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

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//===- GVN.cpp - Eliminate redundant values and loads ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs global value numbering to eliminate fully redundant
// instructions. It also performs simple dead load elimination.
//
// Note that this pass does the value numbering itself; it does not use the
// ValueNumbering analysis passes.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/PHITransAddr.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.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 <vector>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "gvn"
STATISTIC(NumGVNInstr, "Number of instructions deleted");
STATISTIC(NumGVNLoad, "Number of loads deleted");
STATISTIC(NumGVNPRE, "Number of instructions PRE'd");
STATISTIC(NumGVNBlocks, "Number of blocks merged");
STATISTIC(NumGVNSimpl, "Number of instructions simplified");
STATISTIC(NumGVNEqProp, "Number of equalities propagated");
STATISTIC(NumPRELoad, "Number of loads PRE'd");
static cl::opt<bool> EnablePRE("enable-pre",
cl::init(true), cl::Hidden);
static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));
// Maximum allowed recursion depth.
static cl::opt<uint32_t>
MaxRecurseDepth("max-recurse-depth", cl::Hidden, cl::init(1000), cl::ZeroOrMore,
cl::desc("Max recurse depth (default = 1000)"));
//===----------------------------------------------------------------------===//
// ValueTable Class
//===----------------------------------------------------------------------===//
/// This class holds the mapping between values and value numbers. It is used
/// as an efficient mechanism to determine the expression-wise equivalence of
/// two values.
namespace {
struct Expression {
uint32_t opcode;
Type *type;
SmallVector<uint32_t, 4> varargs;
Expression(uint32_t o = ~2U) : opcode(o) { }
bool operator==(const Expression &other) const {
if (opcode != other.opcode)
return false;
if (opcode == ~0U || opcode == ~1U)
return true;
if (type != other.type)
return false;
if (varargs != other.varargs)
return false;
return true;
}
friend hash_code hash_value(const Expression &Value) {
return hash_combine(Value.opcode, Value.type,
hash_combine_range(Value.varargs.begin(),
Value.varargs.end()));
}
};
class ValueTable {
DenseMap<Value*, uint32_t> valueNumbering;
DenseMap<Expression, uint32_t> expressionNumbering;
AliasAnalysis *AA;
MemoryDependenceAnalysis *MD;
DominatorTree *DT;
uint32_t nextValueNumber;
Expression create_expression(Instruction* I);
Expression create_cmp_expression(unsigned Opcode,
CmpInst::Predicate Predicate,
Value *LHS, Value *RHS);
Expression create_extractvalue_expression(ExtractValueInst* EI);
uint32_t lookup_or_add_call(CallInst* C);
public:
ValueTable() : nextValueNumber(1) { }
uint32_t lookup_or_add(Value *V);
uint32_t lookup(Value *V) const;
uint32_t lookup_or_add_cmp(unsigned Opcode, CmpInst::Predicate Pred,
Value *LHS, Value *RHS);
bool exists(Value *V) const;
void add(Value *V, uint32_t num);
void clear();
void erase(Value *v);
void setAliasAnalysis(AliasAnalysis* A) { AA = A; }
AliasAnalysis *getAliasAnalysis() const { return AA; }
void setMemDep(MemoryDependenceAnalysis* M) { MD = M; }
void setDomTree(DominatorTree* D) { DT = D; }
uint32_t getNextUnusedValueNumber() { return nextValueNumber; }
void verifyRemoved(const Value *) const;
};
}
namespace llvm {
template <> struct DenseMapInfo<Expression> {
static inline Expression getEmptyKey() {
return ~0U;
}
static inline Expression getTombstoneKey() {
return ~1U;
}
static unsigned getHashValue(const Expression e) {
using llvm::hash_value;
return static_cast<unsigned>(hash_value(e));
}
static bool isEqual(const Expression &LHS, const Expression &RHS) {
return LHS == RHS;
}
};
}
//===----------------------------------------------------------------------===//
// ValueTable Internal Functions
//===----------------------------------------------------------------------===//
Expression ValueTable::create_expression(Instruction *I) {
Expression e;
e.type = I->getType();
e.opcode = I->getOpcode();
for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
OI != OE; ++OI)
e.varargs.push_back(lookup_or_add(*OI));
if (I->isCommutative()) {
// Ensure that commutative instructions that only differ by a permutation
// of their operands get the same value number by sorting the operand value
// numbers. Since all commutative instructions have two operands it is more
// efficient to sort by hand rather than using, say, std::sort.
assert(I->getNumOperands() == 2 && "Unsupported commutative instruction!");
if (e.varargs[0] > e.varargs[1])
std::swap(e.varargs[0], e.varargs[1]);
}
if (CmpInst *C = dyn_cast<CmpInst>(I)) {
// Sort the operand value numbers so x<y and y>x get the same value number.
CmpInst::Predicate Predicate = C->getPredicate();
if (e.varargs[0] > e.varargs[1]) {
std::swap(e.varargs[0], e.varargs[1]);
Predicate = CmpInst::getSwappedPredicate(Predicate);
}
e.opcode = (C->getOpcode() << 8) | Predicate;
} else if (InsertValueInst *E = dyn_cast<InsertValueInst>(I)) {
for (InsertValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
II != IE; ++II)
e.varargs.push_back(*II);
}
return e;
}
Expression ValueTable::create_cmp_expression(unsigned Opcode,
CmpInst::Predicate Predicate,
Value *LHS, Value *RHS) {
assert((Opcode == Instruction::ICmp || Opcode == Instruction::FCmp) &&
"Not a comparison!");
Expression e;
e.type = CmpInst::makeCmpResultType(LHS->getType());
e.varargs.push_back(lookup_or_add(LHS));
e.varargs.push_back(lookup_or_add(RHS));
// Sort the operand value numbers so x<y and y>x get the same value number.
if (e.varargs[0] > e.varargs[1]) {
std::swap(e.varargs[0], e.varargs[1]);
Predicate = CmpInst::getSwappedPredicate(Predicate);
}
e.opcode = (Opcode << 8) | Predicate;
return e;
}
Expression ValueTable::create_extractvalue_expression(ExtractValueInst *EI) {
assert(EI && "Not an ExtractValueInst?");
Expression e;
e.type = EI->getType();
e.opcode = 0;
IntrinsicInst *I = dyn_cast<IntrinsicInst>(EI->getAggregateOperand());
if (I != nullptr && EI->getNumIndices() == 1 && *EI->idx_begin() == 0 ) {
// EI might be an extract from one of our recognised intrinsics. If it
// is we'll synthesize a semantically equivalent expression instead on
// an extract value expression.
switch (I->getIntrinsicID()) {
case Intrinsic::sadd_with_overflow:
case Intrinsic::uadd_with_overflow:
e.opcode = Instruction::Add;
break;
case Intrinsic::ssub_with_overflow:
case Intrinsic::usub_with_overflow:
e.opcode = Instruction::Sub;
break;
case Intrinsic::smul_with_overflow:
case Intrinsic::umul_with_overflow:
e.opcode = Instruction::Mul;
break;
default:
break;
}
if (e.opcode != 0) {
// Intrinsic recognized. Grab its args to finish building the expression.
assert(I->getNumArgOperands() == 2 &&
"Expect two args for recognised intrinsics.");
e.varargs.push_back(lookup_or_add(I->getArgOperand(0)));
e.varargs.push_back(lookup_or_add(I->getArgOperand(1)));
return e;
}
}
// Not a recognised intrinsic. Fall back to producing an extract value
// expression.
e.opcode = EI->getOpcode();
for (Instruction::op_iterator OI = EI->op_begin(), OE = EI->op_end();
OI != OE; ++OI)
e.varargs.push_back(lookup_or_add(*OI));
for (ExtractValueInst::idx_iterator II = EI->idx_begin(), IE = EI->idx_end();
II != IE; ++II)
e.varargs.push_back(*II);
return e;
}
//===----------------------------------------------------------------------===//
// ValueTable External Functions
//===----------------------------------------------------------------------===//
/// add - Insert a value into the table with a specified value number.
void ValueTable::add(Value *V, uint32_t num) {
valueNumbering.insert(std::make_pair(V, num));
}
uint32_t ValueTable::lookup_or_add_call(CallInst *C) {
if (AA->doesNotAccessMemory(C)) {
Expression exp = create_expression(C);
uint32_t &e = expressionNumbering[exp];
if (!e) e = nextValueNumber++;
valueNumbering[C] = e;
return e;
} else if (AA->onlyReadsMemory(C)) {
Expression exp = create_expression(C);
uint32_t &e = expressionNumbering[exp];
if (!e) {
e = nextValueNumber++;
valueNumbering[C] = e;
return e;
}
if (!MD) {
e = nextValueNumber++;
valueNumbering[C] = e;
return e;
}
MemDepResult local_dep = MD->getDependency(C);
if (!local_dep.isDef() && !local_dep.isNonLocal()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
if (local_dep.isDef()) {
CallInst* local_cdep = cast<CallInst>(local_dep.getInst());
if (local_cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookup_or_add(C->getArgOperand(i));
uint32_t cd_vn = lookup_or_add(local_cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
uint32_t v = lookup_or_add(local_cdep);
valueNumbering[C] = v;
return v;
}
// Non-local case.
const MemoryDependenceAnalysis::NonLocalDepInfo &deps =
MD->getNonLocalCallDependency(CallSite(C));
// FIXME: Move the checking logic to MemDep!
CallInst* cdep = nullptr;
// Check to see if we have a single dominating call instruction that is
// identical to C.
for (unsigned i = 0, e = deps.size(); i != e; ++i) {
const NonLocalDepEntry *I = &deps[i];
if (I->getResult().isNonLocal())
continue;
// We don't handle non-definitions. If we already have a call, reject
// instruction dependencies.
if (!I->getResult().isDef() || cdep != nullptr) {
cdep = nullptr;
break;
}
CallInst *NonLocalDepCall = dyn_cast<CallInst>(I->getResult().getInst());
// FIXME: All duplicated with non-local case.
if (NonLocalDepCall && DT->properlyDominates(I->getBB(), C->getParent())){
cdep = NonLocalDepCall;
continue;
}
cdep = nullptr;
break;
}
if (!cdep) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
if (cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookup_or_add(C->getArgOperand(i));
uint32_t cd_vn = lookup_or_add(cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
uint32_t v = lookup_or_add(cdep);
valueNumbering[C] = v;
return v;
} else {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
/// Returns true if a value number exists for the specified value.
bool ValueTable::exists(Value *V) const { return valueNumbering.count(V) != 0; }
/// lookup_or_add - Returns the value number for the specified value, assigning
/// it a new number if it did not have one before.
uint32_t ValueTable::lookup_or_add(Value *V) {
DenseMap<Value*, uint32_t>::iterator VI = valueNumbering.find(V);
if (VI != valueNumbering.end())
return VI->second;
if (!isa<Instruction>(V)) {
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
}
Instruction* I = cast<Instruction>(V);
Expression exp;
switch (I->getOpcode()) {
case Instruction::Call:
return lookup_or_add_call(cast<CallInst>(I));
case Instruction::Add:
case Instruction::FAdd:
case Instruction::Sub:
case Instruction::FSub:
case Instruction::Mul:
case Instruction::FMul:
case Instruction::UDiv:
case Instruction::SDiv:
case Instruction::FDiv:
case Instruction::URem:
case Instruction::SRem:
case Instruction::FRem:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::ICmp:
case Instruction::FCmp:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::UIToFP:
case Instruction::SIToFP:
case Instruction::FPTrunc:
case Instruction::FPExt:
case Instruction::PtrToInt:
case Instruction::IntToPtr:
case Instruction::BitCast:
case Instruction::Select:
case Instruction::ExtractElement:
case Instruction::InsertElement:
case Instruction::ShuffleVector:
case Instruction::InsertValue:
case Instruction::GetElementPtr:
exp = create_expression(I);
break;
case Instruction::ExtractValue:
exp = create_extractvalue_expression(cast<ExtractValueInst>(I));
break;
default:
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
}
uint32_t& e = expressionNumbering[exp];
if (!e) e = nextValueNumber++;
valueNumbering[V] = e;
return e;
}
/// Returns the value number of the specified value. Fails if
/// the value has not yet been numbered.
uint32_t ValueTable::lookup(Value *V) const {
DenseMap<Value*, uint32_t>::const_iterator VI = valueNumbering.find(V);
assert(VI != valueNumbering.end() && "Value not numbered?");
return VI->second;
}
/// Returns the value number of the given comparison,
/// assigning it a new number if it did not have one before. Useful when
/// we deduced the result of a comparison, but don't immediately have an
/// instruction realizing that comparison to hand.
uint32_t ValueTable::lookup_or_add_cmp(unsigned Opcode,
CmpInst::Predicate Predicate,
Value *LHS, Value *RHS) {
Expression exp = create_cmp_expression(Opcode, Predicate, LHS, RHS);
uint32_t& e = expressionNumbering[exp];
if (!e) e = nextValueNumber++;
return e;
}
/// Remove all entries from the ValueTable.
void ValueTable::clear() {
valueNumbering.clear();
expressionNumbering.clear();
nextValueNumber = 1;
}
/// Remove a value from the value numbering.
void ValueTable::erase(Value *V) {
valueNumbering.erase(V);
}
/// verifyRemoved - Verify that the value is removed from all internal data
/// structures.
void ValueTable::verifyRemoved(const Value *V) const {
for (DenseMap<Value*, uint32_t>::const_iterator
I = valueNumbering.begin(), E = valueNumbering.end(); I != E; ++I) {
assert(I->first != V && "Inst still occurs in value numbering map!");
}
}
//===----------------------------------------------------------------------===//
// GVN Pass
//===----------------------------------------------------------------------===//
namespace {
class GVN;
/// Represents a particular available value that we know how to materialize.
