forked from OSchip/llvm-project
476 lines
16 KiB
C++
476 lines
16 KiB
C++
//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a trivial dead store elimination that only considers
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// basic-block local redundant stores.
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//
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// FIXME: This should eventually be extended to be a post-dominator tree
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// traversal. Doing so would be pretty trivial.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "dse"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/MallocHelper.h"
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#include "llvm/Analysis/MemoryDependenceAnalysis.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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STATISTIC(NumFastStores, "Number of stores deleted");
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STATISTIC(NumFastOther , "Number of other instrs removed");
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namespace {
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struct DSE : public FunctionPass {
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TargetData *TD;
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static char ID; // Pass identification, replacement for typeid
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DSE() : FunctionPass(&ID) {}
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virtual bool runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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Changed |= runOnBasicBlock(*I);
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return Changed;
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}
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bool runOnBasicBlock(BasicBlock &BB);
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bool handleFreeWithNonTrivialDependency(Instruction *F, MemDepResult Dep);
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bool handleEndBlock(BasicBlock &BB);
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bool RemoveUndeadPointers(Value* Ptr, uint64_t killPointerSize,
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BasicBlock::iterator& BBI,
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SmallPtrSet<Value*, 64>& deadPointers);
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void DeleteDeadInstruction(Instruction *I,
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SmallPtrSet<Value*, 64> *deadPointers = 0);
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// getAnalysisUsage - We require post dominance frontiers (aka Control
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// Dependence Graph)
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<DominatorTree>();
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AU.addRequired<AliasAnalysis>();
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AU.addRequired<MemoryDependenceAnalysis>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<AliasAnalysis>();
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AU.addPreserved<MemoryDependenceAnalysis>();
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}
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};
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}
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char DSE::ID = 0;
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static RegisterPass<DSE> X("dse", "Dead Store Elimination");
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FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
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bool DSE::runOnBasicBlock(BasicBlock &BB) {
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MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
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TD = getAnalysisIfAvailable<TargetData>();
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bool MadeChange = false;
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// Do a top-down walk on the BB.
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for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
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Instruction *Inst = BBI++;
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// If we find a store or a free, get its memory dependence.
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if (!isa<StoreInst>(Inst) && !isFreeCall(Inst))
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continue;
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// Don't molest volatile stores or do queries that will return "clobber".
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if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
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if (SI->isVolatile())
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continue;
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MemDepResult InstDep = MD.getDependency(Inst);
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// Ignore non-local stores.
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// FIXME: cross-block DSE would be fun. :)
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if (InstDep.isNonLocal()) continue;
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// Handle frees whose dependencies are non-trivial.
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if (isFreeCall(Inst)) {
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MadeChange |= handleFreeWithNonTrivialDependency(Inst, InstDep);
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continue;
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}
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StoreInst *SI = cast<StoreInst>(Inst);
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// If not a definite must-alias dependency, ignore it.
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if (!InstDep.isDef())
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continue;
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// If this is a store-store dependence, then the previous store is dead so
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// long as this store is at least as big as it.
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if (StoreInst *DepStore = dyn_cast<StoreInst>(InstDep.getInst()))
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if (TD &&
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TD->getTypeStoreSize(DepStore->getOperand(0)->getType()) <=
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TD->getTypeStoreSize(SI->getOperand(0)->getType())) {
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// Delete the store and now-dead instructions that feed it.
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DeleteDeadInstruction(DepStore);
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NumFastStores++;
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MadeChange = true;
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// DeleteDeadInstruction can delete the current instruction in loop
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// cases, reset BBI.
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BBI = Inst;
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if (BBI != BB.begin())
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--BBI;
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continue;
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}
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// If we're storing the same value back to a pointer that we just
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// loaded from, then the store can be removed.
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if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
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if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
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SI->getOperand(0) == DepLoad) {
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// DeleteDeadInstruction can delete the current instruction. Save BBI
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// in case we need it.
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WeakVH NextInst(BBI);
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DeleteDeadInstruction(SI);
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if (NextInst == 0) // Next instruction deleted.
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BBI = BB.begin();
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else if (BBI != BB.begin()) // Revisit this instruction if possible.
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--BBI;
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NumFastStores++;
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MadeChange = true;
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continue;
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}
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}
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}
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// If this block ends in a return, unwind, or unreachable, all allocas are
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// dead at its end, which means stores to them are also dead.
