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llvm-project/llvm/lib/Transforms/Scalar/EarlyCSE.cpp

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//===- EarlyCSE.cpp - Simple and fast CSE pass ----------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs a simple dominator tree walk that eliminates trivially
// redundant instructions.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/EarlyCSE.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/AtomicOrdering.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include <cassert>
#include <deque>
#include <memory>
#include <utility>
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "early-cse"
STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd");
STATISTIC(NumCSE, "Number of instructions CSE'd");
STATISTIC(NumCSECVP, "Number of compare instructions CVP'd");
STATISTIC(NumCSELoad, "Number of load instructions CSE'd");
STATISTIC(NumCSECall, "Number of call instructions CSE'd");
STATISTIC(NumDSE, "Number of trivial dead stores removed");
//===----------------------------------------------------------------------===//
// SimpleValue
//===----------------------------------------------------------------------===//
namespace {
/// \brief Struct representing the available values in the scoped hash table.
struct SimpleValue {
Instruction *Inst;
SimpleValue(Instruction *I) : Inst(I) {
assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
}
bool isSentinel() const {
return Inst == DenseMapInfo<Instruction *>::getEmptyKey() ||
Inst == DenseMapInfo<Instruction *>::getTombstoneKey();
}
static bool canHandle(Instruction *Inst) {
// This can only handle non-void readnone functions.
if (CallInst *CI = dyn_cast<CallInst>(Inst))
return CI->doesNotAccessMemory() && !CI->getType()->isVoidTy();
return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) ||
isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) ||
isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) ||
isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) ||
isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst);
}
};
} // end anonymous namespace
namespace llvm {
template <> struct DenseMapInfo<SimpleValue> {
static inline SimpleValue getEmptyKey() {
return DenseMapInfo<Instruction *>::getEmptyKey();
}
static inline SimpleValue getTombstoneKey() {
return DenseMapInfo<Instruction *>::getTombstoneKey();
}
static unsigned getHashValue(SimpleValue Val);
static bool isEqual(SimpleValue LHS, SimpleValue RHS);
};
} // end namespace llvm
unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
Instruction *Inst = Val.Inst;
// Hash in all of the operands as pointers.
if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(Inst)) {
Value *LHS = BinOp->getOperand(0);
Value *RHS = BinOp->getOperand(1);
if (BinOp->isCommutative() && BinOp->getOperand(0) > BinOp->getOperand(1))
std::swap(LHS, RHS);
return hash_combine(BinOp->getOpcode(), LHS, RHS);
}
if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) {
Value *LHS = CI->getOperand(0);
Value *RHS = CI->getOperand(1);
CmpInst::Predicate Pred = CI->getPredicate();
if (Inst->getOperand(0) > Inst->getOperand(1)) {
std::swap(LHS, RHS);
Pred = CI->getSwappedPredicate();
}
return hash_combine(Inst->getOpcode(), Pred, LHS, RHS);
}
// Hash min/max/abs (cmp + select) to allow for commuted operands.
// Min/max may also have non-canonical compare predicate (eg, the compare for
// smin may use 'sgt' rather than 'slt'), and non-canonical operands in the
// compare.
Value *A, *B;
SelectPatternFlavor SPF = matchSelectPattern(Inst, A, B).Flavor;
// TODO: We should also detect FP min/max.
if (SPF == SPF_SMIN || SPF == SPF_SMAX ||
SPF == SPF_UMIN || SPF == SPF_UMAX ||
SPF == SPF_ABS || SPF == SPF_NABS) {
if (A > B)
std::swap(A, B);
return hash_combine(Inst->getOpcode(), SPF, A, B);
}
if (CastInst *CI = dyn_cast<CastInst>(Inst))
return hash_combine(CI->getOpcode(), CI->getType(), CI->getOperand(0));
if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst))
return hash_combine(EVI->getOpcode(), EVI->getOperand(0),
hash_combine_range(EVI->idx_begin(), EVI->idx_end()));
if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst))
return hash_combine(IVI->getOpcode(), IVI->getOperand(0),
IVI->getOperand(1),
hash_combine_range(IVI->idx_begin(), IVI->idx_end()));
assert((isa<CallInst>(Inst) || isa<BinaryOperator>(Inst) ||
isa<GetElementPtrInst>(Inst) || isa<SelectInst>(Inst) ||
isa<ExtractElementInst>(Inst) || isa<InsertElementInst>(Inst) ||
isa<ShuffleVectorInst>(Inst)) &&
"Invalid/unknown instruction");
// Mix in the opcode.