/// Materialization of an AvailableValue never fails. An AvailableValue is
/// implicitly associated with a rematerialization point which is the
/// location of the instruction from which it was formed.
struct AvailableValue {
enum ValType {
SimpleVal, // A simple offsetted value that is accessed.
LoadVal, // A value produced by a load.
MemIntrin, // A memory intrinsic which is loaded from.
UndefVal // A UndefValue representing a value from dead block (which
// is not yet physically removed from the CFG).
};
/// V - The value that is live out of the block.
PointerIntPair<Value *, 2, ValType> Val;
/// Offset - The byte offset in Val that is interesting for the load query.
unsigned Offset;
static AvailableValue get(Value *V,
unsigned Offset = 0) {
AvailableValue Res;
Res.Val.setPointer(V);
Res.Val.setInt(SimpleVal);
Res.Offset = Offset;
return Res;
}
static AvailableValue getMI(MemIntrinsic *MI,
unsigned Offset = 0) {
AvailableValue Res;
Res.Val.setPointer(MI);
Res.Val.setInt(MemIntrin);
Res.Offset = Offset;
return Res;
}
static AvailableValue getLoad(LoadInst *LI,
unsigned Offset = 0) {
AvailableValue Res;
Res.Val.setPointer(LI);
Res.Val.setInt(LoadVal);
Res.Offset = Offset;
return Res;
}
static AvailableValue getUndef() {
AvailableValue Res;
Res.Val.setPointer(nullptr);
Res.Val.setInt(UndefVal);
Res.Offset = 0;
return Res;
}
bool isSimpleValue() const { return Val.getInt() == SimpleVal; }
bool isCoercedLoadValue() const { return Val.getInt() == LoadVal; }
bool isMemIntrinValue() const { return Val.getInt() == MemIntrin; }
bool isUndefValue() const { return Val.getInt() == UndefVal; }
Value *getSimpleValue() const {
assert(isSimpleValue() && "Wrong accessor");
return Val.getPointer();
}
LoadInst *getCoercedLoadValue() const {
assert(isCoercedLoadValue() && "Wrong accessor");
return cast<LoadInst>(Val.getPointer());
}
MemIntrinsic *getMemIntrinValue() const {
assert(isMemIntrinValue() && "Wrong accessor");
return cast<MemIntrinsic>(Val.getPointer());
}
/// Emit code at the specified insertion point to adjust the value defined
/// here to the specified type. This handles various coercion cases.
Value *MaterializeAdjustedValue(LoadInst *LI, Instruction *InsertPt,
GVN &gvn) const;
};
/// Represents an AvailableValue which can be rematerialized at the end of
/// the associated BasicBlock.
struct AvailableValueInBlock {
/// BB - The basic block in question.
BasicBlock *BB;
/// AV - The actual available value
AvailableValue AV;
static AvailableValueInBlock get(BasicBlock *BB, AvailableValue &&AV) {
AvailableValueInBlock Res;
Res.BB = BB;
Res.AV = std::move(AV);
return Res;
}
static AvailableValueInBlock get(BasicBlock *BB, Value *V,
unsigned Offset = 0) {
return get(BB, AvailableValue::get(V, Offset));
}
static AvailableValueInBlock getMI(BasicBlock *BB, MemIntrinsic *MI,
unsigned Offset = 0) {
return get(BB, AvailableValue::getMI(MI, Offset));
}
static AvailableValueInBlock getLoad(BasicBlock *BB, LoadInst *LI,
unsigned Offset = 0) {
return get(BB, AvailableValue::getLoad(LI, Offset));
}
static AvailableValueInBlock getUndef(BasicBlock *BB) {
return get(BB, AvailableValue::getUndef());
}
/// Emit code at the end of this block to adjust the value defined here to
/// the specified type. This handles various coercion cases.
Value *MaterializeAdjustedValue(LoadInst *LI, GVN &gvn) const {
return AV.MaterializeAdjustedValue(LI, BB->getTerminator(), gvn);
}
};
class GVN : public FunctionPass {
bool NoLoads;
MemoryDependenceAnalysis *MD;
DominatorTree *DT;
const TargetLibraryInfo *TLI;
AssumptionCache *AC;
SetVector<BasicBlock *> DeadBlocks;
ValueTable VN;
/// A mapping from value numbers to lists of Value*'s that
/// have that value number. Use findLeader to query it.
struct LeaderTableEntry {
Value *Val;
const BasicBlock *BB;
LeaderTableEntry *Next;
};
DenseMap<uint32_t, LeaderTableEntry> LeaderTable;
BumpPtrAllocator TableAllocator;
// Block-local map of equivalent values to their leader, does not
// propagate to any successors. Entries added mid-block are applied
// to the remaining instructions in the block.
SmallMapVector<llvm::Value *, llvm::Constant *, 4> ReplaceWithConstMap;
SmallVector<Instruction*, 8> InstrsToErase;
typedef SmallVector<NonLocalDepResult, 64> LoadDepVect;
typedef SmallVector<AvailableValueInBlock, 64> AvailValInBlkVect;
typedef SmallVector<BasicBlock*, 64> UnavailBlkVect;
public:
static char ID; // Pass identification, replacement for typeid
explicit GVN(bool noloads = false)
: FunctionPass(ID), NoLoads(noloads), MD(nullptr) {
initializeGVNPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
/// This removes the specified instruction from
/// our various maps and marks it for deletion.
void markInstructionForDeletion(Instruction *I) {
VN.erase(I);
InstrsToErase.push_back(I);
}
DominatorTree &getDominatorTree() const { return *DT; }
AliasAnalysis *getAliasAnalysis() const { return VN.getAliasAnalysis(); }
MemoryDependenceAnalysis &getMemDep() const { return *MD; }
private:
/// Push a new Value to the LeaderTable onto the list for its value number.
void addToLeaderTable(uint32_t N, Value *V, const BasicBlock *BB) {
LeaderTableEntry &Curr = LeaderTable[N];
if (!Curr.Val) {
Curr.Val = V;
Curr.BB = BB;
return;
}
LeaderTableEntry *Node = TableAllocator.Allocate<LeaderTableEntry>();
Node->Val = V;
Node->BB = BB;
Node->Next = Curr.Next;
Curr.Next = Node;
}
/// Scan the list of values corresponding to a given
/// value number, and remove the given instruction if encountered.
void removeFromLeaderTable(uint32_t N, Instruction *I, BasicBlock *BB) {
LeaderTableEntry* Prev = nullptr;
LeaderTableEntry* Curr = &LeaderTable[N];
while (Curr && (Curr->Val != I || Curr->BB != BB)) {
Prev = Curr;
Curr = Curr->Next;
}
if (!Curr)
return;
if (Prev) {
Prev->Next = Curr->Next;
} else {
if (!Curr->Next) {
Curr->Val = nullptr;
Curr->BB = nullptr;
} else {
LeaderTableEntry* Next = Curr->Next;
Curr->Val = Next->Val;
Curr->BB = Next->BB;
Curr->Next = Next->Next;
}
}
}
// List of critical edges to be split between iterations.
SmallVector<std::pair<TerminatorInst*, unsigned>, 4> toSplit;
// This transformation requires dominator postdominator info
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
if (!NoLoads)
AU.addRequired<MemoryDependenceAnalysis>();
AU.addRequired<AAResultsWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
}
// Helper functions of redundant load elimination
bool processLoad(LoadInst *L);
bool processNonLocalLoad(LoadInst *L);
bool processAssumeIntrinsic(IntrinsicInst *II);
void AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks);
bool PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks);
// Other helper routines
bool processInstruction(Instruction *I);
bool processBlock(BasicBlock *BB);
void dump(DenseMap<uint32_t, Value*> &d);
bool iterateOnFunction(Function &F);
bool performPRE(Function &F);
bool performScalarPRE(Instruction *I);
bool performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
unsigned int ValNo);
Value *findLeader(const BasicBlock *BB, uint32_t num);
void cleanupGlobalSets();
void verifyRemoved(const Instruction *I) const;
bool splitCriticalEdges();
BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);
bool replaceOperandsWithConsts(Instruction *I) const;
bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root,
bool DominatesByEdge);
bool processFoldableCondBr(BranchInst *BI);
void addDeadBlock(BasicBlock *BB);
void assignValNumForDeadCode();
};
char GVN::ID = 0;
}
// The public interface to this file...
FunctionPass *llvm::createGVNPass(bool NoLoads) {
return new GVN(NoLoads);
}
INITIALIZE_PASS_BEGIN(GVN, "gvn", "Global Value Numbering", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
INITIALIZE_PASS_END(GVN, "gvn", "Global Value Numbering", false, false)
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void GVN::dump(DenseMap<uint32_t, Value*>& d) {
errs() << "{\n";
for (DenseMap<uint32_t, Value*>::iterator I = d.begin(),
E = d.end(); I != E; ++I) {
errs() << I->first << "\n";
I->second->dump();
}
errs() << "}\n";
}
#endif
/// Return true if we can prove that the value
/// we're analyzing is fully available in the specified block. As we go, keep
/// track of which blocks we know are fully alive in FullyAvailableBlocks. This
/// map is actually a tri-state map with the following values:
/// 0) we know the block *is not* fully available.
/// 1) we know the block *is* fully available.
/// 2) we do not know whether the block is fully available or not, but we are
/// currently speculating that it will be.
/// 3) we are speculating for this block and have used that to speculate for
/// other blocks.
static bool IsValueFullyAvailableInBlock(BasicBlock *BB,
DenseMap<BasicBlock*, char> &FullyAvailableBlocks,
uint32_t RecurseDepth) {
if (RecurseDepth > MaxRecurseDepth)
return false;
// Optimistically assume that the block is fully available and check to see
// if we already know about this block in one lookup.
std::pair<DenseMap<BasicBlock*, char>::iterator, char> IV =
FullyAvailableBlocks.insert(std::make_pair(BB, 2));
// If the entry already existed for this block, return the precomputed value.
if (!IV.second) {
// If this is a speculative "available" value, mark it as being used for
// speculation of other blocks.
if (IV.first->second == 2)
IV.first->second = 3;
return IV.first->second != 0;
}
// Otherwise, see if it is fully available in all predecessors.
pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
// If this block has no predecessors, it isn't live-in here.
if (PI == PE)
goto SpeculationFailure;
for (; PI != PE; ++PI)
// If the value isn't fully available in one of our predecessors, then it
// isn't fully available in this block either. Undo our previous
// optimistic assumption and bail out.
if (!IsValueFullyAvailableInBlock(*PI, FullyAvailableBlocks,RecurseDepth+1))
goto SpeculationFailure;
return true;
// If we get here, we found out that this is not, after
// all, a fully-available block. We have a problem if we speculated on this and
// used the speculation to mark other blocks as available.
SpeculationFailure:
char &BBVal = FullyAvailableBlocks[BB];
// If we didn't speculate on this, just return with it set to false.
if (BBVal == 2) {
BBVal = 0;
return false;
}
// If we did speculate on this value, we could have blocks set to 1 that are
// incorrect. Walk the (transitive) successors of this block and mark them as
// 0 if set to one.