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if (BB.getTerminator()->getNumSuccessors() == 0)
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MadeChange |= handleEndBlock(BB);
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return MadeChange;
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}
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/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
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/// dependency is a store to a field of that structure.
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bool DSE::handleFreeWithNonTrivialDependency(Instruction *F, MemDepResult Dep) {
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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StoreInst *Dependency = dyn_cast_or_null<StoreInst>(Dep.getInst());
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if (!Dependency || Dependency->isVolatile())
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return false;
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Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject();
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// Check for aliasing.
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if (AA.alias(F->getOperand(1), 1, DepPointer, 1) !=
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AliasAnalysis::MustAlias)
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return false;
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// DCE instructions only used to calculate that store
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DeleteDeadInstruction(Dependency);
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NumFastStores++;
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return true;
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}
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/// handleEndBlock - Remove dead stores to stack-allocated locations in the
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/// function end block. Ex:
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/// %A = alloca i32
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/// ...
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/// store i32 1, i32* %A
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/// ret void
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bool DSE::handleEndBlock(BasicBlock &BB) {
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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bool MadeChange = false;
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// Pointers alloca'd in this function are dead in the end block
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SmallPtrSet<Value*, 64> deadPointers;
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// Find all of the alloca'd pointers in the entry block.
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BasicBlock *Entry = BB.getParent()->begin();
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for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
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if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
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deadPointers.insert(AI);
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// Treat byval arguments the same, stores to them are dead at the end of the
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// function.
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for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
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AE = BB.getParent()->arg_end(); AI != AE; ++AI)
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if (AI->hasByValAttr())
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deadPointers.insert(AI);
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// Scan the basic block backwards
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for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
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--BBI;
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// If we find a store whose pointer is dead.
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if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
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if (!S->isVolatile()) {
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// See through pointer-to-pointer bitcasts
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Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();
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// Alloca'd pointers or byval arguments (which are functionally like
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// alloca's) are valid candidates for removal.
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if (deadPointers.count(pointerOperand)) {
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// DCE instructions only used to calculate that store.
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BBI++;
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DeleteDeadInstruction(S, &deadPointers);
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NumFastStores++;
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MadeChange = true;
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}
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}
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continue;
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}
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// We can also remove memcpy's to local variables at the end of a function.
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if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
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Value *dest = M->getDest()->getUnderlyingObject();
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if (deadPointers.count(dest)) {
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BBI++;
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DeleteDeadInstruction(M, &deadPointers);
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NumFastOther++;
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MadeChange = true;
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continue;
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}
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// Because a memcpy is also a load, we can't skip it if we didn't remove
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// it.
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}
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Value* killPointer = 0;
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uint64_t killPointerSize = ~0UL;
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// If we encounter a use of the pointer, it is no longer considered dead
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if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
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// However, if this load is unused and not volatile, we can go ahead and
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// remove it, and not have to worry about it making our pointer undead!
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if (L->use_empty() && !L->isVolatile()) {
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BBI++;
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DeleteDeadInstruction(L, &deadPointers);
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NumFastOther++;
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MadeChange = true;
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continue;
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}
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killPointer = L->getPointerOperand();
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} else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
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killPointer = V->getOperand(0);
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} else if (isa<MemCpyInst>(BBI) &&
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isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
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killPointer = cast<MemCpyInst>(BBI)->getSource();
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killPointerSize = cast<ConstantInt>(
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cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
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} else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
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deadPointers.erase(A);
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// Dead alloca's can be DCE'd when we reach them
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if (A->use_empty()) {
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BBI++;
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DeleteDeadInstruction(A, &deadPointers);
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NumFastOther++;
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MadeChange = true;
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}
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continue;
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} else if (CallSite::get(BBI).getInstruction() != 0) {
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// If this call does not access memory, it can't
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// be undeadifying any of our pointers.
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CallSite CS = CallSite::get(BBI);
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if (AA.doesNotAccessMemory(CS))
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continue;
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unsigned modRef = 0;
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unsigned other = 0;
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// Remove any pointers made undead by the call from the dead set
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std::vector<Value*> dead;
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for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
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E = deadPointers.end(); I != E; ++I) {
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// HACK: if we detect that our AA is imprecise, it's not
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// worth it to scan the rest of the deadPointers set. Just
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// assume that the AA will return ModRef for everything, and
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// go ahead and bail.