return hash_combine(
Inst->getOpcode(),
hash_combine_range(Inst->value_op_begin(), Inst->value_op_end()));
}
bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) {
Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
if (LHS.isSentinel() || RHS.isSentinel())
return LHSI == RHSI;
if (LHSI->getOpcode() != RHSI->getOpcode())
return false;
if (LHSI->isIdenticalToWhenDefined(RHSI))
return true;
// If we're not strictly identical, we still might be a commutable instruction
if (BinaryOperator *LHSBinOp = dyn_cast<BinaryOperator>(LHSI)) {
if (!LHSBinOp->isCommutative())
return false;
assert(isa<BinaryOperator>(RHSI) &&
"same opcode, but different instruction type?");
BinaryOperator *RHSBinOp = cast<BinaryOperator>(RHSI);
// Commuted equality
return LHSBinOp->getOperand(0) == RHSBinOp->getOperand(1) &&
LHSBinOp->getOperand(1) == RHSBinOp->getOperand(0);
}
if (CmpInst *LHSCmp = dyn_cast<CmpInst>(LHSI)) {
assert(isa<CmpInst>(RHSI) &&
"same opcode, but different instruction type?");
CmpInst *RHSCmp = cast<CmpInst>(RHSI);
// Commuted equality
return LHSCmp->getOperand(0) == RHSCmp->getOperand(1) &&
LHSCmp->getOperand(1) == RHSCmp->getOperand(0) &&
LHSCmp->getSwappedPredicate() == RHSCmp->getPredicate();
}
// Min/max/abs can occur with commuted operands, non-canonical predicates,
// and/or non-canonical operands.
Value *LHSA, *LHSB;
SelectPatternFlavor LSPF = matchSelectPattern(LHSI, LHSA, LHSB).Flavor;
// TODO: We should also detect FP min/max.
if (LSPF == SPF_SMIN || LSPF == SPF_SMAX ||
LSPF == SPF_UMIN || LSPF == SPF_UMAX ||
LSPF == SPF_ABS || LSPF == SPF_NABS) {
Value *RHSA, *RHSB;
SelectPatternFlavor RSPF = matchSelectPattern(RHSI, RHSA, RHSB).Flavor;
return (LSPF == RSPF && ((LHSA == RHSA && LHSB == RHSB) ||
(LHSA == RHSB && LHSB == RHSA)));
}
return false;
}
//===----------------------------------------------------------------------===//
// CallValue
//===----------------------------------------------------------------------===//
namespace {
/// \brief Struct representing the available call values in the scoped hash
/// table.
struct CallValue {
Instruction *Inst;
CallValue(Instruction *I) : Inst(I) {
assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
}
bool isSentinel() const {
return Inst == DenseMapInfo<Instruction *>::getEmptyKey() ||
Inst == DenseMapInfo<Instruction *>::getTombstoneKey();
}
static bool canHandle(Instruction *Inst) {
// Don't value number anything that returns void.
if (Inst->getType()->isVoidTy())
return false;
CallInst *CI = dyn_cast<CallInst>(Inst);
if (!CI || !CI->onlyReadsMemory())
return false;
return true;
}
};
} // end anonymous namespace
namespace llvm {
template <> struct DenseMapInfo<CallValue> {
static inline CallValue getEmptyKey() {
return DenseMapInfo<Instruction *>::getEmptyKey();
}
static inline CallValue getTombstoneKey() {
return DenseMapInfo<Instruction *>::getTombstoneKey();
}
static unsigned getHashValue(CallValue Val);
static bool isEqual(CallValue LHS, CallValue RHS);
};
} // end namespace llvm
unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) {
Instruction *Inst = Val.Inst;
// Hash all of the operands as pointers and mix in the opcode.
return hash_combine(
Inst->getOpcode(),
hash_combine_range(Inst->value_op_begin(), Inst->value_op_end()));
}
bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) {
Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
if (LHS.isSentinel() || RHS.isSentinel())
return LHSI == RHSI;
return LHSI->isIdenticalTo(RHSI);
}
//===----------------------------------------------------------------------===//
// EarlyCSE implementation
//===----------------------------------------------------------------------===//
namespace {
/// \brief A simple and fast domtree-based CSE pass.
///
/// This pass does a simple depth-first walk over the dominator tree,
/// eliminating trivially redundant instructions and using instsimplify to
/// canonicalize things as it goes. It is intended to be fast and catch obvious
/// cases so that instcombine and other passes are more effective. It is
/// expected that a later pass of GVN will catch the interesting/hard cases.
class EarlyCSE {
public:
const TargetLibraryInfo &TLI;
const TargetTransformInfo &TTI;
DominatorTree &DT;
AssumptionCache &AC;
const SimplifyQuery SQ;
MemorySSA *MSSA;
std::unique_ptr<MemorySSAUpdater> MSSAUpdater;
using AllocatorTy =
RecyclingAllocator<BumpPtrAllocator,
ScopedHashTableVal<SimpleValue, Value *>>;
using ScopedHTType =
ScopedHashTable<SimpleValue, Value *, DenseMapInfo<SimpleValue>,
AllocatorTy>;
/// \brief A scoped hash table of the current values of all of our simple
/// scalar expressions.