SmallVector<BasicBlock*, 32> BBWorklist;
BBWorklist.push_back(BB);
do {
BasicBlock *Entry = BBWorklist.pop_back_val();
// Note that this sets blocks to 0 (unavailable) if they happen to not
// already be in FullyAvailableBlocks. This is safe.
char &EntryVal = FullyAvailableBlocks[Entry];
if (EntryVal == 0) continue; // Already unavailable.
// Mark as unavailable.
EntryVal = 0;
BBWorklist.append(succ_begin(Entry), succ_end(Entry));
} while (!BBWorklist.empty());
return false;
}
/// Return true if CoerceAvailableValueToLoadType will succeed.
static bool CanCoerceMustAliasedValueToLoad(Value *StoredVal,
Type *LoadTy,
const DataLayout &DL) {
// If the loaded or stored value is an first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy() ||
StoredVal->getType()->isStructTy() ||
StoredVal->getType()->isArrayTy())
return false;
// The store has to be at least as big as the load.
if (DL.getTypeSizeInBits(StoredVal->getType()) <
DL.getTypeSizeInBits(LoadTy))
return false;
return true;
}
/// If we saw a store of a value to memory, and
/// then a load from a must-aliased pointer of a different type, try to coerce
/// the stored value. LoadedTy is the type of the load we want to replace.
/// IRB is IRBuilder used to insert new instructions.
///
/// If we can't do it, return null.
static Value *CoerceAvailableValueToLoadType(Value *StoredVal, Type *LoadedTy,
IRBuilder<> &IRB,
const DataLayout &DL) {
if (!CanCoerceMustAliasedValueToLoad(StoredVal, LoadedTy, DL))
return nullptr;
// If this is already the right type, just return it.
Type *StoredValTy = StoredVal->getType();
uint64_t StoreSize = DL.getTypeSizeInBits(StoredValTy);
uint64_t LoadSize = DL.getTypeSizeInBits(LoadedTy);
// If the store and reload are the same size, we can always reuse it.
if (StoreSize == LoadSize) {
// Pointer to Pointer -> use bitcast.
if (StoredValTy->getScalarType()->isPointerTy() &&
LoadedTy->getScalarType()->isPointerTy())
return IRB.CreateBitCast(StoredVal, LoadedTy);
// Convert source pointers to integers, which can be bitcast.
if (StoredValTy->getScalarType()->isPointerTy()) {
StoredValTy = DL.getIntPtrType(StoredValTy);
StoredVal = IRB.CreatePtrToInt(StoredVal, StoredValTy);
}
Type *TypeToCastTo = LoadedTy;
if (TypeToCastTo->getScalarType()->isPointerTy())
TypeToCastTo = DL.getIntPtrType(TypeToCastTo);
if (StoredValTy != TypeToCastTo)
StoredVal = IRB.CreateBitCast(StoredVal, TypeToCastTo);
// Cast to pointer if the load needs a pointer type.
if (LoadedTy->getScalarType()->isPointerTy())
StoredVal = IRB.CreateIntToPtr(StoredVal, LoadedTy);
return StoredVal;
}
// If the loaded value is smaller than the available value, then we can
// extract out a piece from it. If the available value is too small, then we
// can't do anything.
assert(StoreSize >= LoadSize && "CanCoerceMustAliasedValueToLoad fail");
// Convert source pointers to integers, which can be manipulated.
if (StoredValTy->getScalarType()->isPointerTy()) {
StoredValTy = DL.getIntPtrType(StoredValTy);
StoredVal = IRB.CreatePtrToInt(StoredVal, StoredValTy);
}
// Convert vectors and fp to integer, which can be manipulated.
if (!StoredValTy->isIntegerTy()) {
StoredValTy = IntegerType::get(StoredValTy->getContext(), StoreSize);
StoredVal = IRB.CreateBitCast(StoredVal, StoredValTy);
}
// If this is a big-endian system, we need to shift the value down to the low
// bits so that a truncate will work.
if (DL.isBigEndian()) {
StoredVal = IRB.CreateLShr(StoredVal, StoreSize - LoadSize, "tmp");
}
// Truncate the integer to the right size now.
Type *NewIntTy = IntegerType::get(StoredValTy->getContext(), LoadSize);
StoredVal = IRB.CreateTrunc(StoredVal, NewIntTy, "trunc");
if (LoadedTy == NewIntTy)
return StoredVal;
// If the result is a pointer, inttoptr.
if (LoadedTy->getScalarType()->isPointerTy())
return IRB.CreateIntToPtr(StoredVal, LoadedTy, "inttoptr");
// Otherwise, bitcast.
return IRB.CreateBitCast(StoredVal, LoadedTy, "bitcast");
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering memory write (store,
/// memset, memcpy, memmove). This means that the write *may* provide bits used
/// by the load but we can't be sure because the pointers don't mustalias.
///
/// Check this case to see if there is anything more we can do before we give
/// up. This returns -1 if we have to give up, or a byte number in the stored
/// value of the piece that feeds the load.
static int AnalyzeLoadFromClobberingWrite(Type *LoadTy, Value *LoadPtr,
Value *WritePtr,
uint64_t WriteSizeInBits,
const DataLayout &DL) {
// If the loaded or stored value is a first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy())
return -1;
int64_t StoreOffset = 0, LoadOffset = 0;
Value *StoreBase =
GetPointerBaseWithConstantOffset(WritePtr, StoreOffset, DL);
Value *LoadBase = GetPointerBaseWithConstantOffset(LoadPtr, LoadOffset, DL);
if (StoreBase != LoadBase)
return -1;
// If the load and store are to the exact same address, they should have been
// a must alias. AA must have gotten confused.
// FIXME: Study to see if/when this happens. One case is forwarding a memset
// to a load from the base of the memset.
#if 0
if (LoadOffset == StoreOffset) {
dbgs() << "STORE/LOAD DEP WITH COMMON POINTER MISSED:\n"
<< "Base = " << *StoreBase << "\n"
<< "Store Ptr = " << *WritePtr << "\n"
<< "Store Offs = " << StoreOffset << "\n"
<< "Load Ptr = " << *LoadPtr << "\n";
abort();
}
#endif
// If the load and store don't overlap at all, the store doesn't provide
// anything to the load. In this case, they really don't alias at all, AA
// must have gotten confused.
uint64_t LoadSize = DL.getTypeSizeInBits(LoadTy);
if ((WriteSizeInBits & 7) | (LoadSize & 7))
return -1;
uint64_t StoreSize = WriteSizeInBits >> 3; // Convert to bytes.
LoadSize >>= 3;
bool isAAFailure = false;
if (StoreOffset < LoadOffset)
isAAFailure = StoreOffset+int64_t(StoreSize) <= LoadOffset;
else
isAAFailure = LoadOffset+int64_t(LoadSize) <= StoreOffset;
if (isAAFailure) {
#if 0
dbgs() << "STORE LOAD DEP WITH COMMON BASE:\n"
<< "Base = " << *StoreBase << "\n"
<< "Store Ptr = " << *WritePtr << "\n"
<< "Store Offs = " << StoreOffset << "\n"
<< "Load Ptr = " << *LoadPtr << "\n";
abort();
#endif
return -1;
}
// If the Load isn't completely contained within the stored bits, we don't
// have all the bits to feed it. We could do something crazy in the future
// (issue a smaller load then merge the bits in) but this seems unlikely to be
// valuable.
if (StoreOffset > LoadOffset ||
StoreOffset+StoreSize < LoadOffset+LoadSize)
return -1;
// Okay, we can do this transformation. Return the number of bytes into the
// store that the load is.
return LoadOffset-StoreOffset;
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering store.
static int AnalyzeLoadFromClobberingStore(Type *LoadTy, Value *LoadPtr,
StoreInst *DepSI) {
// Cannot handle reading from store of first-class aggregate yet.
if (DepSI->getValueOperand()->getType()->isStructTy() ||
DepSI->getValueOperand()->getType()->isArrayTy())
return -1;
const DataLayout &DL = DepSI->getModule()->getDataLayout();
Value *StorePtr = DepSI->getPointerOperand();
uint64_t StoreSize =DL.getTypeSizeInBits(DepSI->getValueOperand()->getType());
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr,
StorePtr, StoreSize, DL);
}
/// This function is called when we have a
/// memdep query of a load that ends up being clobbered by another load. See if
/// the other load can feed into the second load.
static int AnalyzeLoadFromClobberingLoad(Type *LoadTy, Value *LoadPtr,
LoadInst *DepLI, const DataLayout &DL){
// Cannot handle reading from store of first-class aggregate yet.
if (DepLI->getType()->isStructTy() || DepLI->getType()->isArrayTy())
return -1;
Value *DepPtr = DepLI->getPointerOperand();
uint64_t DepSize = DL.getTypeSizeInBits(DepLI->getType());
int R = AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr, DepPtr, DepSize, DL);
if (R != -1) return R;
// If we have a load/load clobber an DepLI can be widened to cover this load,
// then we should widen it!
int64_t LoadOffs = 0;
const Value *LoadBase =
GetPointerBaseWithConstantOffset(LoadPtr, LoadOffs, DL);
unsigned LoadSize = DL.getTypeStoreSize(LoadTy);
unsigned Size = MemoryDependenceAnalysis::getLoadLoadClobberFullWidthSize(
LoadBase, LoadOffs, LoadSize, DepLI);
if (Size == 0) return -1;
// Check non-obvious conditions enforced by MDA which we rely on for being
// able to materialize this potentially available value
assert(DepLI->isSimple() && "Cannot widen volatile/atomic load!");
assert(DepLI->getType()->isIntegerTy() && "Can't widen non-integer load");
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr, DepPtr, Size*8, DL);
}
static int AnalyzeLoadFromClobberingMemInst(Type *LoadTy, Value *LoadPtr,
MemIntrinsic *MI,
const DataLayout &DL) {
// If the mem operation is a non-constant size, we can't handle it.
ConstantInt *SizeCst = dyn_cast<ConstantInt>(MI->getLength());
if (!SizeCst) return -1;
uint64_t MemSizeInBits = SizeCst->getZExtValue()*8;
// If this is memset, we just need to see if the offset is valid in the size
// of the memset..
if (MI->getIntrinsicID() == Intrinsic::memset)
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr, MI->getDest(),
MemSizeInBits, DL);
// If we have a memcpy/memmove, the only case we can handle is if this is a
// copy from constant memory. In that case, we can read directly from the
// constant memory.
MemTransferInst *MTI = cast<MemTransferInst>(MI);
Constant *Src = dyn_cast<Constant>(MTI->getSource());
if (!Src) return -1;
GlobalVariable *GV = dyn_cast<GlobalVariable>(GetUnderlyingObject(Src, DL));
if (!GV || !GV->isConstant()) return -1;
// See if the access is within the bounds of the transfer.
int Offset = AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr,
MI->getDest(), MemSizeInBits, DL);
if (Offset == -1)
return Offset;
unsigned AS = Src->getType()->getPointerAddressSpace();
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src = ConstantExpr::getBitCast(Src,
Type::getInt8PtrTy(Src->getContext(), AS));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Type::getInt8Ty(Src->getContext()), Src,
OffsetCst);
Src = ConstantExpr::getBitCast(Src, PointerType::get(LoadTy, AS));
if (ConstantFoldLoadFromConstPtr(Src, LoadTy, DL))
return Offset;
return -1;
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering store. This means
/// that the store provides bits used by the load but we the pointers don't
/// mustalias. Check this case to see if there is anything more we can do
/// before we give up.
static Value *GetStoreValueForLoad(Value *SrcVal, unsigned Offset,
Type *LoadTy,
Instruction *InsertPt, const DataLayout &DL){
LLVMContext &Ctx = SrcVal->getType()->getContext();
uint64_t StoreSize = (DL.getTypeSizeInBits(SrcVal->getType()) + 7) / 8;
uint64_t LoadSize = (DL.getTypeSizeInBits(LoadTy) + 7) / 8;
IRBuilder<> Builder(InsertPt);
// Compute which bits of the stored value are being used by the load. Convert
// to an integer type to start with.
if (SrcVal->getType()->getScalarType()->isPointerTy())
SrcVal = Builder.CreatePtrToInt(SrcVal,
DL.getIntPtrType(SrcVal->getType()));
if (!SrcVal->getType()->isIntegerTy())
SrcVal = Builder.CreateBitCast(SrcVal, IntegerType::get(Ctx, StoreSize*8));
// Shift the bits to the least significant depending on endianness.
unsigned ShiftAmt;
if (DL.isLittleEndian())
ShiftAmt = Offset*8;
else
ShiftAmt = (StoreSize-LoadSize-Offset)*8;
if (ShiftAmt)
SrcVal = Builder.CreateLShr(SrcVal, ShiftAmt);
if (LoadSize != StoreSize)
SrcVal = Builder.CreateTrunc(SrcVal, IntegerType::get(Ctx, LoadSize*8));
return CoerceAvailableValueToLoadType(SrcVal, LoadTy, Builder, DL);
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering load. This means
/// that the load *may* provide bits used by the load but we can't be sure
/// because the pointers don't mustalias. Check this case to see if there is
/// anything more we can do before we give up.
static Value *GetLoadValueForLoad(LoadInst *SrcVal, unsigned Offset,
Type *LoadTy, Instruction *InsertPt,
GVN &gvn) {
const DataLayout &DL = SrcVal->getModule()->getDataLayout();
// If Offset+LoadTy exceeds the size of SrcVal, then we must be wanting to
// widen SrcVal out to a larger load.
unsigned SrcValSize = DL.getTypeStoreSize(SrcVal->getType());
unsigned LoadSize = DL.getTypeStoreSize(LoadTy);
if (Offset+LoadSize > SrcValSize) {
assert(SrcVal->isSimple() && "Cannot widen volatile/atomic load!");
assert(SrcVal->getType()->isIntegerTy() && "Can't widen non-integer load");
// If we have a load/load clobber an DepLI can be widened to cover this
// load, then we should widen it to the next power of 2 size big enough!
unsigned NewLoadSize = Offset+LoadSize;
if (!isPowerOf2_32(NewLoadSize))
NewLoadSize = NextPowerOf2(NewLoadSize);
Value *PtrVal = SrcVal->getPointerOperand();
// Insert the new load after the old load. This ensures that subsequent
// memdep queries will find the new load. We can't easily remove the old
// load completely because it is already in the value numbering table.
