))
return requestResimplify();
if (ReduceSwitchRange(SI, Builder, DL, TTI))
return requestResimplify();
return false;
}
bool SimplifyCFGOpt::simplifyIndirectBr(IndirectBrInst *IBI) {
BasicBlock *BB = IBI->getParent();
bool Changed = false;
// Eliminate redundant destinations.
SmallPtrSet<Value *, 8> Succs;
SmallSetVector<BasicBlock *, 8> RemovedSuccs;
for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
BasicBlock *Dest = IBI->getDestination(i);
if (!Dest->hasAddressTaken() || !Succs.insert(Dest).second) {
if (!Dest->hasAddressTaken())
RemovedSuccs.insert(Dest);
Dest->removePredecessor(BB);
IBI->removeDestination(i);
--i;
--e;
Changed = true;
}
}
if (DTU) {
std::vector<DominatorTree::UpdateType> Updates;
Updates.reserve(RemovedSuccs.size());
for (auto *RemovedSucc : RemovedSuccs)
Updates.push_back({DominatorTree::Delete, BB, RemovedSucc});
DTU->applyUpdates(Updates);
}
if (IBI->getNumDestinations() == 0) {
// If the indirectbr has no successors, change it to unreachable.
new UnreachableInst(IBI->getContext(), IBI);
EraseTerminatorAndDCECond(IBI);
return true;
}
if (IBI->getNumDestinations() == 1) {
// If the indirectbr has one successor, change it to a direct branch.
BranchInst::Create(IBI->getDestination(0), IBI);
EraseTerminatorAndDCECond(IBI);
return true;
}
if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
if (SimplifyIndirectBrOnSelect(IBI, SI))
return requestResimplify();
}
return Changed;
}
/// Given an block with only a single landing pad and a unconditional branch
/// try to find another basic block which this one can be merged with. This
/// handles cases where we have multiple invokes with unique landing pads, but
/// a shared handler.
///
/// We specifically choose to not worry about merging non-empty blocks
/// here. That is a PRE/scheduling problem and is best solved elsewhere. In
/// practice, the optimizer produces empty landing pad blocks quite frequently
/// when dealing with exception dense code. (see: instcombine, gvn, if-else
/// sinking in this file)
///
/// This is primarily a code size optimization. We need to avoid performing
/// any transform which might inhibit optimization (such as our ability to
/// specialize a particular handler via tail commoning). We do this by not
/// merging any blocks which require us to introduce a phi. Since the same
/// values are flowing through both blocks, we don't lose any ability to
/// specialize. If anything, we make such specialization more likely.
///
/// TODO - This transformation could remove entries from a phi in the target
/// block when the inputs in the phi are the same for the two blocks being
/// merged. In some cases, this could result in removal of the PHI entirely.
static bool TryToMergeLandingPad(LandingPadInst *LPad, BranchInst *BI,
BasicBlock *BB, DomTreeUpdater *DTU) {
auto Succ = BB->getUniqueSuccessor();
assert(Succ);
// If there's a phi in the successor block, we'd likely have to introduce
// a phi into the merged landing pad block.
if (isa<PHINode>(*Succ->begin()))
return false;
for (BasicBlock *OtherPred : predecessors(Succ)) {
if (BB == OtherPred)
continue;
BasicBlock::iterator I = OtherPred->begin();
LandingPadInst *LPad2 = dyn_cast<LandingPadInst>(I);
if (!LPad2 || !LPad2->isIdenticalTo(LPad))
continue;
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
BranchInst *BI2 = dyn_cast<BranchInst>(I);
if (!BI2 || !BI2->isIdenticalTo(BI))
continue;
std::vector<DominatorTree::UpdateType> Updates;
// We've found an identical block. Update our predecessors to take that
// path instead and make ourselves dead.