///
/// As we walk down the domtree, we look to see if instructions are in this:
/// if so, we replace them with what we find, otherwise we insert them so
/// that dominated values can succeed in their lookup.
ScopedHTType AvailableValues;
/// A scoped hash table of the current values of previously encounted memory
/// locations.
///
/// This allows us to get efficient access to dominating loads or stores when
/// we have a fully redundant load. In addition to the most recent load, we
/// keep track of a generation count of the read, which is compared against
/// the current generation count. The current generation count is incremented
/// after every possibly writing memory operation, which ensures that we only
/// CSE loads with other loads that have no intervening store. Ordering
/// events (such as fences or atomic instructions) increment the generation
/// count as well; essentially, we model these as writes to all possible
/// locations. Note that atomic and/or volatile loads and stores can be
/// present the table; it is the responsibility of the consumer to inspect
/// the atomicity/volatility if needed.
struct LoadValue {
Instruction *DefInst = nullptr;
unsigned Generation = 0;
int MatchingId = -1;
bool IsAtomic = false;
bool IsInvariant = false;
LoadValue() = default;
LoadValue(Instruction *Inst, unsigned Generation, unsigned MatchingId,
bool IsAtomic, bool IsInvariant)
: DefInst(Inst), Generation(Generation), MatchingId(MatchingId),
IsAtomic(IsAtomic), IsInvariant(IsInvariant) {}
};
using LoadMapAllocator =
RecyclingAllocator<BumpPtrAllocator,
ScopedHashTableVal<Value *, LoadValue>>;
using LoadHTType =
ScopedHashTable<Value *, LoadValue, DenseMapInfo<Value *>,
LoadMapAllocator>;
LoadHTType AvailableLoads;
/// \brief A scoped hash table of the current values of read-only call
/// values.
///
/// It uses the same generation count as loads.
using CallHTType =
ScopedHashTable<CallValue, std::pair<Instruction *, unsigned>>;
CallHTType AvailableCalls;
/// \brief This is the current generation of the memory value.
unsigned CurrentGeneration = 0;
/// \brief Set up the EarlyCSE runner for a particular function.
EarlyCSE(const DataLayout &DL, const TargetLibraryInfo &TLI,
const TargetTransformInfo &TTI, DominatorTree &DT,
AssumptionCache &AC, MemorySSA *MSSA)
: TLI(TLI), TTI(TTI), DT(DT), AC(AC), SQ(DL, &TLI, &DT, &AC), MSSA(MSSA),
MSSAUpdater(llvm::make_unique<MemorySSAUpdater>(MSSA)) {}
bool run();
private:
// Almost a POD, but needs to call the constructors for the scoped hash
// tables so that a new scope gets pushed on. These are RAII so that the
// scope gets popped when the NodeScope is destroyed.
class NodeScope {
public:
NodeScope(ScopedHTType &AvailableValues, LoadHTType &AvailableLoads,
CallHTType &AvailableCalls)
: Scope(AvailableValues), LoadScope(AvailableLoads),
CallScope(AvailableCalls) {}
NodeScope(const NodeScope &) = delete;
NodeScope &operator=(const NodeScope &) = delete;
private:
ScopedHTType::ScopeTy Scope;
LoadHTType::ScopeTy LoadScope;
CallHTType::ScopeTy CallScope;
};
// Contains all the needed information to create a stack for doing a depth
// first traversal of the tree. This includes scopes for values, loads, and
// calls as well as the generation. There is a child iterator so that the
// children do not need to be store separately.
class StackNode {
public:
StackNode(ScopedHTType &AvailableValues, LoadHTType &AvailableLoads,
CallHTType &AvailableCalls, unsigned cg, DomTreeNode *n,
DomTreeNode::iterator child, DomTreeNode::iterator end)
: CurrentGeneration(cg), ChildGeneration(cg), Node(n), ChildIter(child),
EndIter(end), Scopes(AvailableValues, AvailableLoads, AvailableCalls)
{}
StackNode(const StackNode &) = delete;
StackNode &operator=(const StackNode &) = delete;
// Accessors.
unsigned currentGeneration() { return CurrentGeneration; }
unsigned childGeneration() { return ChildGeneration; }
void childGeneration(unsigned generation) { ChildGeneration = generation; }
DomTreeNode *node() { return Node; }
DomTreeNode::iterator childIter() { return ChildIter; }
DomTreeNode *nextChild() {
DomTreeNode *child = *ChildIter;
++ChildIter;
return child;
}
DomTreeNode::iterator end() { return EndIter; }
bool isProcessed() { return Processed; }
void process() { Processed = true; }
private:
unsigned CurrentGeneration;
unsigned ChildGeneration;
DomTreeNode *Node;
DomTreeNode::iterator ChildIter;
DomTreeNode::iterator EndIter;
NodeScope Scopes;
bool Processed = false;
};
/// \brief Wrapper class to handle memory instructions, including loads,
/// stores and intrinsic loads and stores defined by the target.