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if (modRef >= 16 && other == 0) {
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deadPointers.clear();
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return MadeChange;
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}
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// Get size information for the alloca
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unsigned pointerSize = ~0U;
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if (TD) {
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if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
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if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
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pointerSize = C->getZExtValue() *
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TD->getTypeAllocSize(A->getAllocatedType());
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} else {
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const PointerType* PT = cast<PointerType>(
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cast<Argument>(*I)->getType());
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pointerSize = TD->getTypeAllocSize(PT->getElementType());
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}
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}
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// See if the call site touches it
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AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
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if (A == AliasAnalysis::ModRef)
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modRef++;
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else
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other++;
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if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
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dead.push_back(*I);
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}
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for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
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I != E; ++I)
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deadPointers.erase(*I);
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continue;
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} else if (isInstructionTriviallyDead(BBI)) {
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// For any non-memory-affecting non-terminators, DCE them as we reach them
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Instruction *Inst = BBI;
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BBI++;
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DeleteDeadInstruction(Inst, &deadPointers);
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NumFastOther++;
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MadeChange = true;
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continue;
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}
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if (!killPointer)
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continue;
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killPointer = killPointer->getUnderlyingObject();
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// Deal with undead pointers
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MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
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deadPointers);
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}
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return MadeChange;
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}
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/// RemoveUndeadPointers - check for uses of a pointer that make it
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/// undead when scanning for dead stores to alloca's.
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bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
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BasicBlock::iterator &BBI,
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SmallPtrSet<Value*, 64>& deadPointers) {
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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// If the kill pointer can be easily reduced to an alloca,
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// don't bother doing extraneous AA queries.
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if (deadPointers.count(killPointer)) {
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deadPointers.erase(killPointer);
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return false;
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}
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// A global can't be in the dead pointer set.
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if (isa<GlobalValue>(killPointer))
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return false;
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bool MadeChange = false;
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SmallVector<Value*, 16> undead;
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for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
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E = deadPointers.end(); I != E; ++I) {
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// Get size information for the alloca.
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unsigned pointerSize = ~0U;
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if (TD) {
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if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
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if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
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pointerSize = C->getZExtValue() *
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TD->getTypeAllocSize(A->getAllocatedType());
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} else {
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const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
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pointerSize = TD->getTypeAllocSize(PT->getElementType());
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}
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}
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// See if this pointer could alias it
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AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
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killPointer, killPointerSize);
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// If it must-alias and a store, we can delete it
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if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
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StoreInst* S = cast<StoreInst>(BBI);
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// Remove it!
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BBI++;
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DeleteDeadInstruction(S, &deadPointers);
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NumFastStores++;
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MadeChange = true;
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continue;
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// Otherwise, it is undead
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} else if (A != AliasAnalysis::NoAlias)
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undead.push_back(*I);
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}
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for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
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I != E; ++I)
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deadPointers.erase(*I);
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return MadeChange;
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}
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/// DeleteDeadInstruction - Delete this instruction. Before we do, go through
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/// and zero out all the operands of this instruction. If any of them become
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/// dead, delete them and the computation tree that feeds them.
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///
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/// If ValueSet is non-null, remove any deleted instructions from it as well.
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///
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void DSE::DeleteDeadInstruction(Instruction *I,
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SmallPtrSet<Value*, 64> *ValueSet) {
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SmallVector<Instruction*, 32> NowDeadInsts;
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NowDeadInsts.push_back(I);
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--NumFastOther;
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// Before we touch this instruction, remove it from memdep!
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MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
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while (!NowDeadInsts.empty()) {
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Instruction *DeadInst = NowDeadInsts.back();
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NowDeadInsts.pop_back();
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++NumFastOther;
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// This instruction is dead, zap it, in stages. Start by removing it from
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// MemDep, which needs to know the operands and needs it to be in the
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// function.
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MDA.removeInstruction(DeadInst);
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for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
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Value *Op = DeadInst->getOperand(op);
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DeadInst->setOperand(op, 0);
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// If this operand just became dead, add it to the NowDeadInsts list.
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if (!Op->use_empty()) continue;
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if (Instruction *OpI = dyn_cast<Instruction>(Op))
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if (isInstructionTriviallyDead(OpI))
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NowDeadInsts.push_back(OpI);
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}
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DeadInst->eraseFromParent();
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if (ValueSet) ValueSet->erase(DeadInst);
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}
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}
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