SmallPtrSet<BasicBlock *, 16> Preds;
Preds.insert(pred_begin(BB), pred_end(BB));
for (BasicBlock *Pred : Preds) {
InvokeInst *II = cast<InvokeInst>(Pred->getTerminator());
assert(II->getNormalDest() != BB && II->getUnwindDest() == BB &&
"unexpected successor");
II->setUnwindDest(OtherPred);
Updates.push_back({DominatorTree::Insert, Pred, OtherPred});
Updates.push_back({DominatorTree::Delete, Pred, BB});
}
// The debug info in OtherPred doesn't cover the merged control flow that
// used to go through BB. We need to delete it or update it.
for (auto I = OtherPred->begin(), E = OtherPred->end(); I != E;) {
Instruction &Inst = *I;
I++;
if (isa<DbgInfoIntrinsic>(Inst))
Inst.eraseFromParent();
}
SmallPtrSet<BasicBlock *, 16> Succs;
Succs.insert(succ_begin(BB), succ_end(BB));
for (BasicBlock *Succ : Succs) {
Succ->removePredecessor(BB);
Updates.push_back({DominatorTree::Delete, BB, Succ});
}
IRBuilder<> Builder(BI);
Builder.CreateUnreachable();
BI->eraseFromParent();
if (DTU)
DTU->applyUpdates(Updates);
return true;
}
return false;
}
bool SimplifyCFGOpt::simplifyBranch(BranchInst *Branch, IRBuilder<> &Builder) {
return Branch->isUnconditional() ? simplifyUncondBranch(Branch, Builder)
: simplifyCondBranch(Branch, Builder);
}
bool SimplifyCFGOpt::simplifyUncondBranch(BranchInst *BI,
IRBuilder<> &Builder) {
BasicBlock *BB = BI->getParent();
BasicBlock *Succ = BI->getSuccessor(0);
// If the Terminator is the only non-phi instruction, simplify the block.
// If LoopHeader is provided, check if the block or its successor is a loop
// header. (This is for early invocations before loop simplify and
// vectorization to keep canonical loop forms for nested loops. These blocks
// can be eliminated when the pass is invoked later in the back-end.)
// Note that if BB has only one predecessor then we do not introduce new
// backedge, so we can eliminate BB.
bool NeedCanonicalLoop =
Options.NeedCanonicalLoop &&
(!LoopHeaders.empty() && BB->hasNPredecessorsOrMore(2) &&
(is_contained(LoopHeaders, BB) || is_contained(LoopHeaders, Succ)));
BasicBlock::iterator I = BB->getFirstNonPHIOrDbg(true)->getIterator();
if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
!NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB, DTU))
return true;
// If the only instruction in the block is a seteq/setne comparison against a
// constant, try to simplify the block.
if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
if (I->isTerminator() &&
tryToSimplifyUncondBranchWithICmpInIt(ICI, Builder))
return true;
}
// See if we can merge an empty landing pad block with another which is
// equivalent.
if (LandingPadInst *LPad = dyn_cast<LandingPadInst>(I)) {
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
if (I->isTerminator() && TryToMergeLandingPad(LPad, BI, BB, DTU))
return true;
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and our successor, fold the comparison into the
// predecessor and use logical operations to update the incoming value
// for PHI nodes in common successor.
if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
Options.BonusInstThreshold, true))
return requestResimplify();
return false;
}
static BasicBlock *allPredecessorsComeFromSameSource(BasicBlock *BB) {
BasicBlock *PredPred = nullptr;
for (auto *P : predecessors(BB)) {
BasicBlock *PPred = P->getSinglePredecessor();
if (!PPred || (PredPred && PredPred != PPred))
return nullptr;
PredPred = PPred;
}
return PredPred;
}
bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
BasicBlock *BB = BI->getParent();
if (!Options.SimplifyCondBranch)
return false;
// Conditional branch
if (isValueEqualityComparison(BI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this
// switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
return requestResimplify();
// This block must be empty, except for the setcond inst, if it exists.
// Ignore dbg and pseudo intrinsics.
auto I = BB->instructionsWithoutDebug(true).begin();
if (&*I == BI) {
if (FoldValueComparisonIntoPredecessors(BI, Builder))
return requestResimplify();
} else if (&*I == cast<Instruction>(BI->getCondition())) {
++I;
if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
return requestResimplify();
}
}
// Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
if (SimplifyBranchOnICmpChain(BI, Builder, DL))
return true;
// If this basic block has dominating predecessor blocks and the dominating
// blocks' conditions imply BI's condition, we know the direction of BI.
Optional<bool> Imp = isImpliedByDomCondition(BI->getCondition(), BI, DL);
if (Imp) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
ConstantInt *TorF = *Imp ? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
BI->setCondition(TorF);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return requestResimplify();
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI, DTU, /*MSSAU=*/nullptr, &TTI,
Options.BonusInstThreshold, true))
return requestResimplify();
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
// there is any identical code in the "then" and "else" blocks. If so, we
// can hoist it up to the branching block.
if (BI->getSuccessor(0)->getSinglePredecessor()) {
if (BI->getSuccessor(1)->getSinglePredecessor()) {
if (HoistCommon && Options.HoistCommonInsts)
if (HoistThenElseCodeToIf(BI, TTI))
return requestResimplify();
} else {
// If Successor #1 has multiple preds, we may be able to conditionally
// execute Successor #0 if it branches to Successor #1.