class ParseMemoryInst {
public:
ParseMemoryInst(Instruction *Inst, const TargetTransformInfo &TTI)
: Inst(Inst) {
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst))
if (TTI.getTgtMemIntrinsic(II, Info))
IsTargetMemInst = true;
}
bool isLoad() const {
if (IsTargetMemInst) return Info.ReadMem;
return isa<LoadInst>(Inst);
}
bool isStore() const {
if (IsTargetMemInst) return Info.WriteMem;
return isa<StoreInst>(Inst);
}
bool isAtomic() const {
if (IsTargetMemInst)
return Info.Ordering != AtomicOrdering::NotAtomic;
return Inst->isAtomic();
}
bool isUnordered() const {
if (IsTargetMemInst)
return Info.isUnordered();
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->isUnordered();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->isUnordered();
}
// Conservative answer
return !Inst->isAtomic();
}
bool isVolatile() const {
if (IsTargetMemInst)
return Info.IsVolatile;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->isVolatile();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->isVolatile();
}
// Conservative answer
return true;
}
bool isInvariantLoad() const {
if (auto *LI = dyn_cast<LoadInst>(Inst))
return LI->getMetadata(LLVMContext::MD_invariant_load) != nullptr;
return false;
}
bool isMatchingMemLoc(const ParseMemoryInst &Inst) const {
return (getPointerOperand() == Inst.getPointerOperand() &&
getMatchingId() == Inst.getMatchingId());
}
bool isValid() const { return getPointerOperand() != nullptr; }
// For regular (non-intrinsic) loads/stores, this is set to -1. For
// intrinsic loads/stores, the id is retrieved from the corresponding
// field in the MemIntrinsicInfo structure. That field contains
// non-negative values only.
int getMatchingId() const {
if (IsTargetMemInst) return Info.MatchingId;
return -1;
}
Value *getPointerOperand() const {
if (IsTargetMemInst) return Info.PtrVal;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->getPointerOperand();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->getPointerOperand();
}
return nullptr;
}
bool mayReadFromMemory() const {
if (IsTargetMemInst) return Info.ReadMem;
return Inst->mayReadFromMemory();
}
bool mayWriteToMemory() const {
if (IsTargetMemInst) return Info.WriteMem;
return Inst->mayWriteToMemory();
}
private:
bool IsTargetMemInst = false;
MemIntrinsicInfo Info;
Instruction *Inst;
};
bool processNode(DomTreeNode *Node);
Value *getOrCreateResult(Value *Inst, Type *ExpectedType) const {
if (auto *LI = dyn_cast<LoadInst>(Inst))
return LI;
if (auto *SI = dyn_cast<StoreInst>(Inst))
return SI->getValueOperand();
assert(isa<IntrinsicInst>(Inst) && "Instruction not supported");
return TTI.getOrCreateResultFromMemIntrinsic(cast<IntrinsicInst>(Inst),
ExpectedType);
}
bool isSameMemGeneration(unsigned EarlierGeneration, unsigned LaterGeneration,
Instruction *EarlierInst, Instruction *LaterInst);
void removeMSSA(Instruction *Inst) {
if (!MSSA)
return;
// Removing a store here can leave MemorySSA in an unoptimized state by
// creating MemoryPhis that have identical arguments and by creating
// MemoryUses whose defining access is not an actual clobber. We handle the
// phi case eagerly here. The non-optimized MemoryUse case is lazily
// updated by MemorySSA getClobberingMemoryAccess.
if (MemoryAccess *MA = MSSA->getMemoryAccess(Inst)) {
// Optimize MemoryPhi nodes that may become redundant by having all the
// same input values once MA is removed.
SmallSetVector<MemoryPhi *, 4> PhisToCheck;
SmallVector<MemoryAccess *, 8> WorkQueue;
WorkQueue.push_back(MA);
// Process MemoryPhi nodes in FIFO order using a ever-growing vector since
// we shouldn't be processing that many phis and this will avoid an
// allocation in almost all cases.
for (unsigned I = 0; I < WorkQueue.size(); ++I) {
MemoryAccess *WI = WorkQueue[I];
for (auto *U : WI->users())
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(U))
PhisToCheck.insert(MP);
MSSAUpdater->removeMemoryAccess(WI);
for (MemoryPhi *MP : PhisToCheck) {
MemoryAccess *FirstIn = MP->getIncomingValue(0);
if (llvm::all_of(MP->incoming_values(),
[=](Use &In) { return In == FirstIn; }))
WorkQueue.push_back(MP);
}
PhisToCheck.clear();
}
}
}
};
} // end anonymous namespace
/// Determine if the memory referenced by LaterInst is from the same heap
/// version as EarlierInst.