IRBuilder<> Builder(SrcVal->getParent(), ++BasicBlock::iterator(SrcVal));
Type *DestPTy =
IntegerType::get(LoadTy->getContext(), NewLoadSize*8);
DestPTy = PointerType::get(DestPTy,
PtrVal->getType()->getPointerAddressSpace());
Builder.SetCurrentDebugLocation(SrcVal->getDebugLoc());
PtrVal = Builder.CreateBitCast(PtrVal, DestPTy);
LoadInst *NewLoad = Builder.CreateLoad(PtrVal);
NewLoad->takeName(SrcVal);
NewLoad->setAlignment(SrcVal->getAlignment());
DEBUG(dbgs() << "GVN WIDENED LOAD: " << *SrcVal << "\n");
DEBUG(dbgs() << "TO: " << *NewLoad << "\n");
// Replace uses of the original load with the wider load. On a big endian
// system, we need to shift down to get the relevant bits.
Value *RV = NewLoad;
if (DL.isBigEndian())
RV = Builder.CreateLShr(RV,
NewLoadSize*8-SrcVal->getType()->getPrimitiveSizeInBits());
RV = Builder.CreateTrunc(RV, SrcVal->getType());
SrcVal->replaceAllUsesWith(RV);
// We would like to use gvn.markInstructionForDeletion here, but we can't
// because the load is already memoized into the leader map table that GVN
// tracks. It is potentially possible to remove the load from the table,
// but then there all of the operations based on it would need to be
// rehashed. Just leave the dead load around.
gvn.getMemDep().removeInstruction(SrcVal);
SrcVal = NewLoad;
}
return GetStoreValueForLoad(SrcVal, Offset, LoadTy, InsertPt, DL);
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering mem intrinsic.
static Value *GetMemInstValueForLoad(MemIntrinsic *SrcInst, unsigned Offset,
Type *LoadTy, Instruction *InsertPt,
const DataLayout &DL){
LLVMContext &Ctx = LoadTy->getContext();
uint64_t LoadSize = DL.getTypeSizeInBits(LoadTy)/8;
IRBuilder<> Builder(InsertPt);
// We know that this method is only called when the mem transfer fully
// provides the bits for the load.
if (MemSetInst *MSI = dyn_cast<MemSetInst>(SrcInst)) {
// memset(P, 'x', 1234) -> splat('x'), even if x is a variable, and
// independently of what the offset is.
Value *Val = MSI->getValue();
if (LoadSize != 1)
Val = Builder.CreateZExt(Val, IntegerType::get(Ctx, LoadSize*8));
Value *OneElt = Val;
// Splat the value out to the right number of bits.
for (unsigned NumBytesSet = 1; NumBytesSet != LoadSize; ) {
// If we can double the number of bytes set, do it.
if (NumBytesSet*2 <= LoadSize) {
Value *ShVal = Builder.CreateShl(Val, NumBytesSet*8);
Val = Builder.CreateOr(Val, ShVal);
NumBytesSet <<= 1;
continue;
}
// Otherwise insert one byte at a time.
Value *ShVal = Builder.CreateShl(Val, 1*8);
Val = Builder.CreateOr(OneElt, ShVal);
++NumBytesSet;
}
return CoerceAvailableValueToLoadType(Val, LoadTy, Builder, DL);
}
// Otherwise, this is a memcpy/memmove from a constant global.
MemTransferInst *MTI = cast<MemTransferInst>(SrcInst);
Constant *Src = cast<Constant>(MTI->getSource());
unsigned AS = Src->getType()->getPointerAddressSpace();
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src = ConstantExpr::getBitCast(Src,
Type::getInt8PtrTy(Src->getContext(), AS));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Type::getInt8Ty(Src->getContext()), Src,
OffsetCst);
Src = ConstantExpr::getBitCast(Src, PointerType::get(LoadTy, AS));
return ConstantFoldLoadFromConstPtr(Src, LoadTy, DL);
}
/// Given a set of loads specified by ValuesPerBlock,
/// construct SSA form, allowing us to eliminate LI. This returns the value
/// that should be used at LI's definition site.
static Value *ConstructSSAForLoadSet(LoadInst *LI,
SmallVectorImpl<AvailableValueInBlock> &ValuesPerBlock,
GVN &gvn) {
// Check for the fully redundant, dominating load case. In this case, we can
// just use the dominating value directly.
if (ValuesPerBlock.size() == 1 &&
gvn.getDominatorTree().properlyDominates(ValuesPerBlock[0].BB,
LI->getParent())) {
assert(!ValuesPerBlock[0].AV.isUndefValue() &&
"Dead BB dominate this block");
return ValuesPerBlock[0].MaterializeAdjustedValue(LI, gvn);
}
// Otherwise, we have to construct SSA form.
SmallVector<PHINode*, 8> NewPHIs;
SSAUpdater SSAUpdate(&NewPHIs);
SSAUpdate.Initialize(LI->getType(), LI->getName());
for (const AvailableValueInBlock &AV : ValuesPerBlock) {
BasicBlock *BB = AV.BB;
if (SSAUpdate.HasValueForBlock(BB))
continue;
SSAUpdate.AddAvailableValue(BB, AV.MaterializeAdjustedValue(LI, gvn));
}
// Perform PHI construction.
return SSAUpdate.GetValueInMiddleOfBlock(LI->getParent());
}
Value *AvailableValue::MaterializeAdjustedValue(LoadInst *LI,
Instruction *InsertPt,
GVN &gvn) const {
Value *Res;
Type *LoadTy = LI->getType();
const DataLayout &DL = LI->getModule()->getDataLayout();
if (isSimpleValue()) {
Res = getSimpleValue();
if (Res->getType() != LoadTy) {
Res = GetStoreValueForLoad(Res, Offset, LoadTy, InsertPt, DL);
DEBUG(dbgs() << "GVN COERCED NONLOCAL VAL:\nOffset: " << Offset << " "
<< *getSimpleValue() << '\n'
<< *Res << '\n' << "\n\n\n");
}
} else if (isCoercedLoadValue()) {
LoadInst *Load = getCoercedLoadValue();
if (Load->getType() == LoadTy && Offset == 0) {
Res = Load;
} else {
Res = GetLoadValueForLoad(Load, Offset, LoadTy, InsertPt, gvn);
DEBUG(dbgs() << "GVN COERCED NONLOCAL LOAD:\nOffset: " << Offset << " "
<< *getCoercedLoadValue() << '\n'
<< *Res << '\n' << "\n\n\n");
}
} else if (isMemIntrinValue()) {
Res = GetMemInstValueForLoad(getMemIntrinValue(), Offset, LoadTy,
InsertPt, DL);
DEBUG(dbgs() << "GVN COERCED NONLOCAL MEM INTRIN:\nOffset: " << Offset
<< " " << *getMemIntrinValue() << '\n'
<< *Res << '\n' << "\n\n\n");
} else {
assert(isUndefValue() && "Should be UndefVal");
DEBUG(dbgs() << "GVN COERCED NONLOCAL Undef:\n";);
return UndefValue::get(LoadTy);
}
assert(Res && "failed to materialize?");
return Res;
}
static bool isLifetimeStart(const Instruction *Inst) {
if (const IntrinsicInst* II = dyn_cast<IntrinsicInst>(Inst))
return II->getIntrinsicID() == Intrinsic::lifetime_start;
return false;
}
void GVN::AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks) {
// Filter out useless results (non-locals, etc). Keep track of the blocks
// where we have a value available in repl, also keep track of whether we see
// dependencies that produce an unknown value for the load (such as a call
// that could potentially clobber the load).
unsigned NumDeps = Deps.size();
const DataLayout &DL = LI->getModule()->getDataLayout();
for (unsigned i = 0, e = NumDeps; i != e; ++i) {
BasicBlock *DepBB = Deps[i].getBB();
MemDepResult DepInfo = Deps[i].getResult();
if (DeadBlocks.count(DepBB)) {
// Dead dependent mem-op disguise as a load evaluating the same value
// as the load in question.
ValuesPerBlock.push_back(AvailableValueInBlock::getUndef(DepBB));
continue;
}
if (!DepInfo.isDef() && !DepInfo.isClobber()) {
UnavailableBlocks.push_back(DepBB);
continue;
}
if (DepInfo.isClobber()) {
// The address being loaded in this non-local block may not be the same as
// the pointer operand of the load if PHI translation occurs. Make sure
// to consider the right address.
Value *Address = Deps[i].getAddress();
// If the dependence is to a store that writes to a superset of the bits
// read by the load, we can extract the bits we need for the load from the
// stored value.
if (StoreInst *DepSI = dyn_cast<StoreInst>(DepInfo.getInst())) {
if (Address) {
int Offset =
AnalyzeLoadFromClobberingStore(LI->getType(), Address, DepSI);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
DepSI->getValueOperand(),
Offset));
continue;
}
}
}
// Check to see if we have something like this:
// load i32* P
// load i8* (P+1)
// if we have this, replace the later with an extraction from the former.
if (LoadInst *DepLI = dyn_cast<LoadInst>(DepInfo.getInst())) {
// If this is a clobber and L is the first instruction in its block, then
// we have the first instruction in the entry block.
if (DepLI != LI && Address) {
int Offset =
AnalyzeLoadFromClobberingLoad(LI->getType(), Address, DepLI, DL);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::getLoad(DepBB,DepLI,
Offset));
continue;
}
}
}
// If the clobbering value is a memset/memcpy/memmove, see if we can
// forward a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(DepInfo.getInst())) {
if (Address) {
int Offset = AnalyzeLoadFromClobberingMemInst(LI->getType(), Address,
DepMI, DL);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::getMI(DepBB, DepMI,
Offset));
continue;
}
}
}
UnavailableBlocks.push_back(DepBB);
continue;
}
// DepInfo.isDef() here
Instruction *DepInst = DepInfo.getInst();
// Loading the allocation -> undef.
if (isa<AllocaInst>(DepInst) || isMallocLikeFn(DepInst, TLI) ||
// Loading immediately after lifetime begin -> undef.
isLifetimeStart(DepInst)) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
UndefValue::get(LI->getType())));
continue;
}
// Loading from calloc (which zero initializes memory) -> zero
if (isCallocLikeFn(DepInst, TLI)) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(
DepBB, Constant::getNullValue(LI->getType())));
continue;
}
if (StoreInst *S = dyn_cast<StoreInst>(DepInst)) {
// Reject loads and stores that are to the same address but are of
// different types if we have to.
if (S->getValueOperand()->getType() != LI->getType()) {
// If the stored value is larger or equal to the loaded value, we can
// reuse it.
if (!CanCoerceMustAliasedValueToLoad(S->getValueOperand(),
LI->getType(), DL)) {
UnavailableBlocks.push_back(DepBB);
continue;
}
}
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
S->getValueOperand()));
continue;
}
if (LoadInst *LD = dyn_cast<LoadInst>(DepInst)) {
// If the types mismatch and we can't handle it, reject reuse of the load.
if (LD->getType() != LI->getType()) {
// If the stored value is larger or equal to the loaded value, we can
// reuse it.
if (!CanCoerceMustAliasedValueToLoad(LD, LI->getType(), DL)) {
UnavailableBlocks.push_back(DepBB);
continue;
}
}
ValuesPerBlock.push_back(AvailableValueInBlock::getLoad(DepBB, LD));
continue;
}
UnavailableBlocks.push_back(DepBB);
}
}
bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks) {
// Okay, we have *some* definitions of the value. This means that the value
// is available in some of our (transitive) predecessors. Lets think about
// doing PRE of this load. This will involve inserting a new load into the
// predecessor when it's not available. We could do this in general, but
// prefer to not increase code size. As such, we only do this when we know
// that we only have to insert *one* load (which means we're basically moving
// the load, not inserting a new one).