Instruction *Succ0TI = BI->getSuccessor(0)->getTerminator();
if (Succ0TI->getNumSuccessors() == 1 &&
Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0), TTI))
return requestResimplify();
}
} else if (BI->getSuccessor(1)->getSinglePredecessor()) {
// If Successor #0 has multiple preds, we may be able to conditionally
// execute Successor #1 if it branches to Successor #0.
Instruction *Succ1TI = BI->getSuccessor(1)->getTerminator();
if (Succ1TI->getNumSuccessors() == 1 &&
Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1), TTI))
return requestResimplify();
}
// If this is a branch on a phi node in the current block, thread control
// through this block if any PHI node entries are constants.
if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
if (PN->getParent() == BI->getParent())
if (FoldCondBranchOnPHI(BI, DTU, DL, Options.AC))
return requestResimplify();
// Scan predecessor blocks for conditional branches.
for (BasicBlock *Pred : predecessors(BB))
if (BranchInst *PBI = dyn_cast<BranchInst>(Pred->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (SimplifyCondBranchToCondBranch(PBI, BI, DTU, DL, TTI))
return requestResimplify();
// Look for diamond patterns.
if (MergeCondStores)
if (BasicBlock *PrevBB = allPredecessorsComeFromSameSource(BB))
if (BranchInst *PBI = dyn_cast<BranchInst>(PrevBB->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (mergeConditionalStores(PBI, BI, DTU, DL, TTI))
return requestResimplify();
return false;
}
/// Check if passing a value to an instruction will cause undefined behavior.
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValueMayBeModified) {
Constant *C = dyn_cast<Constant>(V);
if (!C)
return false;
if (I->use_empty())
return false;
if (C->isNullValue() || isa<UndefValue>(C)) {
// Only look at the first use, avoid hurting compile time with long uselists
User *Use = *I->user_begin();
// Now make sure that there are no instructions in between that can alter
// control flow (eg. calls)
for (BasicBlock::iterator
i = ++BasicBlock::iterator(I),
UI = BasicBlock::iterator(dyn_cast<Instruction>(Use));
i != UI; ++i)
if (i == I->getParent()->end() || i->mayHaveSideEffects())
return false;
// Look through GEPs. A load from a GEP derived from NULL is still undefined
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
if (GEP->getPointerOperand() == I) {
if (!GEP->isInBounds() || !GEP->hasAllZeroIndices())
PtrValueMayBeModified = true;
return passingValueIsAlwaysUndefined(V, GEP, PtrValueMayBeModified);
}
// Look through bitcasts.
if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
return passingValueIsAlwaysUndefined(V, BC, PtrValueMayBeModified);
// Load from null is undefined.
if (LoadInst *LI = dyn_cast<LoadInst>(Use))
if (!LI->isVolatile())
return !NullPointerIsDefined(LI->getFunction(),
LI->getPointerAddressSpace());
// Store to null is undefined.
if (StoreInst *SI = dyn_cast<StoreInst>(Use))
if (!SI->isVolatile())
return (!NullPointerIsDefined(SI->getFunction(),
SI->getPointerAddressSpace())) &&
SI->getPointerOperand() == I;
if (auto *CB = dyn_cast<CallBase>(Use)) {
if (C->isNullValue() && NullPointerIsDefined(CB->getFunction()))
return false;
// A call to null is undefined.
if (CB->getCalledOperand() == I)
return true;
if (C->isNullValue()) {
for (const llvm::Use &Arg : CB->args())
if (Arg == I) {
unsigned ArgIdx = CB->getArgOperandNo(&Arg);
if (CB->isPassingUndefUB(ArgIdx) &&
CB->paramHasAttr(ArgIdx, Attribute::NonNull)) {
// Passing null to a nonnnull+noundef argument is undefined.
return !PtrValueMayBeModified;
}
}
} else if (isa<UndefValue>(C)) {
// Passing undef to a noundef argument is undefined.
for (const llvm::Use &Arg : CB->args())
if (Arg == I) {
unsigned ArgIdx = CB->getArgOperandNo(&Arg);
if (CB->isPassingUndefUB(ArgIdx)) {
// Passing undef to a noundef argument is undefined.
