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//===- DeadStoreElimination.cpp - Dead Store Elimination ------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by Owen Anderson and 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 "fdse"
#include "llvm/Transforms/Scalar.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 FDSE : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
FDSE() : 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, StoreInst* dependency,
SetVector<Instruction*>& possiblyDead);
bool handleEndBlock(BasicBlock& BB, SetVector<Instruction*>& possiblyDead);
void DeleteDeadInstructionChains(Instruction *I,
SetVector<Instruction*> &DeadInsts);
// 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 FDSE::ID = 0;
RegisterPass<FDSE> X("fdse", "Fast Dead Store Elimination");
}
FunctionPass *llvm::createFastDeadStoreEliminationPass() { return new FDSE(); }
bool FDSE::runOnBasicBlock(BasicBlock &BB) {
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
// 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)) {
Value* pointer = 0;
if (StoreInst* S = dyn_cast<StoreInst>(BBI))
pointer = S->getPointerOperand();
else if (FreeInst* F = dyn_cast<FreeInst>(BBI))
pointer = F->getPointerOperand();
assert(pointer && "Not a free or a store?");
StoreInst*& last = lastStore[pointer];
// ... to a pointer that has been stored to before...
if (last) {
// ... and no other memory dependencies are between them....
if (MD.getDependency(BBI) == last) {
// 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);
last->eraseFromParent();
NumFastStores++;
MadeChange = true;
// If this is a free, check for a non-trivial dependency
} else if (FreeInst* F = dyn_cast<FreeInst>(BBI))
MadeChange |= handleFreeWithNonTrivialDependency(F, last, possiblyDead);
}
// Update our most-recent-store map
if (StoreInst* S = dyn_cast<StoreInst>(BBI))
last = S;
else
last = 0;
}
}
// 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 FDSE::handleFreeWithNonTrivialDependency(FreeInst* F, StoreInst* dependency,
SetVector<Instruction*>& possiblyDead) {
TargetData &TD = getAnalysis<TargetData>();
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
Value* depPointer = dependency->getPointerOperand();
unsigned depPointerSize = TD.getTypeSize(dependency->getOperand(0)->getType());
// 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);
dependency->eraseFromParent();
NumFastStores++;
return true;
}
return false;
}
void FDSE::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;
}
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