/// This is currently called in two scenarios:
///
/// load p
/// ...
/// load p
///
/// and
///
/// x = load p
/// ...
/// store x, p
///
/// in both cases we want to verify that there are no possible writes to the
/// memory referenced by p between the earlier and later instruction.
bool EarlyCSE::isSameMemGeneration(unsigned EarlierGeneration,
unsigned LaterGeneration,
Instruction *EarlierInst,
Instruction *LaterInst) {
// Check the simple memory generation tracking first.
if (EarlierGeneration == LaterGeneration)
return true;
if (!MSSA)
return false;
// If MemorySSA has determined that one of EarlierInst or LaterInst does not
// read/write memory, then we can safely return true here.
// FIXME: We could be more aggressive when checking doesNotAccessMemory(),
// onlyReadsMemory(), mayReadFromMemory(), and mayWriteToMemory() in this pass
// by also checking the MemorySSA MemoryAccess on the instruction. Initial
// experiments suggest this isn't worthwhile, at least for C/C++ code compiled
// with the default optimization pipeline.
auto *EarlierMA = MSSA->getMemoryAccess(EarlierInst);
if (!EarlierMA)
return true;
auto *LaterMA = MSSA->getMemoryAccess(LaterInst);
if (!LaterMA)
return true;
// Since we know LaterDef dominates LaterInst and EarlierInst dominates
// LaterInst, if LaterDef dominates EarlierInst then it can't occur between
// EarlierInst and LaterInst and neither can any other write that potentially
// clobbers LaterInst.
MemoryAccess *LaterDef =
MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
return MSSA->dominates(LaterDef, EarlierMA);
}
bool EarlyCSE::processNode(DomTreeNode *Node) {
bool Changed = false;
BasicBlock *BB = Node->getBlock();
// If this block has a single predecessor, then the predecessor is the parent
// of the domtree node and all of the live out memory values are still current
// in this block. If this block has multiple predecessors, then they could
// have invalidated the live-out memory values of our parent value. For now,
// just be conservative and invalidate memory if this block has multiple
// predecessors.
if (!BB->getSinglePredecessor())
++CurrentGeneration;
// If this node has a single predecessor which ends in a conditional branch,
// we can infer the value of the branch condition given that we took this
// path. We need the single predecessor to ensure there's not another path
// which reaches this block where the condition might hold a different
// value. Since we're adding this to the scoped hash table (like any other
// def), it will have been popped if we encounter a future merge block.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
auto *BI = dyn_cast<BranchInst>(Pred->getTerminator());
if (BI && BI->isConditional()) {
auto *CondInst = dyn_cast<Instruction>(BI->getCondition());
if (CondInst && SimpleValue::canHandle(CondInst)) {
assert(BI->getSuccessor(0) == BB || BI->getSuccessor(1) == BB);
auto *TorF = (BI->getSuccessor(0) == BB)
? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
AvailableValues.insert(CondInst, TorF);
DEBUG(dbgs() << "EarlyCSE CVP: Add conditional value for '"
<< CondInst->getName() << "' as " << *TorF << " in "
<< BB->getName() << "\n");
// Replace all dominated uses with the known value.
if (unsigned Count = replaceDominatedUsesWith(
CondInst, TorF, DT, BasicBlockEdge(Pred, BB))) {
Changed = true;
NumCSECVP += Count;
}
}
}
}
/// LastStore - Keep track of the last non-volatile store that we saw... for
/// as long as there in no instruction that reads memory. If we see a store
/// to the same location, we delete the dead store. This zaps trivial dead
/// stores which can occur in bitfield code among other things.
Instruction *LastStore = nullptr;
// See if any instructions in the block can be eliminated. If so, do it. If
// not, add them to AvailableValues.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
Instruction *Inst = &*I++;
// Dead instructions should just be removed.
if (isInstructionTriviallyDead(Inst, &TLI)) {
DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n');
salvageDebugInfo(*Inst);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumSimplify;
continue;
}
// Skip assume intrinsics, they don't really have side effects (although
// they're marked as such to ensure preservation of control dependencies),
// and this pass will not bother with its removal. However, we should mark
// its condition as true for all dominated blocks.
if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
auto *CondI =
dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0));
if (CondI && SimpleValue::canHandle(CondI)) {
DEBUG(dbgs() << "EarlyCSE considering assumption: " << *Inst << '\n');
AvailableValues.insert(CondI, ConstantInt::getTrue(BB->getContext()));
} else
DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
continue;
}
// Skip sideeffect intrinsics, for the same reason as assume intrinsics.
