llvm-6502/lib/Transforms/Scalar/DeadStoreElimination.cpp
2010-11-30 05:30:45 +00:00

594 lines
20 KiB
C++

//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a trivial dead store elimination that only considers
// basic-block local redundant stores.
//
// FIXME: This should eventually be extended to be a post-dominator tree
// traversal. Doing so would be pretty trivial.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "dse"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");
namespace {
struct DSE : public FunctionPass {
TargetData *TD;
static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass(ID) {
initializeDSEPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F) {
bool Changed = false;
DominatorTree &DT = getAnalysis<DominatorTree>();
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
// Only check non-dead blocks. Dead blocks may have strange pointer
// cycles that will confuse alias analysis.
if (DT.isReachableFromEntry(I))
Changed |= runOnBasicBlock(*I);
return Changed;
}
bool runOnBasicBlock(BasicBlock &BB);
bool HandleFree(CallInst *F);
bool handleEndBlock(BasicBlock &BB);
bool RemoveUndeadPointers(Value *Ptr, uint64_t killPointerSize,
BasicBlock::iterator &BBI,
SmallPtrSet<Value*, 64> &deadPointers);
void DeleteDeadInstruction(Instruction *I,
SmallPtrSet<Value*, 64> *deadPointers = 0);
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<DominatorTree>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
uint64_t getPointerSize(Value *V) const;
};
}
char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
/// hasMemoryWrite - Does this instruction write some memory? This only returns
/// true for things that we can analyze with other helpers below.
static bool hasMemoryWrite(Instruction *I) {
if (isa<StoreInst>(I))
return true;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default:
return false;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
case Intrinsic::init_trampoline:
case Intrinsic::lifetime_end:
return true;
}
}
return false;
}
/// isRemovable - If the value of this instruction and the memory it writes to
/// is unused, may we delete this instruction?
static bool isRemovable(Instruction *I) {
assert(hasMemoryWrite(I));
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
return II->getIntrinsicID() != Intrinsic::lifetime_end;
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return !SI->isVolatile();
return true;
}
/// getPointerOperand - Return the pointer that is being written to.
static Value *getPointerOperand(Instruction *I) {
assert(hasMemoryWrite(I));
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->getPointerOperand();
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
return MI->getArgOperand(0);
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(false && "Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return II->getArgOperand(0);
case Intrinsic::lifetime_end:
return II->getArgOperand(1);
}
}
/// getStoreSize - Return the length in bytes of the write by the clobbering
/// instruction. If variable or unknown, returns AliasAnalysis::UnknownSize.
static uint64_t getStoreSize(Instruction *I, const TargetData *TD) {
assert(hasMemoryWrite(I));
if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (!TD) return AliasAnalysis::UnknownSize;
return TD->getTypeStoreSize(SI->getOperand(0)->getType());
}
Value *Len;
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
Len = MI->getLength();
} else {
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(false && "Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return AliasAnalysis::UnknownSize;
case Intrinsic::lifetime_end:
Len = II->getArgOperand(0);
break;
}
}
if (ConstantInt *LenCI = dyn_cast<ConstantInt>(Len))
if (!LenCI->isAllOnesValue())
return LenCI->getZExtValue();
return AliasAnalysis::UnknownSize;
}
/// isStoreAtLeastAsWideAs - Return true if the size of the store in I1 is
/// greater than or equal to the store in I2. This returns false if we don't
/// know.
///
static bool isStoreAtLeastAsWideAs(Instruction *I1, Instruction *I2,
const TargetData *TD) {
const Type *I1Ty = getPointerOperand(I1)->getType();
const Type *I2Ty = getPointerOperand(I2)->getType();
// Exactly the same type, must have exactly the same size.
if (I1Ty == I2Ty) return true;
uint64_t I1Size = getStoreSize(I1, TD);
uint64_t I2Size = getStoreSize(I2, TD);
return I1Size != AliasAnalysis::UnknownSize &&
I2Size != AliasAnalysis::UnknownSize &&
I1Size >= I2Size;
}
bool DSE::runOnBasicBlock(BasicBlock &BB) {
MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
TD = getAnalysisIfAvailable<TargetData>();
bool MadeChange = false;
// Do a top-down walk on the BB.
for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
Instruction *Inst = BBI++;
// Handle 'free' calls specially.
if (CallInst *F = isFreeCall(Inst)) {
MadeChange |= HandleFree(F);
continue;
}
// If we find something that writes memory, get its memory dependence.
if (!hasMemoryWrite(Inst))
continue;
MemDepResult InstDep = MD.getDependency(Inst);
// Ignore non-local store liveness.
// FIXME: cross-block DSE would be fun. :)
if (InstDep.isNonLocal()) continue;
// If we're storing the same value back to a pointer that we just
// loaded from, then the store can be removed.
