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e9295510b5
llvm-mirror/lib/Transforms/Scalar/DeadStoreElimination.cpp
Chris Lattner e9295510b5 Reimplement the internal abstraction used by MemDep in terms 
of a pointer/int pair instead of a manually bitmangled pointer.
This forces clients to think a little more about checking the 
appropriate pieces and will be useful for internal 
implementation improvements later.

I'm not particularly happy with this.  After going through this
I don't think that the clients of memdep should be exposed to
the internal type at all.  I'll fix this in a subsequent commit.

This has no functionality change.

llvm-svn: 60230
2008-11-29 01:43:36 +00:00

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//===- 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/MemoryDependenceAnalysis.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");
namespace {
struct VISIBILITY_HIDDEN DSE : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass(&ID) {}
virtual bool runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
Changed |= runOnBasicBlock(*I);
return Changed;
}
typedef MemoryDependenceAnalysis::DepResultTy DepResultTy;
bool runOnBasicBlock(BasicBlock &BB);
bool handleFreeWithNonTrivialDependency(FreeInst *F, DepResultTy Dep);
bool handleEndBlock(BasicBlock &BB);
bool RemoveUndeadPointers(Value* pointer, 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<TargetData>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<AliasAnalysis>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
};
}
char DSE::ID = 0;
static RegisterPass<DSE> X("dse", "Dead Store Elimination");
FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
bool DSE::runOnBasicBlock(BasicBlock &BB) {
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
TargetData &TD = getAnalysis<TargetData>();
// Record the last-seen store to this pointer
DenseMap<Value*, StoreInst*> lastStore;
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++;
// If we find a store or a free...
if (!isa<StoreInst>(Inst) && !isa<FreeInst>(Inst))
continue;
Value* pointer = 0;
if (StoreInst* S = dyn_cast<StoreInst>(Inst)) {
if (S->isVolatile())
continue;
pointer = S->getPointerOperand();
} else {
pointer = cast<FreeInst>(Inst)->getPointerOperand();
}
pointer = pointer->stripPointerCasts();
StoreInst *&last = lastStore[pointer];
// ... to a pointer that has been stored to before...
if (last) {
DepResultTy dep = MD.getDependency(Inst);
bool deletedStore = false;
// ... and no other memory dependencies are between them....
while (dep.getInt() == MemoryDependenceAnalysis::Normal &&
isa<StoreInst>(dep.getPointer())) {
if (dep.getPointer() != last ||
TD.getTypeStoreSize(last->getOperand(0)->getType()) >
TD.getTypeStoreSize(Inst->getOperand(0)->getType())) {
dep = MD.getDependency(Inst, dep.getPointer());
continue;
}
// Delete the store and now-dead instructions that feed it.
DeleteDeadInstruction(last);
NumFastStores++;
deletedStore = true;
MadeChange = true;
break;
}
// If we deleted a store, reinvestigate this instruction.
if (deletedStore) {
if (!isa<TerminatorInst>(BB.begin()))
--BBI;
continue;
}
}
// Handle frees whose dependencies are non-trivial.
if (FreeInst* F = dyn_cast<FreeInst>(Inst)) {
MadeChange |= handleFreeWithNonTrivialDependency(F, MD.getDependency(F));
// No known stores after the free.
last = 0;
} else {
StoreInst* S = cast<StoreInst>(Inst);
// If we're storing the same value back to a pointer that we just
// loaded from, then the store can be removed;
if (LoadInst* L = dyn_cast<LoadInst>(S->getOperand(0))) {
// FIXME: Don't do dep query if Parents don't match and other stuff!
DepResultTy dep = MD.getDependency(S);
DominatorTree& DT = getAnalysis<DominatorTree>();
if (!S->isVolatile() && S->getParent() == L->getParent() &&
S->getPointerOperand() == L->getPointerOperand() &&
(dep.getInt() == MemoryDependenceAnalysis::None ||
dep.getInt() == MemoryDependenceAnalysis::NonLocal ||
DT.dominates(dep.getPointer(), L))) {
DeleteDeadInstruction(S);
if (!isa<TerminatorInst>(BB.begin()))
--BBI;
NumFastStores++;
MadeChange = true;
} else
// Update our most-recent-store map.
last = S;
} else
// Update our most-recent-store map.
last = S;
}
}
// 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;
}
/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
/// dependency is a store to a field of that structure.
bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, DepResultTy dep) {
TargetData &TD = getAnalysis<TargetData>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
if (dep.getInt() == MemoryDependenceAnalysis::None ||
dep.getInt() == MemoryDependenceAnalysis::NonLocal)
return false;
StoreInst* dependency = dyn_cast<StoreInst>(dep.getPointer());
if (!dependency)
return false;
else if (dependency->isVolatile())
return false;
Value* depPointer = dependency->getPointerOperand();
const Type* depType = dependency->getOperand(0)->getType();
unsigned depPointerSize = TD.getTypeStoreSize(depType);
// Check for aliasing
AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0U,
depPointer, depPointerSize);
if (A != AliasAnalysis::MustAlias)
return false;
// DCE instructions only used to calculate that store
DeleteDeadInstruction(dependency);
NumFastStores++;
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) {
TargetData &TD = getAnalysis<TargetData>();
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 (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
if (!S->isVolatile()) {
// See through pointer-to-pointer bitcasts
Value* pointerOperand = S->getPointerOperand()->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.
BBI++;
DeleteDeadInstruction(S, &deadPointers);
NumFastStores++;
MadeChange = true;
}
}
continue;
}
// We can also remove memcpy's to local variables at the end of a function.
if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
Value *dest = M->getDest()->getUnderlyingObject();
if (deadPointers.count(dest)) {
BBI++;
DeleteDeadInstruction(M, &deadPointers);
NumFastOther++;
MadeChange = true;
continue;
}
// Because a memcpy is also a load, we can't skip it if we didn't remove
// it.
}
Value* killPointer = 0;
uint64_t killPointerSize = ~0UL;
// 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<MemCpyInst>(BBI) &&
isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
killPointer = cast<MemCpyInst>(BBI)->getSource();
killPointerSize = cast<ConstantInt>(
cast<MemCpyInst>(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::get(BBI).getInstruction() != 0) {
// If this call does not access memory, it can't
// be undeadifying any of our pointers.
CallSite CS = CallSite::get(BBI);
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;
}
// Get size information for the alloca
unsigned pointerSize = ~0U;
if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
pointerSize = C->getZExtValue() *
TD.getABITypeSize(A->getAllocatedType());
} else {
const PointerType* PT = cast<PointerType>(
cast<Argument>(*I)->getType());
pointerSize = TD.getABITypeSize(PT->getElementType());
}
// See if the call site touches it
AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
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) {
TargetData &TD = getAnalysis<TargetData>();
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) {
// Get size information for the alloca.
unsigned pointerSize = ~0U;
if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
pointerSize = C->getZExtValue() *
TD.getABITypeSize(A->getAllocatedType());
} else {
const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
pointerSize = TD.getABITypeSize(PT->getElementType());
}
// See if this pointer could alias it
AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
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>();
while (!NowDeadInsts.empty()) {
Instruction *DeadInst = NowDeadInsts.back();
NowDeadInsts.pop_back();
++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);
}
}
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