llvm-6502/lib/Transforms/Scalar/DeadStoreElimination.cpp
2007-12-29 20:36:04 +00:00

463 lines
16 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/Pass.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.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((intptr_t)&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;
}
bool runOnBasicBlock(BasicBlock &BB);
bool handleFreeWithNonTrivialDependency(FreeInst* F,
Instruction* dependency,
SetVector<Instruction*>& possiblyDead);
bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
bool RemoveUndeadPointers(Value* pointer,
BasicBlock::iterator& BBI,
SmallPtrSet<AllocaInst*, 64>& deadPointers,
SetVector<Instruction*>& possiblyDead);
void DeleteDeadInstructionChains(Instruction *I,
SetVector<Instruction*> &DeadInsts);
/// Find the base pointer that a pointer came from
/// Because this is used to find pointers that originate
/// from allocas, it is safe to ignore GEP indices, since
/// either the store will be in the alloca, and thus dead,
/// or beyond the end of the alloca, and thus undefined.
void TranslatePointerBitCasts(Value*& v, bool zeroGepsOnly = false) {
assert(isa<PointerType>(v->getType()) &&
"Translating a non-pointer type?");
while (true) {
if (BitCastInst* C = dyn_cast<BitCastInst>(v))
v = C->getOperand(0);
else if (GetElementPtrInst* G = dyn_cast<GetElementPtrInst>(v))
if (!zeroGepsOnly || G->hasAllZeroIndices()) {
v = G->getOperand(0);
} else {
break;
}
else
break;
}
}
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<TargetData>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addPreserved<AliasAnalysis>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
};
char DSE::ID = 0;
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;
// Record instructions possibly made dead by deleting a store
SetVector<Instruction*> possiblyDead;
bool MadeChange = false;
// Do a top-down walk on the BB
for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end();
BBI != BBE; ++BBI) {
// If we find a store or a free...
if (!isa<StoreInst>(BBI) && !isa<FreeInst>(BBI))
continue;
Value* pointer = 0;
if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
if (!S->isVolatile())
pointer = S->getPointerOperand();
else
continue;
} else
pointer = cast<FreeInst>(BBI)->getPointerOperand();
TranslatePointerBitCasts(pointer, true);
StoreInst*& last = lastStore[pointer];
bool deletedStore = false;
// ... to a pointer that has been stored to before...
if (last) {
Instruction* dep = MD.getDependency(BBI);
// ... and no other memory dependencies are between them....
while (dep != MemoryDependenceAnalysis::None &&
dep != MemoryDependenceAnalysis::NonLocal &&
isa<StoreInst>(dep)) {
if (dep != last ||
TD.getTypeStoreSize(last->getOperand(0)->getType()) >
TD.getTypeStoreSize(BBI->getOperand(0)->getType())) {
dep = MD.getDependency(BBI, dep);
continue;
}
// Remove it!
MD.removeInstruction(last);
// DCE instructions only used to calculate that store
if (Instruction* D = dyn_cast<Instruction>(last->getOperand(0)))
possiblyDead.insert(D);
if (Instruction* D = dyn_cast<Instruction>(last->getOperand(1)))
possiblyDead.insert(D);
last->eraseFromParent();
NumFastStores++;
deletedStore = true;
MadeChange = true;
break;
}
}
// Handle frees whose dependencies are non-trivial.
if (FreeInst* F = dyn_cast<FreeInst>(BBI)) {
if (!deletedStore)
MadeChange |= handleFreeWithNonTrivialDependency(F,
MD.getDependency(F),
possiblyDead);
// No known stores after the free
last = 0;
} else {
// Update our most-recent-store map.
last = cast<StoreInst>(BBI);
}
}
// If this block ends in a return, unwind, unreachable, and eventually
// tailcall, then all allocas are dead at its end.
if (BB.getTerminator()->getNumSuccessors() == 0)
MadeChange |= handleEndBlock(BB, possiblyDead);
// Do a trivial DCE
while (!possiblyDead.empty()) {
Instruction *I = possiblyDead.back();
possiblyDead.pop_back();
DeleteDeadInstructionChains(I, possiblyDead);
}
return MadeChange;
}
/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
/// dependency is a store to a field of that structure
bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, Instruction* dep,
SetVector<Instruction*>& possiblyDead) {
TargetData &TD = getAnalysis<TargetData>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
if (dep == MemoryDependenceAnalysis::None ||
dep == MemoryDependenceAnalysis::NonLocal)
return false;
StoreInst* dependency = dyn_cast<StoreInst>(dep);
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(), ~0UL,
depPointer, depPointerSize);
if (A == AliasAnalysis::MustAlias) {
// Remove it!