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
SI->getOperand(0) == DepLoad && !SI->isVolatile()) {
// DeleteDeadInstruction can delete the current instruction. Save BBI
// in case we need it.
WeakVH NextInst(BBI);
DeleteDeadInstruction(SI);
if (NextInst == 0) // Next instruction deleted.
BBI = BB.begin();
else if (BBI != BB.begin()) // Revisit this instruction if possible.
--BBI;
++NumFastStores;
MadeChange = true;
continue;
}
}
}
if (!InstDep.isDef()) {
// If this is a may-aliased store that is clobbering the store value, we
// can keep searching past it for another must-aliased pointer that stores
// to the same location. For example, in:
// store -> P
// store -> Q
// store -> P
// we can remove the first store to P even though we don't know if P and Q
// alias.
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
AliasAnalysis::Location Loc = AA.getLocation(SI);
while (InstDep.isClobber() && InstDep.getInst() != &BB.front()) {
// Can't look past this instruction if it might read 'Loc'.
if (AA.getModRefInfo(InstDep.getInst(), Loc) & AliasAnalysis::Ref)
break;
InstDep = MD.getPointerDependencyFrom(Loc, false,
InstDep.getInst(), &BB);
}
}
}
// If this is a store-store dependence, then the previous store is dead so
// long as this store is at least as big as it.
if (InstDep.isDef() && hasMemoryWrite(InstDep.getInst())) {
Instruction *DepStore = InstDep.getInst();
if (!isRemovable(DepStore) ||
!isStoreAtLeastAsWideAs(Inst, DepStore, TD))
continue;
// Delete the store and now-dead instructions that feed it.
DeleteDeadInstruction(DepStore);
++NumFastStores;
MadeChange = true;
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
--BBI;
continue;
}
}
// If this block ends in a return, unwind, or unreachable, all allocas are
// dead at its end, which means stores to them are also dead.
if (BB.getTerminator()->getNumSuccessors() == 0)
MadeChange |= handleEndBlock(BB);
return MadeChange;
}
/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
MemDepResult Dep = MD.getDependency(F);
do {
if (Dep.isNonLocal()) return false;
Instruction *Dependency = Dep.getInst();
if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
return false;
Value *DepPointer = getPointerOperand(Dependency)->getUnderlyingObject();
// Check for aliasing.
if (AA.alias(F->getArgOperand(0), 1, DepPointer, 1) !=
AliasAnalysis::MustAlias)
return false;
// DCE instructions only used to calculate that store
DeleteDeadInstruction(Dependency);
++NumFastStores;
// Inst's old Dependency is now deleted. Compute the next dependency,
// which may also be dead, as in
// s[0] = 0;
// s[1] = 0; // This has just been deleted.
// free(s);
Dep = MD.getDependency(F);
} while (!Dep.isNonLocal());
return true;
}
/// handleEndBlock - Remove dead stores to stack-allocated locations in the
/// function end block. Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
bool DSE::handleEndBlock(BasicBlock &BB) {
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
bool MadeChange = false;
// Pointers alloca'd in this function are dead in the end block
SmallPtrSet<Value*, 64> deadPointers;
// Find all of the alloca'd pointers in the entry block.
BasicBlock *Entry = BB.getParent()->begin();
for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
deadPointers.insert(AI);
// Treat byval arguments the same, stores to them are dead at the end of the
// function.
for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
AE = BB.getParent()->arg_end(); AI != AE; ++AI)
if (AI->hasByValAttr())
deadPointers.insert(AI);
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
--BBI;
// If we find a store whose pointer is dead.
if (hasMemoryWrite(BBI)) {
if (isRemovable(BBI)) {
// See through pointer-to-pointer bitcasts
Value *pointerOperand = getPointerOperand(BBI)->getUnderlyingObject();
// Alloca'd pointers or byval arguments (which are functionally like
// alloca's) are valid candidates for removal.
if (deadPointers.count(pointerOperand)) {
// DCE instructions only used to calculate that store.
Instruction *Dead = BBI;
++BBI;
DeleteDeadInstruction(Dead, &deadPointers);
++NumFastStores;
MadeChange = true;
continue;
}
}
// Because a memcpy or memmove is also a load, we can't skip it if we
// didn't remove it.
if (!isa<MemTransferInst>(BBI))
continue;
}
Value *killPointer = 0;
uint64_t killPointerSize = AliasAnalysis::UnknownSize;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
// However, if this load is unused and not volatile, we can go ahead and
// remove it, and not have to worry about it making our pointer undead!