return true;
}
}
}
}
}
return false;
}
/// If BB has an incoming value that will always trigger undefined behavior
/// (eg. null pointer dereference), remove the branch leading here.
static bool removeUndefIntroducingPredecessor(BasicBlock *BB,
DomTreeUpdater *DTU) {
for (PHINode &PHI : BB->phis())
for (unsigned i = 0, e = PHI.getNumIncomingValues(); i != e; ++i)
if (passingValueIsAlwaysUndefined(PHI.getIncomingValue(i), &PHI)) {
BasicBlock *Predecessor = PHI.getIncomingBlock(i);
Instruction *T = Predecessor->getTerminator();
IRBuilder<> Builder(T);
if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
BB->removePredecessor(Predecessor);
// Turn uncoditional branches into unreachables and remove the dead
// destination from conditional branches.
if (BI->isUnconditional())
Builder.CreateUnreachable();
else
Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
: BI->getSuccessor(0));
BI->eraseFromParent();
if (DTU)
DTU->applyUpdates({{DominatorTree::Delete, Predecessor, BB}});
return true;
}
// TODO: SwitchInst.
}
return false;
}
bool SimplifyCFGOpt::simplifyOnceImpl(BasicBlock *BB) {
bool Changed = false;
assert(BB && BB->getParent() && "Block not embedded in function!");
assert(BB->getTerminator() && "Degenerate basic block encountered!");
// Remove basic blocks that have no predecessors (except the entry block)...
// or that just have themself as a predecessor. These are unreachable.
if ((pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) ||
BB->getSinglePredecessor() == BB) {
LLVM_DEBUG(dbgs() << "Removing BB: \n" << *BB);
DeleteDeadBlock(BB, DTU);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB, /*DeleteDeadConditions=*/true,
/*TLI=*/nullptr, DTU);
// Check for and eliminate duplicate PHI nodes in this block.
Changed |= EliminateDuplicatePHINodes(BB);
// Check for and remove branches that will always cause undefined behavior.
Changed |= removeUndefIntroducingPredecessor(BB, DTU);
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
if (MergeBlockIntoPredecessor(BB, DTU))
return true;
if (SinkCommon && Options.SinkCommonInsts)
Changed |= SinkCommonCodeFromPredecessors(BB, DTU);
IRBuilder<> Builder(BB);
if (Options.FoldTwoEntryPHINode) {
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (auto *PN = dyn_cast<PHINode>(BB->begin()))
if (PN->getNumIncomingValues() == 2)
Changed |= FoldTwoEntryPHINode(PN, TTI, DTU, DL);
}
Instruction *Terminator = BB->getTerminator();
Builder.SetInsertPoint(Terminator);
switch (Terminator->getOpcode()) {
case Instruction::Br:
Changed |= simplifyBranch(cast<BranchInst>(Terminator), Builder);
break;
case Instruction::Ret:
Changed |= simplifyReturn(cast<ReturnInst>(Terminator), Builder);
break;
case Instruction::Resume:
Changed |= simplifyResume(cast<ResumeInst>(Terminator), Builder);
break;
case Instruction::CleanupRet:
Changed |= simplifyCleanupReturn(cast<CleanupReturnInst>(Terminator));
break;
case Instruction::Switch:
Changed |= simplifySwitch(cast<SwitchInst>(Terminator), Builder);
break;
case Instruction::Unreachable:
Changed |= simplifyUnreachable(cast<UnreachableInst>(Terminator));
break;
case Instruction::IndirectBr:
Changed |= simplifyIndirectBr(cast<IndirectBrInst>(Terminator));
break;
}
return Changed;
}
bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
bool Changed = simplifyOnceImpl(BB);
return Changed;
}
bool SimplifyCFGOpt::run(BasicBlock *BB) {
bool Changed = false;
// Repeated simplify BB as long as resimplification is requested.
do {
Resimplify = false;
// Perform one round of simplifcation. Resimplify flag will be set if
// another iteration is requested.
Changed |= simplifyOnce(BB);
} while (Resimplify);
return Changed;
}
bool llvm::simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
DomTreeUpdater *DTU, const SimplifyCFGOptions &Options,
ArrayRef<WeakVH> LoopHeaders) {
return SimplifyCFGOpt(TTI, DTU, BB->getModule()->getDataLayout(), LoopHeaders,
Options)
.run(BB);
}
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