if (match(Inst, m_Intrinsic<Intrinsic::sideeffect>())) {
DEBUG(dbgs() << "EarlyCSE skipping sideeffect: " << *Inst << '\n');
continue;
}
// Skip invariant.start intrinsics since they only read memory, and we can
// forward values across it. Also, we dont need to consume the last store
// since the semantics of invariant.start allow us to perform DSE of the
// last store, if there was a store following invariant.start. Consider:
//
// store 30, i8* p
// invariant.start(p)
// store 40, i8* p
// We can DSE the store to 30, since the store 40 to invariant location p
// causes undefined behaviour.
if (match(Inst, m_Intrinsic<Intrinsic::invariant_start>()))
continue;
if (match(Inst, m_Intrinsic<Intrinsic::experimental_guard>())) {
if (auto *CondI =
dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0))) {
if (SimpleValue::canHandle(CondI)) {
// Do we already know the actual value of this condition?
if (auto *KnownCond = AvailableValues.lookup(CondI)) {
// Is the condition known to be true?
if (isa<ConstantInt>(KnownCond) &&
cast<ConstantInt>(KnownCond)->isOne()) {
DEBUG(dbgs() << "EarlyCSE removing guard: " << *Inst << '\n');
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
continue;
} else
// Use the known value if it wasn't true.
cast<CallInst>(Inst)->setArgOperand(0, KnownCond);
}
// The condition we're on guarding here is true for all dominated
// locations.
AvailableValues.insert(CondI, ConstantInt::getTrue(BB->getContext()));
}
}
// Guard intrinsics read all memory, but don't write any memory.
// Accordingly, don't update the generation but consume the last store (to
// avoid an incorrect DSE).
LastStore = nullptr;
continue;
}
// If the instruction can be simplified (e.g. X+0 = X) then replace it with
// its simpler value.
if (Value *V = SimplifyInstruction(Inst, SQ)) {
DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n');
bool Killed = false;
if (!Inst->use_empty()) {
Inst->replaceAllUsesWith(V);
Changed = true;
}
if (isInstructionTriviallyDead(Inst, &TLI)) {
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
Killed = true;
}
if (Changed)
++NumSimplify;
if (Killed)
continue;
}
// If this is a simple instruction that we can value number, process it.
if (SimpleValue::canHandle(Inst)) {
// See if the instruction has an available value. If so, use it.
if (Value *V = AvailableValues.lookup(Inst)) {
DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n');
if (auto *I = dyn_cast<Instruction>(V))
I->andIRFlags(Inst);
Inst->replaceAllUsesWith(V);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSE;
continue;
}
// Otherwise, just remember that this value is available.
AvailableValues.insert(Inst, Inst);
continue;
}
ParseMemoryInst MemInst(Inst, TTI);
// If this is a non-volatile load, process it.
if (MemInst.isValid() && MemInst.isLoad()) {
// (conservatively) we can't peak past the ordering implied by this
// operation, but we can add this load to our set of available values
if (MemInst.isVolatile() || !MemInst.isUnordered()) {
LastStore = nullptr;
++CurrentGeneration;
}
// If we have an available version of this load, and if it is the right
// generation or the load is known to be from an invariant location,
// replace this instruction.
//
// If either the dominating load or the current load are invariant, then
// we can assume the current load loads the same value as the dominating
// load.
LoadValue InVal = AvailableLoads.lookup(MemInst.getPointerOperand());
if (InVal.DefInst != nullptr &&
InVal.MatchingId == MemInst.getMatchingId() &&
// We don't yet handle removing loads with ordering of any kind.
!MemInst.isVolatile() && MemInst.isUnordered() &&
// We can't replace an atomic load with one which isn't also atomic.
InVal.IsAtomic >= MemInst.isAtomic() &&
(InVal.IsInvariant || MemInst.isInvariantLoad() ||
isSameMemGeneration(InVal.Generation, CurrentGeneration,
InVal.DefInst, Inst))) {
Value *Op = getOrCreateResult(InVal.DefInst, Inst->getType());
if (Op != nullptr) {
DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst
<< " to: " << *InVal.DefInst << '\n');
if (!Inst->use_empty())
Inst->replaceAllUsesWith(Op);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSELoad;
continue;
}
}
// Otherwise, remember that we have this instruction.