SmallPtrSet<BasicBlock *, 4> Blockers(UnavailableBlocks.begin(),
UnavailableBlocks.end());
// Let's find the first basic block with more than one predecessor. Walk
// backwards through predecessors if needed.
BasicBlock *LoadBB = LI->getParent();
BasicBlock *TmpBB = LoadBB;
while (TmpBB->getSinglePredecessor()) {
TmpBB = TmpBB->getSinglePredecessor();
if (TmpBB == LoadBB) // Infinite (unreachable) loop.
return false;
if (Blockers.count(TmpBB))
return false;
// If any of these blocks has more than one successor (i.e. if the edge we
// just traversed was critical), then there are other paths through this
// block along which the load may not be anticipated. Hoisting the load
// above this block would be adding the load to execution paths along
// which it was not previously executed.
if (TmpBB->getTerminator()->getNumSuccessors() != 1)
return false;
}
assert(TmpBB);
LoadBB = TmpBB;
// Check to see how many predecessors have the loaded value fully
// available.
MapVector<BasicBlock *, Value *> PredLoads;
DenseMap<BasicBlock*, char> FullyAvailableBlocks;
for (const AvailableValueInBlock &AV : ValuesPerBlock)
FullyAvailableBlocks[AV.BB] = true;
for (BasicBlock *UnavailableBB : UnavailableBlocks)
FullyAvailableBlocks[UnavailableBB] = false;
SmallVector<BasicBlock *, 4> CriticalEdgePred;
for (BasicBlock *Pred : predecessors(LoadBB)) {
// If any predecessor block is an EH pad that does not allow non-PHI
// instructions before the terminator, we can't PRE the load.
if (Pred->getTerminator()->isEHPad()) {
DEBUG(dbgs()
<< "COULD NOT PRE LOAD BECAUSE OF AN EH PAD PREDECESSOR '"
<< Pred->getName() << "': " << *LI << '\n');
return false;
}
if (IsValueFullyAvailableInBlock(Pred, FullyAvailableBlocks, 0)) {
continue;
}
if (Pred->getTerminator()->getNumSuccessors() != 1) {
if (isa<IndirectBrInst>(Pred->getTerminator())) {
DEBUG(dbgs() << "COULD NOT PRE LOAD BECAUSE OF INDBR CRITICAL EDGE '"
<< Pred->getName() << "': " << *LI << '\n');
return false;
}
if (LoadBB->isEHPad()) {
DEBUG(dbgs()
<< "COULD NOT PRE LOAD BECAUSE OF AN EH PAD CRITICAL EDGE '"
<< Pred->getName() << "': " << *LI << '\n');
return false;
}
CriticalEdgePred.push_back(Pred);
} else {
// Only add the predecessors that will not be split for now.
PredLoads[Pred] = nullptr;
}
}
// Decide whether PRE is profitable for this load.
unsigned NumUnavailablePreds = PredLoads.size() + CriticalEdgePred.size();
assert(NumUnavailablePreds != 0 &&
"Fully available value should already be eliminated!");
// If this load is unavailable in multiple predecessors, reject it.
// FIXME: If we could restructure the CFG, we could make a common pred with
// all the preds that don't have an available LI and insert a new load into
// that one block.
if (NumUnavailablePreds != 1)
return false;
// Split critical edges, and update the unavailable predecessors accordingly.
for (BasicBlock *OrigPred : CriticalEdgePred) {
BasicBlock *NewPred = splitCriticalEdges(OrigPred, LoadBB);
assert(!PredLoads.count(OrigPred) && "Split edges shouldn't be in map!");
PredLoads[NewPred] = nullptr;
DEBUG(dbgs() << "Split critical edge " << OrigPred->getName() << "->"
<< LoadBB->getName() << '\n');
}
// Check if the load can safely be moved to all the unavailable predecessors.
bool CanDoPRE = true;
const DataLayout &DL = LI->getModule()->getDataLayout();
SmallVector<Instruction*, 8> NewInsts;
for (auto &PredLoad : PredLoads) {
BasicBlock *UnavailablePred = PredLoad.first;
// Do PHI translation to get its value in the predecessor if necessary. The
// returned pointer (if non-null) is guaranteed to dominate UnavailablePred.
// If all preds have a single successor, then we know it is safe to insert
// the load on the pred (?!?), so we can insert code to materialize the
// pointer if it is not available.
PHITransAddr Address(LI->getPointerOperand(), DL, AC);
Value *LoadPtr = nullptr;
LoadPtr = Address.PHITranslateWithInsertion(LoadBB, UnavailablePred,
*DT, NewInsts);
// If we couldn't find or insert a computation of this phi translated value,
// we fail PRE.
if (!LoadPtr) {
DEBUG(dbgs() << "COULDN'T INSERT PHI TRANSLATED VALUE OF: "
<< *LI->getPointerOperand() << "\n");
CanDoPRE = false;
break;
}
PredLoad.second = LoadPtr;
}
if (!CanDoPRE) {
while (!NewInsts.empty()) {
Instruction *I = NewInsts.pop_back_val();
if (MD) MD->removeInstruction(I);
I->eraseFromParent();
}
// HINT: Don't revert the edge-splitting as following transformation may
// also need to split these critical edges.
return !CriticalEdgePred.empty();
}
// Okay, we can eliminate this load by inserting a reload in the predecessor
// and using PHI construction to get the value in the other predecessors, do
// it.
DEBUG(dbgs() << "GVN REMOVING PRE LOAD: " << *LI << '\n');
DEBUG(if (!NewInsts.empty())
dbgs() << "INSERTED " << NewInsts.size() << " INSTS: "
<< *NewInsts.back() << '\n');
// Assign value numbers to the new instructions.
for (Instruction *I : NewInsts) {
// FIXME: We really _ought_ to insert these value numbers into their
// parent's availability map. However, in doing so, we risk getting into
// ordering issues. If a block hasn't been processed yet, we would be
// marking a value as AVAIL-IN, which isn't what we intend.
VN.lookup_or_add(I);
}
for (const auto &PredLoad : PredLoads) {
BasicBlock *UnavailablePred = PredLoad.first;
Value *LoadPtr = PredLoad.second;
Instruction *NewLoad = new LoadInst(LoadPtr, LI->getName()+".pre", false,
LI->getAlignment(),
UnavailablePred->getTerminator());
// Transfer the old load's AA tags to the new load.
AAMDNodes Tags;
LI->getAAMetadata(Tags);
if (Tags)
NewLoad->setAAMetadata(Tags);
if (auto *MD = LI->getMetadata(LLVMContext::MD_invariant_load))
NewLoad->setMetadata(LLVMContext::MD_invariant_load, MD);
if (auto *InvGroupMD = LI->getMetadata(LLVMContext::MD_invariant_group))
NewLoad->setMetadata(LLVMContext::MD_invariant_group, InvGroupMD);
// Transfer DebugLoc.
NewLoad->setDebugLoc(LI->getDebugLoc());
// Add the newly created load.
ValuesPerBlock.push_back(AvailableValueInBlock::get(UnavailablePred,
NewLoad));
MD->invalidateCachedPointerInfo(LoadPtr);
DEBUG(dbgs() << "GVN INSERTED " << *NewLoad << '\n');
}
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, *this);
LI->replaceAllUsesWith(V);
if (isa<PHINode>(V))
V->takeName(LI);
if (Instruction *I = dyn_cast<Instruction>(V))
I->setDebugLoc(LI->getDebugLoc());
if (V->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
markInstructionForDeletion(LI);
++NumPRELoad;
return true;
}
/// Attempt to eliminate a load whose dependencies are
/// non-local by performing PHI construction.
bool GVN::processNonLocalLoad(LoadInst *LI) {
// non-local speculations are not allowed under asan.
if (LI->getParent()->getParent()->hasFnAttribute(Attribute::SanitizeAddress))
return false;
// Step 1: Find the non-local dependencies of the load.
LoadDepVect Deps;
MD->getNonLocalPointerDependency(LI, Deps);
// If we had to process more than one hundred blocks to find the
// dependencies, this load isn't worth worrying about. Optimizing
// it will be too expensive.
unsigned NumDeps = Deps.size();
if (NumDeps > 100)
return false;
// If we had a phi translation failure, we'll have a single entry which is a
// clobber in the current block. Reject this early.
if (NumDeps == 1 &&
!Deps[0].getResult().isDef() && !Deps[0].getResult().isClobber()) {
DEBUG(
dbgs() << "GVN: non-local load ";
LI->printAsOperand(dbgs());
dbgs() << " has unknown dependencies\n";
);
return false;
}
// If this load follows a GEP, see if we can PRE the indices before analyzing.
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0))) {
for (GetElementPtrInst::op_iterator OI = GEP->idx_begin(),
OE = GEP->idx_end();
OI != OE; ++OI)
if (Instruction *I = dyn_cast<Instruction>(OI->get()))
performScalarPRE(I);
}
// Step 2: Analyze the availability of the load
AvailValInBlkVect ValuesPerBlock;
UnavailBlkVect UnavailableBlocks;
AnalyzeLoadAvailability(LI, Deps, ValuesPerBlock, UnavailableBlocks);
// If we have no predecessors that produce a known value for this load, exit
// early.
if (ValuesPerBlock.empty())
return false;
// Step 3: Eliminate fully redundancy.
//
// If all of the instructions we depend on produce a known value for this
// load, then it is fully redundant and we can use PHI insertion to compute
// its value. Insert PHIs and remove the fully redundant value now.
if (UnavailableBlocks.empty()) {
DEBUG(dbgs() << "GVN REMOVING NONLOCAL LOAD: " << *LI << '\n');
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, *this);
LI->replaceAllUsesWith(V);
if (isa<PHINode>(V))
V->takeName(LI);
if (Instruction *I = dyn_cast<Instruction>(V))
if (LI->getDebugLoc())
I->setDebugLoc(LI->getDebugLoc());
if (V->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
markInstructionForDeletion(LI);
++NumGVNLoad;
return true;
}
// Step 4: Eliminate partial redundancy.
if (!EnablePRE || !EnableLoadPRE)
return false;
return PerformLoadPRE(LI, ValuesPerBlock, UnavailableBlocks);
}
bool GVN::processAssumeIntrinsic(IntrinsicInst *IntrinsicI) {
assert(IntrinsicI->getIntrinsicID() == Intrinsic::assume &&
"This function can only be called with llvm.assume intrinsic");
Value *V = IntrinsicI->getArgOperand(0);
if (ConstantInt *Cond = dyn_cast<ConstantInt>(V)) {
if (Cond->isZero()) {
Type *Int8Ty = Type::getInt8Ty(V->getContext());
// Insert a new store to null instruction before the load to indicate that
// this code is not reachable. FIXME: We could insert unreachable
// instruction directly because we can modify the CFG.
new StoreInst(UndefValue::get(Int8Ty),
Constant::getNullValue(Int8Ty->getPointerTo()),
IntrinsicI);
}
markInstructionForDeletion(IntrinsicI);
return false;
}
Constant *True = ConstantInt::getTrue(V->getContext());
bool Changed = false;
for (BasicBlock *Successor : successors(IntrinsicI->getParent())) {
BasicBlockEdge Edge(IntrinsicI->getParent(), Successor);
// This property is only true in dominated successors, propagateEquality
// will check dominance for us.