if (L->use_empty() && !L->isVolatile()) {
++BBI;
DeleteDeadInstruction(L, &deadPointers);
++NumFastOther;
MadeChange = true;
continue;
}
killPointer = L->getPointerOperand();
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
killPointer = V->getOperand(0);
} else if (isa<MemTransferInst>(BBI) &&
isa<ConstantInt>(cast<MemTransferInst>(BBI)->getLength())) {
killPointer = cast<MemTransferInst>(BBI)->getSource();
killPointerSize = cast<ConstantInt>(
cast<MemTransferInst>(BBI)->getLength())->getZExtValue();
} else if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
deadPointers.erase(A);
// Dead alloca's can be DCE'd when we reach them
if (A->use_empty()) {
++BBI;
DeleteDeadInstruction(A, &deadPointers);
++NumFastOther;
MadeChange = true;
}
continue;
} else if (CallSite CS = cast<Value>(BBI)) {
// If this call does not access memory, it can't
// be undeadifying any of our pointers.
if (AA.doesNotAccessMemory(CS))
continue;
unsigned modRef = 0;
unsigned other = 0;
// Remove any pointers made undead by the call from the dead set
std::vector<Value*> dead;
for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
E = deadPointers.end(); I != E; ++I) {
// HACK: if we detect that our AA is imprecise, it's not
// worth it to scan the rest of the deadPointers set. Just
// assume that the AA will return ModRef for everything, and
// go ahead and bail.
if (modRef >= 16 && other == 0) {
deadPointers.clear();
return MadeChange;
}
// See if the call site touches it
AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I,
getPointerSize(*I));
if (A == AliasAnalysis::ModRef)
++modRef;
else
++other;
if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
dead.push_back(*I);
}
for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
I != E; ++I)
deadPointers.erase(*I);
continue;
} else if (isInstructionTriviallyDead(BBI)) {
// For any non-memory-affecting non-terminators, DCE them as we reach them
Instruction *Inst = BBI;
++BBI;
DeleteDeadInstruction(Inst, &deadPointers);
++NumFastOther;
MadeChange = true;
continue;
}
if (!killPointer)
continue;
killPointer = killPointer->getUnderlyingObject();
// Deal with undead pointers
MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
deadPointers);
}
return MadeChange;
}
/// RemoveUndeadPointers - check for uses of a pointer that make it
/// undead when scanning for dead stores to alloca's.
bool DSE::RemoveUndeadPointers(Value *killPointer, uint64_t killPointerSize,
BasicBlock::iterator &BBI,
SmallPtrSet<Value*, 64> &deadPointers) {
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
// If the kill pointer can be easily reduced to an alloca,
// don't bother doing extraneous AA queries.
if (deadPointers.count(killPointer)) {
deadPointers.erase(killPointer);
return false;
}
// A global can't be in the dead pointer set.
if (isa<GlobalValue>(killPointer))
return false;
bool MadeChange = false;
SmallVector<Value*, 16> undead;
for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
E = deadPointers.end(); I != E; ++I) {
// See if this pointer could alias it
AliasAnalysis::AliasResult A = AA.alias(*I, getPointerSize(*I),
killPointer, killPointerSize);
// If it must-alias and a store, we can delete it
if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
StoreInst *S = cast<StoreInst>(BBI);
// Remove it!
++BBI;
DeleteDeadInstruction(S, &deadPointers);
++NumFastStores;
MadeChange = true;
continue;
// Otherwise, it is undead
} else if (A != AliasAnalysis::NoAlias)
undead.push_back(*I);
}
for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
I != E; ++I)
deadPointers.erase(*I);
return MadeChange;
}
/// DeleteDeadInstruction - Delete this instruction. Before we do, go through
/// and zero out all the operands of this instruction. If any of them become
/// dead, delete them and the computation tree that feeds them.
///
/// If ValueSet is non-null, remove any deleted instructions from it as well.
///
void DSE::DeleteDeadInstruction(Instruction *I,
SmallPtrSet<Value*, 64> *ValueSet) {
SmallVector<Instruction*, 32> NowDeadInsts;
NowDeadInsts.push_back(I);
--NumFastOther;
// Before we touch this instruction, remove it from memdep!
MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
++NumFastOther;
// This instruction is dead, zap it, in stages. Start by removing it from
// MemDep, which needs to know the operands and needs it to be in the
// function.
MDA.removeInstruction(DeadInst);
for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
Value *Op = DeadInst->getOperand(op);
DeadInst->setOperand(op, 0);
// If this operand just became dead, add it to the NowDeadInsts list.
if (!Op->use_empty()) continue;
if (Instruction *OpI = dyn_cast<Instruction>(Op))
if (isInstructionTriviallyDead(OpI))
NowDeadInsts.push_back(OpI);
}
DeadInst->eraseFromParent();
if (ValueSet) ValueSet->erase(DeadInst);
} while (!NowDeadInsts.empty());
}
uint64_t DSE::getPointerSize(Value *V) const {
if (TD) {
if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
// Get size information for the alloca
if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
} else {
assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
const PointerType *PT = cast<PointerType>(V->getType());
return TD->getTypeAllocSize(PT->getElementType());
}
}
return AliasAnalysis::UnknownSize;
}