MD.removeInstruction(dependency);
// DCE instructions only used to calculate that store
if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(0)))
possiblyDead.insert(D);
if (Instruction* D = dyn_cast<Instruction>(dependency->getOperand(1)))
possiblyDead.insert(D);
dependency->eraseFromParent();
NumFastStores++;
return true;
}
return false;
}
/// 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,
SetVector<Instruction*>& possiblyDead) {
TargetData &TD = getAnalysis<TargetData>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
bool MadeChange = false;
// Pointers alloca'd in this function are dead in the end block
SmallPtrSet<AllocaInst*, 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);
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
--BBI;
if (deadPointers.empty())
break;
// If we find a store whose pointer is dead...
if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
if (!S->isVolatile()) {
Value* pointerOperand = S->getPointerOperand();
// See through pointer-to-pointer bitcasts
TranslatePointerBitCasts(pointerOperand);
if (isa<AllocaInst>(pointerOperand) &&
deadPointers.count(cast<AllocaInst>(pointerOperand))) {
// Remove it!
MD.removeInstruction(S);
// DCE instructions only used to calculate that store
if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
possiblyDead.insert(D);
if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
possiblyDead.insert(D);
BBI++;
S->eraseFromParent();
NumFastStores++;
MadeChange = true;
}
}
continue;
}
Value* killPointer = 0;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst* L = dyn_cast<LoadInst>(BBI)) {
killPointer = L->getPointerOperand();
} else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
killPointer = V->getOperand(0);
} else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
deadPointers.erase(A);
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<Instruction*> dead;
for (SmallPtrSet<AllocaInst*, 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 = ~0UL;
if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
pointerSize = C->getZExtValue() * \
TD.getABITypeSize((*I)->getAllocatedType());
// 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<Instruction*>::iterator I = dead.begin(), E = dead.end();
I != E; ++I)
if (AllocaInst *AI = dyn_cast<AllocaInst>(*I))
deadPointers.erase(AI);
continue;
}
if (!killPointer)
continue;
TranslatePointerBitCasts(killPointer);
// Deal with undead pointers
MadeChange |= RemoveUndeadPointers(killPointer, BBI,
deadPointers, possiblyDead);
}
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,
BasicBlock::iterator& BBI,
SmallPtrSet<AllocaInst*, 64>& deadPointers,
SetVector<Instruction*>& possiblyDead) {
TargetData &TD = getAnalysis<TargetData>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
// If the kill pointer can be easily reduced to an alloca,
// don't bother doing extraneous AA queries
if (AllocaInst* A = dyn_cast<AllocaInst>(killPointer)) {
if (deadPointers.count(A))
deadPointers.erase(A);
return false;
} else if (isa<GlobalValue>(killPointer)) {
// A global can't be in the dead pointer set
return false;
}
bool MadeChange = false;
std::vector<Instruction*> undead;
for (SmallPtrSet<AllocaInst*, 64>::iterator I = deadPointers.begin(),
E = deadPointers.end(); I != E; ++I) {
// Get size information for the alloca
unsigned pointerSize = ~0UL;
if (ConstantInt* C = dyn_cast<ConstantInt>((*I)->getArraySize()))
pointerSize = C->getZExtValue() * \
TD.getABITypeSize((*I)->getAllocatedType());
// See if this pointer could alias it
AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
killPointer, ~0UL);
// 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!
MD.removeInstruction(S);
// DCE instructions only used to calculate that store
if (Instruction* D = dyn_cast<Instruction>(S->getOperand(0)))
possiblyDead.insert(D);
if (Instruction* D = dyn_cast<Instruction>(S->getOperand(1)))
possiblyDead.insert(D);
BBI++;
S->eraseFromParent();
NumFastStores++;
MadeChange = true;
continue;
// Otherwise, it is undead
} else if (A != AliasAnalysis::NoAlias)
undead.push_back(*I);
}
for (std::vector<Instruction*>::iterator I = undead.begin(), E = undead.end();
I != E; ++I)
if (AllocaInst *AI = dyn_cast<AllocaInst>(*I))
deadPointers.erase(AI);
return MadeChange;
}
/// DeleteDeadInstructionChains - takes an instruction and a setvector of
/// dead instructions. If I is dead, it is erased, and its operands are
/// checked for deadness. If they are dead, they are added to the dead
/// setvector.
void DSE::DeleteDeadInstructionChains(Instruction *I,
SetVector<Instruction*> &DeadInsts) {
// Instruction must be dead.
if (!I->use_empty() || !isInstructionTriviallyDead(I)) return;
// Let the memory dependence know
getAnalysis<MemoryDependenceAnalysis>().removeInstruction(I);
// See if this made any operands dead. We do it this way in case the
// instruction uses the same operand twice. We don't want to delete a
// value then reference it.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
if (I->getOperand(i)->hasOneUse())
if (Instruction* Op = dyn_cast<Instruction>(I->getOperand(i)))
DeadInsts.insert(Op); // Attempt to nuke it later.
I->setOperand(i, 0); // Drop from the operand list.
}
I->eraseFromParent();
++NumFastOther;
}