Changed |= propagateEquality(V, True, Edge, false);
}
// We can replace assume value with true, which covers cases like this:
// call void @llvm.assume(i1 %cmp)
// br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
ReplaceWithConstMap[V] = True;
// If one of *cmp *eq operand is const, adding it to map will cover this:
// %cmp = fcmp oeq float 3.000000e+00, %0 ; const on lhs could happen
// call void @llvm.assume(i1 %cmp)
// ret float %0 ; will change it to ret float 3.000000e+00
if (auto *CmpI = dyn_cast<CmpInst>(V)) {
if (CmpI->getPredicate() == CmpInst::Predicate::ICMP_EQ ||
CmpI->getPredicate() == CmpInst::Predicate::FCMP_OEQ ||
(CmpI->getPredicate() == CmpInst::Predicate::FCMP_UEQ &&
CmpI->getFastMathFlags().noNaNs())) {
Value *CmpLHS = CmpI->getOperand(0);
Value *CmpRHS = CmpI->getOperand(1);
if (isa<Constant>(CmpLHS))
std::swap(CmpLHS, CmpRHS);
auto *RHSConst = dyn_cast<Constant>(CmpRHS);
// If only one operand is constant.
if (RHSConst != nullptr && !isa<Constant>(CmpLHS))
ReplaceWithConstMap[CmpLHS] = RHSConst;
}
}
return Changed;
}
static void patchReplacementInstruction(Instruction *I, Value *Repl) {
// Patch the replacement so that it is not more restrictive than the value
// being replaced.
BinaryOperator *Op = dyn_cast<BinaryOperator>(I);
BinaryOperator *ReplOp = dyn_cast<BinaryOperator>(Repl);
if (Op && ReplOp)
ReplOp->andIRFlags(Op);
if (Instruction *ReplInst = dyn_cast<Instruction>(Repl)) {
// FIXME: If both the original and replacement value are part of the
// same control-flow region (meaning that the execution of one
// guarantees the execution of the other), then we can combine the
// noalias scopes here and do better than the general conservative
// answer used in combineMetadata().
// In general, GVN unifies expressions over different control-flow
// regions, and so we need a conservative combination of the noalias
// scopes.
static const unsigned KnownIDs[] = {
LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
LLVMContext::MD_noalias, LLVMContext::MD_range,
LLVMContext::MD_fpmath, LLVMContext::MD_invariant_load,
LLVMContext::MD_invariant_group};
combineMetadata(ReplInst, I, KnownIDs);
}
}
static void patchAndReplaceAllUsesWith(Instruction *I, Value *Repl) {
patchReplacementInstruction(I, Repl);
I->replaceAllUsesWith(Repl);
}
/// Attempt to eliminate a load, first by eliminating it
/// locally, and then attempting non-local elimination if that fails.
bool GVN::processLoad(LoadInst *L) {
if (!MD)
return false;
if (!L->isSimple())
return false;
if (L->use_empty()) {
markInstructionForDeletion(L);
return true;
}
// ... to a pointer that has been loaded from before...
MemDepResult Dep = MD->getDependency(L);
const DataLayout &DL = L->getModule()->getDataLayout();
// If it is defined in another block, try harder.
if (Dep.isNonLocal())
return processNonLocalLoad(L);
// Only handle the local case below
if (!Dep.isDef() && !Dep.isClobber()) {
// This might be a NonFuncLocal or an Unknown
DEBUG(
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load ";
L->printAsOperand(dbgs());
dbgs() << " has unknown dependence\n";
);
return false;
}
// If we have a clobber and target data is around, see if this is a clobber
// that we can fix up through code synthesis.
if (Dep.isClobber()) {
// Check to see if we have something like this:
// store i32 123, i32* %P
// %A = bitcast i32* %P to i8*
// %B = gep i8* %A, i32 1
// %C = load i8* %B
//
// We could do that by recognizing if the clobber instructions are obviously
// a common base + constant offset, and if the previous store (or memset)
// completely covers this load. This sort of thing can happen in bitfield
// access code.
Value *AvailVal = nullptr;
if (StoreInst *DepSI = dyn_cast<StoreInst>(Dep.getInst())) {
int Offset = AnalyzeLoadFromClobberingStore(
L->getType(), L->getPointerOperand(), DepSI);
if (Offset != -1)
AvailVal = GetStoreValueForLoad(DepSI->getValueOperand(), Offset,
L->getType(), L, DL);
}
// Check to see if we have something like this:
// load i32* P
// load i8* (P+1)
// if we have this, replace the later with an extraction from the former.
if (LoadInst *DepLI = dyn_cast<LoadInst>(Dep.getInst())) {
// If this is a clobber and L is the first instruction in its block, then
// we have the first instruction in the entry block.
if (DepLI == L)
return false;
int Offset = AnalyzeLoadFromClobberingLoad(
L->getType(), L->getPointerOperand(), DepLI, DL);
if (Offset != -1)
AvailVal = GetLoadValueForLoad(DepLI, Offset, L->getType(), L, *this);
}
// If the clobbering value is a memset/memcpy/memmove, see if we can forward
// a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(Dep.getInst())) {
int Offset = AnalyzeLoadFromClobberingMemInst(
L->getType(), L->getPointerOperand(), DepMI, DL);
if (Offset != -1)
AvailVal = GetMemInstValueForLoad(DepMI, Offset, L->getType(), L, DL);
}
if (AvailVal) {
DEBUG(dbgs() << "GVN COERCED INST:\n" << *Dep.getInst() << '\n'
<< *AvailVal << '\n' << *L << "\n\n\n");
// Replace the load!
L->replaceAllUsesWith(AvailVal);
if (AvailVal->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(AvailVal);
markInstructionForDeletion(L);
++NumGVNLoad;
return true;
}
// If the value isn't available, don't do anything!
DEBUG(
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load ";
L->printAsOperand(dbgs());
Instruction *I = Dep.getInst();
dbgs() << " is clobbered by " << *I << '\n';
);
return false;
}
assert(Dep.isDef() && "expected from control flow");
Instruction *DepInst = Dep.getInst();
Value *AvailableValue = nullptr;
if (StoreInst *DepSI = dyn_cast<StoreInst>(DepInst)) {
Value *StoredVal = DepSI->getValueOperand();
// The store and load are to a must-aliased pointer, but they may not
// actually have the same type. See if we know how to reuse the stored
// value (depending on its type).
if (StoredVal->getType() != L->getType()) {
IRBuilder<> Builder(L);
StoredVal =
CoerceAvailableValueToLoadType(StoredVal, L->getType(), Builder, DL);
if (!StoredVal)
return false;
DEBUG(dbgs() << "GVN COERCED STORE:\n" << *DepSI << '\n' << *StoredVal
<< '\n' << *L << "\n\n\n");
}
AvailableValue = StoredVal;
}
if (LoadInst *DepLI = dyn_cast<LoadInst>(DepInst)) {
AvailableValue = DepLI;
// The loads are of a must-aliased pointer, but they may not actually have
// the same type. See if we know how to reuse the previously loaded value
// (depending on its type).
if (DepLI->getType() != L->getType()) {
IRBuilder<> Builder(L);
AvailableValue =
CoerceAvailableValueToLoadType(DepLI, L->getType(), Builder, DL);
if (!AvailableValue)
return false;
DEBUG(dbgs() << "GVN COERCED LOAD:\n" << *DepLI << "\n" << *AvailableValue
<< "\n" << *L << "\n\n\n");
}
}
// If this load really doesn't depend on anything, then we must be loading an
// undef value. This can happen when loading for a fresh allocation with no
// intervening stores, for example.
if (isa<AllocaInst>(DepInst) || isMallocLikeFn(DepInst, TLI) ||
isLifetimeStart(DepInst))
AvailableValue = UndefValue::get(L->getType());
// If this load follows a calloc (which zero initializes memory),
// then the loaded value is zero
if (isCallocLikeFn(DepInst, TLI))
AvailableValue = Constant::getNullValue(L->getType());
if (AvailableValue) {
// Do the actual replacement
patchAndReplaceAllUsesWith(L, AvailableValue);
markInstructionForDeletion(L);
++NumGVNLoad;
// Tell MDA to rexamine the reused pointer since we might have more
// information after forwarding it.
if (MD && AvailableValue->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(AvailableValue);
return true;
}
return false;
}
// In order to find a leader for a given value number at a
// specific basic block, we first obtain the list of all Values for that number,
// and then scan the list to find one whose block dominates the block in
// question. This is fast because dominator tree queries consist of only
// a few comparisons of DFS numbers.
Value *GVN::findLeader(const BasicBlock *BB, uint32_t num) {
LeaderTableEntry Vals = LeaderTable[num];
if (!Vals.Val) return nullptr;
Value *Val = nullptr;
if (DT->dominates(Vals.BB, BB)) {
Val = Vals.Val;
if (isa<Constant>(Val)) return Val;
}
LeaderTableEntry* Next = Vals.Next;
while (Next) {
if (DT->dominates(Next->BB, BB)) {
if (isa<Constant>(Next->Val)) return Next->Val;
if (!Val) Val = Next->Val;
}
Next = Next->Next;
}
return Val;
}
/// There is an edge from 'Src' to 'Dst'. Return
/// true if every path from the entry block to 'Dst' passes via this edge. In
/// particular 'Dst' must not be reachable via another edge from 'Src'.
static bool isOnlyReachableViaThisEdge(const BasicBlockEdge &E,
DominatorTree *DT) {
// While in theory it is interesting to consider the case in which Dst has
// more than one predecessor, because Dst might be part of a loop which is
// only reachable from Src, in practice it is pointless since at the time
// GVN runs all such loops have preheaders, which means that Dst will have
// been changed to have only one predecessor, namely Src.
const BasicBlock *Pred = E.getEnd()->getSinglePredecessor();
const BasicBlock *Src = E.getStart();
assert((!Pred || Pred == Src) && "No edge between these basic blocks!");
(void)Src;
return Pred != nullptr;
}
// Tries to replace instruction with const, using information from
// ReplaceWithConstMap.
bool GVN::replaceOperandsWithConsts(Instruction *Instr) const {
bool Changed = false;
for (unsigned OpNum = 0; OpNum < Instr->getNumOperands(); ++OpNum) {
Value *Operand = Instr->getOperand(OpNum);
auto it = ReplaceWithConstMap.find(Operand);
if (it != ReplaceWithConstMap.end()) {
assert(!isa<Constant>(Operand) &&
"Replacing constants with constants is invalid");
DEBUG(dbgs() << "GVN replacing: " << *Operand << " with " << *it->second
<< " in instruction " << *Instr << '\n');
Instr->setOperand(OpNum, it->second);
Changed = true;
}
}
return Changed;
}
/// The given values are known to be equal in every block
/// dominated by 'Root'. Exploit this, for example by replacing 'LHS' with
/// 'RHS' everywhere in the scope. Returns whether a change was made.
/// If DominatesByEdge is false, then it means that we will propagate the RHS
/// value starting from the end of Root.Start.
bool GVN::propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root,
bool DominatesByEdge) {
SmallVector<std::pair<Value*, Value*>, 4> Worklist;
Worklist.push_back(std::make_pair(LHS, RHS));
bool Changed = false;
// For speed, compute a conservative fast approximation to
// DT->dominates(Root, Root.getEnd());
bool RootDominatesEnd = isOnlyReachableViaThisEdge(Root, DT);
while (!Worklist.empty()) {
std::pair<Value*, Value*> Item = Worklist.pop_back_val();
LHS = Item.first; RHS = Item.second;
if (LHS == RHS)
continue;
assert(LHS->getType() == RHS->getType() && "Equality but unequal types!");
// Don't try to propagate equalities between constants.
if (isa<Constant>(LHS) && isa<Constant>(RHS))
continue;
// Prefer a constant on the right-hand side, or an Argument if no constants.
if (isa<Constant>(LHS) || (isa<Argument>(LHS) && !isa<Constant>(RHS)))
std::swap(LHS, RHS);
assert((isa<Argument>(LHS) || isa<Instruction>(LHS)) && "Unexpected value!");
// If there is no obvious reason to prefer the left-hand side over the
// right-hand side, ensure the longest lived term is on the right-hand side,
// so the shortest lived term will be replaced by the longest lived.
// This tends to expose more simplifications.
uint32_t LVN = VN.lookup_or_add(LHS);
if ((isa<Argument>(LHS) && isa<Argument>(RHS)) ||
(isa<Instruction>(LHS) && isa<Instruction>(RHS))) {
// Move the 'oldest' value to the right-hand side, using the value number
// as a proxy for age.
uint32_t RVN = VN.lookup_or_add(RHS);
if (LVN < RVN) {
std::swap(LHS, RHS);
LVN = RVN;
}
}
// If value numbering later sees that an instruction in the scope is equal
// to 'LHS' then ensure it will be turned into 'RHS'. In order to preserve
// the invariant that instructions only occur in the leader table for their
// own value number (this is used by removeFromLeaderTable), do not do this
// if RHS is an instruction (if an instruction in the scope is morphed into
// LHS then it will be turned into RHS by the next GVN iteration anyway, so
// using the leader table is about compiling faster, not optimizing better).