AvailableLoads.insert(
MemInst.getPointerOperand(),
LoadValue(Inst, CurrentGeneration, MemInst.getMatchingId(),
MemInst.isAtomic(), MemInst.isInvariantLoad()));
LastStore = nullptr;
continue;
}
// If this instruction may read from memory or throw (and potentially read
// from memory in the exception handler), forget LastStore. Load/store
// intrinsics will indicate both a read and a write to memory. The target
// may override this (e.g. so that a store intrinsic does not read from
// memory, and thus will be treated the same as a regular store for
// commoning purposes).
if ((Inst->mayReadFromMemory() || Inst->mayThrow()) &&
!(MemInst.isValid() && !MemInst.mayReadFromMemory()))
LastStore = nullptr;
// If this is a read-only call, process it.
if (CallValue::canHandle(Inst)) {
// If we have an available version of this call, and if it is the right
// generation, replace this instruction.
std::pair<Instruction *, unsigned> InVal = AvailableCalls.lookup(Inst);
if (InVal.first != nullptr &&
isSameMemGeneration(InVal.second, CurrentGeneration, InVal.first,
Inst)) {
DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst
<< " to: " << *InVal.first << '\n');
if (!Inst->use_empty())
Inst->replaceAllUsesWith(InVal.first);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSECall;
continue;
}
// Otherwise, remember that we have this instruction.
AvailableCalls.insert(
Inst, std::pair<Instruction *, unsigned>(Inst, CurrentGeneration));
continue;
}
// A release fence requires that all stores complete before it, but does
// not prevent the reordering of following loads 'before' the fence. As a
// result, we don't need to consider it as writing to memory and don't need
// to advance the generation. We do need to prevent DSE across the fence,
// but that's handled above.
if (FenceInst *FI = dyn_cast<FenceInst>(Inst))
if (FI->getOrdering() == AtomicOrdering::Release) {
assert(Inst->mayReadFromMemory() && "relied on to prevent DSE above");
continue;
}
// write back DSE - If we write back the same value we just loaded from
// the same location and haven't passed any intervening writes or ordering
// operations, we can remove the write. The primary benefit is in allowing
// the available load table to remain valid and value forward past where
// the store originally was.
if (MemInst.isValid() && MemInst.isStore()) {
LoadValue InVal = AvailableLoads.lookup(MemInst.getPointerOperand());
if (InVal.DefInst &&
InVal.DefInst == getOrCreateResult(Inst, InVal.DefInst->getType()) &&
InVal.MatchingId == MemInst.getMatchingId() &&
// We don't yet handle removing stores with ordering of any kind.
!MemInst.isVolatile() && MemInst.isUnordered() &&
isSameMemGeneration(InVal.Generation, CurrentGeneration,
InVal.DefInst, Inst)) {
// It is okay to have a LastStore to a different pointer here if MemorySSA
// tells us that the load and store are from the same memory generation.
// In that case, LastStore should keep its present value since we're
// removing the current store.
assert((!LastStore ||
ParseMemoryInst(LastStore, TTI).getPointerOperand() ==
MemInst.getPointerOperand() ||
MSSA) &&
"can't have an intervening store if not using MemorySSA!");
DEBUG(dbgs() << "EarlyCSE DSE (writeback): " << *Inst << '\n');
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumDSE;
// We can avoid incrementing the generation count since we were able
// to eliminate this store.
continue;
}
}
// Okay, this isn't something we can CSE at all. Check to see if it is
// something that could modify memory. If so, our available memory values
// cannot be used so bump the generation count.
if (Inst->mayWriteToMemory()) {
++CurrentGeneration;
if (MemInst.isValid() && MemInst.isStore()) {
// We do a trivial form of DSE if there are two stores to the same
// location with no intervening loads. Delete the earlier store.
// At the moment, we don't remove ordered stores, but do remove
// unordered atomic stores. There's no special requirement (for
// unordered atomics) about removing atomic stores only in favor of
// other atomic stores since we we're going to execute the non-atomic
// one anyway and the atomic one might never have become visible.
if (LastStore) {
ParseMemoryInst LastStoreMemInst(LastStore, TTI);
assert(LastStoreMemInst.isUnordered() &&
!LastStoreMemInst.isVolatile() &&
"Violated invariant");
if (LastStoreMemInst.isMatchingMemLoc(MemInst)) {
DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore
<< " due to: " << *Inst << '\n');
removeMSSA(LastStore);
LastStore->eraseFromParent();
Changed = true;
++NumDSE;
LastStore = nullptr;
}
// fallthrough - we can exploit information about this store
}
// Okay, we just invalidated anything we knew about loaded values. Try
// to salvage *something* by remembering that the stored value is a live
// version of the pointer. It is safe to forward from volatile stores
// to non-volatile loads, so we don't have to check for volatility of
// the store.