// The leader table only tracks basic blocks, not edges. Only add to if we
// have the simple case where the edge dominates the end.
if (RootDominatesEnd && !isa<Instruction>(RHS))
addToLeaderTable(LVN, RHS, Root.getEnd());
// Replace all occurrences of 'LHS' with 'RHS' everywhere in the scope. As
// LHS always has at least one use that is not dominated by Root, this will
// never do anything if LHS has only one use.
if (!LHS->hasOneUse()) {
unsigned NumReplacements =
DominatesByEdge
? replaceDominatedUsesWith(LHS, RHS, *DT, Root)
: replaceDominatedUsesWith(LHS, RHS, *DT, Root.getStart());
Changed |= NumReplacements > 0;
NumGVNEqProp += NumReplacements;
}
// Now try to deduce additional equalities from this one. For example, if
// the known equality was "(A != B)" == "false" then it follows that A and B
// are equal in the scope. Only boolean equalities with an explicit true or
// false RHS are currently supported.
if (!RHS->getType()->isIntegerTy(1))
// Not a boolean equality - bail out.
continue;
ConstantInt *CI = dyn_cast<ConstantInt>(RHS);
if (!CI)
// RHS neither 'true' nor 'false' - bail out.
continue;
// Whether RHS equals 'true'. Otherwise it equals 'false'.
bool isKnownTrue = CI->isAllOnesValue();
bool isKnownFalse = !isKnownTrue;
// If "A && B" is known true then both A and B are known true. If "A || B"
// is known false then both A and B are known false.
Value *A, *B;
if ((isKnownTrue && match(LHS, m_And(m_Value(A), m_Value(B)))) ||
(isKnownFalse && match(LHS, m_Or(m_Value(A), m_Value(B))))) {
Worklist.push_back(std::make_pair(A, RHS));
Worklist.push_back(std::make_pair(B, RHS));
continue;
}
// If we are propagating an equality like "(A == B)" == "true" then also
// propagate the equality A == B. When propagating a comparison such as
// "(A >= B)" == "true", replace all instances of "A < B" with "false".
if (CmpInst *Cmp = dyn_cast<CmpInst>(LHS)) {
Value *Op0 = Cmp->getOperand(0), *Op1 = Cmp->getOperand(1);
// If "A == B" is known true, or "A != B" is known false, then replace
// A with B everywhere in the scope.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::ICMP_EQ) ||
(isKnownFalse && Cmp->getPredicate() == CmpInst::ICMP_NE))
Worklist.push_back(std::make_pair(Op0, Op1));
// Handle the floating point versions of equality comparisons too.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) ||
(isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) {
// Floating point -0.0 and 0.0 compare equal, so we can only
// propagate values if we know that we have a constant and that
// its value is non-zero.
// FIXME: We should do this optimization if 'no signed zeros' is
// applicable via an instruction-level fast-math-flag or some other
// indicator that relaxed FP semantics are being used.
if (isa<ConstantFP>(Op1) && !cast<ConstantFP>(Op1)->isZero())
Worklist.push_back(std::make_pair(Op0, Op1));
}
// If "A >= B" is known true, replace "A < B" with false everywhere.
CmpInst::Predicate NotPred = Cmp->getInversePredicate();
Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);
// Since we don't have the instruction "A < B" immediately to hand, work
// out the value number that it would have and use that to find an
// appropriate instruction (if any).
uint32_t NextNum = VN.getNextUnusedValueNumber();
uint32_t Num = VN.lookup_or_add_cmp(Cmp->getOpcode(), NotPred, Op0, Op1);
// If the number we were assigned was brand new then there is no point in
// looking for an instruction realizing it: there cannot be one!
if (Num < NextNum) {
Value *NotCmp = findLeader(Root.getEnd(), Num);
if (NotCmp && isa<Instruction>(NotCmp)) {
unsigned NumReplacements =
DominatesByEdge
? replaceDominatedUsesWith(NotCmp, NotVal, *DT, Root)
: replaceDominatedUsesWith(NotCmp, NotVal, *DT,
Root.getStart());
Changed |= NumReplacements > 0;
NumGVNEqProp += NumReplacements;
}
}
// Ensure that any instruction in scope that gets the "A < B" value number
// is replaced with false.
// The leader table only tracks basic blocks, not edges. Only add to if we
// have the simple case where the edge dominates the end.
if (RootDominatesEnd)
addToLeaderTable(Num, NotVal, Root.getEnd());
continue;
}
}
return Changed;
}
/// When calculating availability, handle an instruction
/// by inserting it into the appropriate sets
bool GVN::processInstruction(Instruction *I) {
// Ignore dbg info intrinsics.
if (isa<DbgInfoIntrinsic>(I))
return false;
// If the instruction can be easily simplified then do so now in preference
// to value numbering it. Value numbering often exposes redundancies, for
// example if it determines that %y is equal to %x then the instruction
// "%z = and i32 %x, %y" becomes "%z = and i32 %x, %x" which we now simplify.
const DataLayout &DL = I->getModule()->getDataLayout();
if (Value *V = SimplifyInstruction(I, DL, TLI, DT, AC)) {
I->replaceAllUsesWith(V);
if (MD && V->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
markInstructionForDeletion(I);
++NumGVNSimpl;
return true;
}
if (IntrinsicInst *IntrinsicI = dyn_cast<IntrinsicInst>(I))
if (IntrinsicI->getIntrinsicID() == Intrinsic::assume)
return processAssumeIntrinsic(IntrinsicI);
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
if (processLoad(LI))
return true;
unsigned Num = VN.lookup_or_add(LI);
addToLeaderTable(Num, LI, LI->getParent());
return false;
}
// For conditional branches, we can perform simple conditional propagation on
// the condition value itself.
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
if (!BI->isConditional())
return false;
if (isa<Constant>(BI->getCondition()))
return processFoldableCondBr(BI);
Value *BranchCond = BI->getCondition();
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
// Avoid multiple edges early.
if (TrueSucc == FalseSucc)
return false;
BasicBlock *Parent = BI->getParent();
bool Changed = false;
Value *TrueVal = ConstantInt::getTrue(TrueSucc->getContext());
BasicBlockEdge TrueE(Parent, TrueSucc);
Changed |= propagateEquality(BranchCond, TrueVal, TrueE, true);
Value *FalseVal = ConstantInt::getFalse(FalseSucc->getContext());
BasicBlockEdge FalseE(Parent, FalseSucc);
Changed |= propagateEquality(BranchCond, FalseVal, FalseE, true);
return Changed;
}
// For switches, propagate the case values into the case destinations.
if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
Value *SwitchCond = SI->getCondition();
BasicBlock *Parent = SI->getParent();
bool Changed = false;
// Remember how many outgoing edges there are to every successor.
SmallDenseMap<BasicBlock *, unsigned, 16> SwitchEdges;
for (unsigned i = 0, n = SI->getNumSuccessors(); i != n; ++i)
++SwitchEdges[SI->getSuccessor(i)];
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
BasicBlock *Dst = i.getCaseSuccessor();
// If there is only a single edge, propagate the case value into it.
if (SwitchEdges.lookup(Dst) == 1) {
BasicBlockEdge E(Parent, Dst);
Changed |= propagateEquality(SwitchCond, i.getCaseValue(), E, true);
}
}
return Changed;
}
// Instructions with void type don't return a value, so there's
// no point in trying to find redundancies in them.
if (I->getType()->isVoidTy())
return false;
uint32_t NextNum = VN.getNextUnusedValueNumber();
unsigned Num = VN.lookup_or_add(I);
// Allocations are always uniquely numbered, so we can save time and memory
// by fast failing them.
if (isa<AllocaInst>(I) || isa<TerminatorInst>(I) || isa<PHINode>(I)) {
addToLeaderTable(Num, I, I->getParent());
return false;
}
// If the number we were assigned was a brand new VN, then we don't
// need to do a lookup to see if the number already exists
// somewhere in the domtree: it can't!
if (Num >= NextNum) {
addToLeaderTable(Num, I, I->getParent());
return false;
}
// Perform fast-path value-number based elimination of values inherited from
// dominators.
Value *Repl = findLeader(I->getParent(), Num);
if (!Repl) {
// Failure, just remember this instance for future use.
addToLeaderTable(Num, I, I->getParent());
return false;
} else if (Repl == I) {
// If I was the result of a shortcut PRE, it might already be in the table
// and the best replacement for itself. Nothing to do.
return false;
}
// Remove it!
patchAndReplaceAllUsesWith(I, Repl);
if (MD && Repl->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(Repl);
markInstructionForDeletion(I);
return true;
}
/// runOnFunction - This is the main transformation entry point for a function.
bool GVN::runOnFunction(Function& F) {
if (skipOptnoneFunction(F))
return false;
if (!NoLoads)
MD = &getAnalysis<MemoryDependenceAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
VN.setAliasAnalysis(&getAnalysis<AAResultsWrapperPass>().getAAResults());
VN.setMemDep(MD);
VN.setDomTree(DT);
bool Changed = false;
bool ShouldContinue = true;
// Merge unconditional branches, allowing PRE to catch more
// optimization opportunities.
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ) {
BasicBlock *BB = &*FI++;
bool removedBlock =
MergeBlockIntoPredecessor(BB, DT, /* LoopInfo */ nullptr, MD);
if (removedBlock) ++NumGVNBlocks;
Changed |= removedBlock;
}
unsigned Iteration = 0;
while (ShouldContinue) {
DEBUG(dbgs() << "GVN iteration: " << Iteration << "\n");
ShouldContinue = iterateOnFunction(F);
Changed |= ShouldContinue;
++Iteration;
}
if (EnablePRE) {
// Fabricate val-num for dead-code in order to suppress assertion in
// performPRE().
assignValNumForDeadCode();
bool PREChanged = true;
while (PREChanged) {
PREChanged = performPRE(F);
Changed |= PREChanged;
}
}
// FIXME: Should perform GVN again after PRE does something. PRE can move
// computations into blocks where they become fully redundant. Note that
// we can't do this until PRE's critical edge splitting updates memdep.
// Actually, when this happens, we should just fully integrate PRE into GVN.
cleanupGlobalSets();
// Do not cleanup DeadBlocks in cleanupGlobalSets() as it's called for each
// iteration.
DeadBlocks.clear();
return Changed;
}
bool GVN::processBlock(BasicBlock *BB) {
// FIXME: Kill off InstrsToErase by doing erasing eagerly in a helper function
// (and incrementing BI before processing an instruction).
assert(InstrsToErase.empty() &&
"We expect InstrsToErase to be empty across iterations");
if (DeadBlocks.count(BB))
return false;
// Clearing map before every BB because it can be used only for single BB.
ReplaceWithConstMap.clear();
bool ChangedFunction = false;
for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
BI != BE;) {
if (!ReplaceWithConstMap.empty())
ChangedFunction |= replaceOperandsWithConsts(&*BI);
ChangedFunction |= processInstruction(&*BI);
if (InstrsToErase.empty()) {
++BI;
continue;
}
// If we need some instructions deleted, do it now.
NumGVNInstr += InstrsToErase.size();
// Avoid iterator invalidation.
bool AtStart = BI == BB->begin();
if (!AtStart)
--BI;
for (SmallVectorImpl<Instruction *>::iterator I = InstrsToErase.begin(),
E = InstrsToErase.end(); I != E; ++I) {
DEBUG(dbgs() << "GVN removed: " << **I << '\n');
if (MD) MD->removeInstruction(*I);
DEBUG(verifyRemoved(*I));
(*I)->eraseFromParent();
}
InstrsToErase.clear();
if (AtStart)
BI = BB->begin();
else
++BI;
}
return ChangedFunction;
}
// Instantiate an expression in a predecessor that lacked it.
bool GVN::performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
unsigned int ValNo) {
// Because we are going top-down through the block, all value numbers
// will be available in the predecessor by the time we need them. Any
// that weren't originally present will have been instantiated earlier
// in this loop.
bool success = true;
for (unsigned i = 0, e = Instr->getNumOperands(); i != e; ++i) {
Value *Op = Instr->getOperand(i);
if (isa<Argument>(Op) || isa<Constant>(Op) || isa<GlobalValue>(Op))
continue;
// This could be a newly inserted instruction, in which case, we won't
// find a value number, and should give up before we hurt ourselves.
// FIXME: Rewrite the infrastructure to let it easier to value number
// and process newly inserted instructions.
if (!VN.exists(Op)) {
success = false;
break;
}
if (Value *V = findLeader(Pred, VN.lookup(Op))) {
Instr->setOperand(i, V);
} else {
success = false;
break;
}
}
// Fail out if we encounter an operand that is not available in
// the PRE predecessor. This is typically because of loads which
// are not value numbered precisely.
if (!success)
return false;
Instr->insertBefore(Pred->getTerminator());
Instr->setName(Instr->getName() + ".pre");
Instr->setDebugLoc(Instr->getDebugLoc());
VN.add(Instr, ValNo);
// Update the availability map to include the new instruction.
addToLeaderTable(ValNo, Instr, Pred);
return true;
}
bool GVN::performScalarPRE(Instruction *CurInst) {
SmallVector<std::pair<Value*, BasicBlock*>, 8> predMap;
if (isa<AllocaInst>(CurInst) || isa<TerminatorInst>(CurInst) ||
isa<PHINode>(CurInst) || CurInst->getType()->isVoidTy() ||
CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
isa<DbgInfoIntrinsic>(CurInst))
return false;
// Don't do PRE on compares. The PHI would prevent CodeGenPrepare from
// sinking the compare again, and it would force the code generator to
// move the i1 from processor flags or predicate registers into a general
// purpose register.
if (isa<CmpInst>(CurInst))
return false;
// We don't currently value number ANY inline asm calls.
if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
if (CallI->isInlineAsm())
return false;
uint32_t ValNo = VN.lookup(CurInst);
// Look for the predecessors for PRE opportunities. We're
// only trying to solve the basic diamond case, where
// a value is computed in the successor and one predecessor,
// but not the other. We also explicitly disallow cases
// where the successor is its own predecessor, because they're
// more complicated to get right.
unsigned NumWith = 0;
unsigned NumWithout = 0;
BasicBlock *PREPred = nullptr;
BasicBlock *CurrentBlock = CurInst->getParent();
predMap.clear();
for (BasicBlock *P : predecessors(CurrentBlock)) {
// We're not interested in PRE where the block is its
// own predecessor, or in blocks with predecessors
// that are not reachable.
if (P == CurrentBlock) {
NumWithout = 2;
break;
} else if (!DT->isReachableFromEntry(P)) {
NumWithout = 2;
break;
}
Value *predV = findLeader(P, ValNo);
if (!predV) {
predMap.push_back(std::make_pair(static_cast<Value *>(nullptr), P));
PREPred = P;
++NumWithout;
} else if (predV == CurInst) {
/* CurInst dominates this predecessor. */
NumWithout = 2;
break;
} else {
predMap.push_back(std::make_pair(predV, P));
++NumWith;
}
}
// Don't do PRE when it might increase code size, i.e. when
// we would need to insert instructions in more than one pred.
if (NumWithout > 1 || NumWith == 0)
return false;
// We may have a case where all predecessors have the instruction,
// and we just need to insert a phi node. Otherwise, perform
// insertion.
Instruction *PREInstr = nullptr;
if (NumWithout != 0) {
// Don't do PRE across indirect branch.
if (isa<IndirectBrInst>(PREPred->getTerminator()))
return false;
// We can't do PRE safely on a critical edge, so instead we schedule
// the edge to be split and perform the PRE the next time we iterate
// on the function.
unsigned SuccNum = GetSuccessorNumber(PREPred, CurrentBlock);
if (isCriticalEdge(PREPred->getTerminator(), SuccNum)) {
toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
return false;
}
// We need to insert somewhere, so let's give it a shot
PREInstr = CurInst->clone();
if (!performScalarPREInsertion(PREInstr, PREPred, ValNo)) {
// If we failed insertion, make sure we remove the instruction.
DEBUG(verifyRemoved(PREInstr));
delete PREInstr;
return false;
}
}
// Either we should have filled in the PRE instruction, or we should
// not have needed insertions.
assert (PREInstr != nullptr || NumWithout == 0);
++NumGVNPRE;
// Create a PHI to make the value available in this block.
PHINode *Phi =
PHINode::Create(CurInst->getType(), predMap.size(),
CurInst->getName() + ".pre-phi", &CurrentBlock->front());
for (unsigned i = 0, e = predMap.size(); i != e; ++i) {
if (Value *V = predMap[i].first)
Phi->addIncoming(V, predMap[i].second);
else
Phi->addIncoming(PREInstr, PREPred);
}
VN.add(Phi, ValNo);
addToLeaderTable(ValNo, Phi, CurrentBlock);
Phi->setDebugLoc(CurInst->getDebugLoc());
CurInst->replaceAllUsesWith(Phi);
if (MD && Phi->getType()->getScalarType()->isPointerTy())
MD->invalidateCachedPointerInfo(Phi);
VN.erase(CurInst);
removeFromLeaderTable(ValNo, CurInst, CurrentBlock);
DEBUG(dbgs() << "GVN PRE removed: " << *CurInst << '\n');
if (MD)
MD->removeInstruction(CurInst);
DEBUG(verifyRemoved(CurInst));
CurInst->eraseFromParent();
++NumGVNInstr;
return true;
}
/// Perform a purely local form of PRE that looks for diamond
/// control flow patterns and attempts to perform simple PRE at the join point.
bool GVN::performPRE(Function &F) {
bool Changed = false;
for (BasicBlock *CurrentBlock : depth_first(&F.getEntryBlock())) {
// Nothing to PRE in the entry block.
if (CurrentBlock == &F.getEntryBlock())
continue;
// Don't perform PRE on an EH pad.
if (CurrentBlock->isEHPad())
continue;
for (BasicBlock::iterator BI = CurrentBlock->begin(),
BE = CurrentBlock->end();
BI != BE;) {
Instruction *CurInst = &*BI++;
Changed |= performScalarPRE(CurInst);
}
}
if (splitCriticalEdges())
Changed = true;
return Changed;
}
/// Split the critical edge connecting the given two blocks, and return
/// the block inserted to the critical edge.
BasicBlock *GVN::splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ) {
BasicBlock *BB =
SplitCriticalEdge(Pred, Succ, CriticalEdgeSplittingOptions(DT));
if (MD)
MD->invalidateCachedPredecessors();
return BB;
}
/// Split critical edges found during the previous
/// iteration that may enable further optimization.
bool GVN::splitCriticalEdges() {
if (toSplit.empty())
return false;
do {
std::pair<TerminatorInst*, unsigned> Edge = toSplit.pop_back_val();
SplitCriticalEdge(Edge.first, Edge.second,
CriticalEdgeSplittingOptions(DT));
} while (!toSplit.empty());
if (MD) MD->invalidateCachedPredecessors();
return true;
}
/// Executes one iteration of GVN
bool GVN::iterateOnFunction(Function &F) {
cleanupGlobalSets();
// Top-down walk of the dominator tree
bool Changed = false;
// Save the blocks this function have before transformation begins. GVN may
// split critical edge, and hence may invalidate the RPO/DT iterator.
//
std::vector<BasicBlock *> BBVect;
BBVect.reserve(256);
// Needed for value numbering with phi construction to work.
ReversePostOrderTraversal<Function *> RPOT(&F);
for (ReversePostOrderTraversal<Function *>::rpo_iterator RI = RPOT.begin(),
RE = RPOT.end();
RI != RE; ++RI)
BBVect.push_back(*RI);
for (std::vector<BasicBlock *>::iterator I = BBVect.begin(), E = BBVect.end();
I != E; I++)
Changed |= processBlock(*I);
return Changed;
}
void GVN::cleanupGlobalSets() {
VN.clear();
LeaderTable.clear();
TableAllocator.Reset();
}
/// Verify that the specified instruction does not occur in our
/// internal data structures.
void GVN::verifyRemoved(const Instruction *Inst) const {
VN.verifyRemoved(Inst);
// Walk through the value number scope to make sure the instruction isn't
// ferreted away in it.
for (DenseMap<uint32_t, LeaderTableEntry>::const_iterator
I = LeaderTable.begin(), E = LeaderTable.end(); I != E; ++I) {
const LeaderTableEntry *Node = &I->second;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
while (Node->Next) {
Node = Node->Next;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
}
}
}
/// BB is declared dead, which implied other blocks become dead as well. This
/// function is to add all these blocks to "DeadBlocks". For the dead blocks'
/// live successors, update their phi nodes by replacing the operands
/// corresponding to dead blocks with UndefVal.
void GVN::addDeadBlock(BasicBlock *BB) {
SmallVector<BasicBlock *, 4> NewDead;
SmallSetVector<BasicBlock *, 4> DF;
NewDead.push_back(BB);
while (!NewDead.empty()) {
BasicBlock *D = NewDead.pop_back_val();
if (DeadBlocks.count(D))
continue;
// All blocks dominated by D are dead.
SmallVector<BasicBlock *, 8> Dom;
DT->getDescendants(D, Dom);
DeadBlocks.insert(Dom.begin(), Dom.end());
// Figure out the dominance-frontier(D).
for (BasicBlock *B : Dom) {
for (BasicBlock *S : successors(B)) {
if (DeadBlocks.count(S))
continue;
bool AllPredDead = true;
for (BasicBlock *P : predecessors(S))
if (!DeadBlocks.count(P)) {
AllPredDead = false;
break;
}
if (!AllPredDead) {
// S could be proved dead later on. That is why we don't update phi
// operands at this moment.
DF.insert(S);
} else {
// While S is not dominated by D, it is dead by now. This could take
// place if S already have a dead predecessor before D is declared
// dead.
NewDead.push_back(S);
}
}
}
}
// For the dead blocks' live successors, update their phi nodes by replacing
// the operands corresponding to dead blocks with UndefVal.
for(SmallSetVector<BasicBlock *, 4>::iterator I = DF.begin(), E = DF.end();
I != E; I++) {
BasicBlock *B = *I;
if (DeadBlocks.count(B))
continue;
SmallVector<BasicBlock *, 4> Preds(pred_begin(B), pred_end(B));
for (BasicBlock *P : Preds) {
if (!DeadBlocks.count(P))
continue;
if (isCriticalEdge(P->getTerminator(), GetSuccessorNumber(P, B))) {
if (BasicBlock *S = splitCriticalEdges(P, B))
DeadBlocks.insert(P = S);
}
for (BasicBlock::iterator II = B->begin(); isa<PHINode>(II); ++II) {
PHINode &Phi = cast<PHINode>(*II);
Phi.setIncomingValue(Phi.getBasicBlockIndex(P),
UndefValue::get(Phi.getType()));
}
}
}
}
// If the given branch is recognized as a foldable branch (i.e. conditional
// branch with constant condition), it will perform following analyses and
// transformation.
// 1) If the dead out-coming edge is a critical-edge, split it. Let
// R be the target of the dead out-coming edge.
// 1) Identify the set of dead blocks implied by the branch's dead outcoming
// edge. The result of this step will be {X| X is dominated by R}
// 2) Identify those blocks which haves at least one dead predecessor. The
// result of this step will be dominance-frontier(R).
// 3) Update the PHIs in DF(R) by replacing the operands corresponding to
// dead blocks with "UndefVal" in an hope these PHIs will optimized away.
//
// Return true iff *NEW* dead code are found.
bool GVN::processFoldableCondBr(BranchInst *BI) {
if (!BI || BI->isUnconditional())
return false;
// If a branch has two identical successors, we cannot declare either dead.
if (BI->getSuccessor(0) == BI->getSuccessor(1))
return false;
ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
if (!Cond)
return false;
BasicBlock *DeadRoot = Cond->getZExtValue() ?
BI->getSuccessor(1) : BI->getSuccessor(0);
if (DeadBlocks.count(DeadRoot))
return false;
if (!DeadRoot->getSinglePredecessor())
DeadRoot = splitCriticalEdges(BI->getParent(), DeadRoot);
addDeadBlock(DeadRoot);
return true;
}
// performPRE() will trigger assert if it comes across an instruction without
// associated val-num. As it normally has far more live instructions than dead
// instructions, it makes more sense just to "fabricate" a val-number for the
// dead code than checking if instruction involved is dead or not.
void GVN::assignValNumForDeadCode() {
for (BasicBlock *BB : DeadBlocks) {
for (Instruction &Inst : *BB) {
unsigned ValNum = VN.lookup_or_add(&Inst);
addToLeaderTable(ValNum, &Inst, BB);
}
}
}
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