AvailableLoads.insert(
MemInst.getPointerOperand(),
LoadValue(Inst, CurrentGeneration, MemInst.getMatchingId(),
MemInst.isAtomic(), /*IsInvariant=*/false));
// Remember that this was the last unordered store we saw for DSE. We
// don't yet handle DSE on ordered or volatile stores since we don't
// have a good way to model the ordering requirement for following
// passes once the store is removed. We could insert a fence, but
// since fences are slightly stronger than stores in their ordering,
// it's not clear this is a profitable transform. Another option would
// be to merge the ordering with that of the post dominating store.
if (MemInst.isUnordered() && !MemInst.isVolatile())
LastStore = Inst;
else
LastStore = nullptr;
}
}
}
return Changed;
}
bool EarlyCSE::run() {
// Note, deque is being used here because there is significant performance
// gains over vector when the container becomes very large due to the
// specific access patterns. For more information see the mailing list
// discussion on this:
// http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20120116/135228.html
std::deque<StackNode *> nodesToProcess;
bool Changed = false;
// Process the root node.
nodesToProcess.push_back(new StackNode(
AvailableValues, AvailableLoads, AvailableCalls, CurrentGeneration,
DT.getRootNode(), DT.getRootNode()->begin(), DT.getRootNode()->end()));
// Save the current generation.
unsigned LiveOutGeneration = CurrentGeneration;
// Process the stack.
while (!nodesToProcess.empty()) {
// Grab the first item off the stack. Set the current generation, remove
// the node from the stack, and process it.
StackNode *NodeToProcess = nodesToProcess.back();
// Initialize class members.
CurrentGeneration = NodeToProcess->currentGeneration();
// Check if the node needs to be processed.
if (!NodeToProcess->isProcessed()) {
// Process the node.
Changed |= processNode(NodeToProcess->node());
NodeToProcess->childGeneration(CurrentGeneration);
NodeToProcess->process();
} else if (NodeToProcess->childIter() != NodeToProcess->end()) {
// Push the next child onto the stack.
DomTreeNode *child = NodeToProcess->nextChild();
nodesToProcess.push_back(
new StackNode(AvailableValues, AvailableLoads, AvailableCalls,
NodeToProcess->childGeneration(), child, child->begin(),
child->end()));
} else {
// It has been processed, and there are no more children to process,
// so delete it and pop it off the stack.
delete NodeToProcess;
nodesToProcess.pop_back();
}
} // while (!nodes...)
// Reset the current generation.
CurrentGeneration = LiveOutGeneration;
return Changed;
}
PreservedAnalyses EarlyCSEPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &AC = AM.getResult<AssumptionAnalysis>(F);
auto *MSSA =
UseMemorySSA ? &AM.getResult<MemorySSAAnalysis>(F).getMSSA() : nullptr;
EarlyCSE CSE(F.getParent()->getDataLayout(), TLI, TTI, DT, AC, MSSA);
if (!CSE.run())
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
if (UseMemorySSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
namespace {
/// \brief A simple and fast domtree-based CSE pass.
///
/// This pass does a simple depth-first walk over the dominator tree,
/// eliminating trivially redundant instructions and using instsimplify to
/// canonicalize things as it goes. It is intended to be fast and catch obvious
/// cases so that instcombine and other passes are more effective. It is
/// expected that a later pass of GVN will catch the interesting/hard cases.
template<bool UseMemorySSA>
class EarlyCSELegacyCommonPass : public FunctionPass {
public:
static char ID;
EarlyCSELegacyCommonPass() : FunctionPass(ID) {
if (UseMemorySSA)
initializeEarlyCSEMemSSALegacyPassPass(*PassRegistry::getPassRegistry());
else
initializeEarlyCSELegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto *MSSA =
UseMemorySSA ? &getAnalysis<MemorySSAWrapperPass>().getMSSA() : nullptr;
EarlyCSE CSE(F.getParent()->getDataLayout(), TLI, TTI, DT, AC, MSSA);
return CSE.run();
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
if (UseMemorySSA) {
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
}
AU.addPreserved<GlobalsAAWrapperPass>();
AU.setPreservesCFG();
}
};
} // end anonymous namespace
using EarlyCSELegacyPass = EarlyCSELegacyCommonPass</*UseMemorySSA=*/false>;
template<>
char EarlyCSELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(EarlyCSELegacyPass, "early-cse", "Early CSE", false,
false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(EarlyCSELegacyPass, "early-cse", "Early CSE", false, false)
using EarlyCSEMemSSALegacyPass =
EarlyCSELegacyCommonPass</*UseMemorySSA=*/true>;
template<>
char EarlyCSEMemSSALegacyPass::ID = 0;
FunctionPass *llvm::createEarlyCSEPass(bool UseMemorySSA) {
if (UseMemorySSA)
return new EarlyCSEMemSSALegacyPass();
else
return new EarlyCSELegacyPass();
}
INITIALIZE_PASS_BEGIN(EarlyCSEMemSSALegacyPass, "early-cse-memssa",
"Early CSE w/ MemorySSA", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_END(EarlyCSEMemSSALegacyPass, "early-cse-memssa",
"Early CSE w/ MemorySSA", false, false)
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