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reference to a pointer, so that it can handle the case where DataLayout is not available and behave conservatively. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174024 91177308-0d34-0410-b5e6-96231b3b80d8
901 lines
32 KiB
C++
901 lines
32 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/ADT/STLExtras.h"
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#include "llvm/ADT/SetVector.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/CaptureTracking.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/MemoryBuiltins.h"
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#include "llvm/Analysis/MemoryDependenceAnalysis.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/GlobalVariable.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Target/TargetLibraryInfo.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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AliasAnalysis *AA;
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MemoryDependenceAnalysis *MD;
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DominatorTree *DT;
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const TargetLibraryInfo *TLI;
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static char ID; // Pass identification, replacement for typeid
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DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) {
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initializeDSEPass(*PassRegistry::getPassRegistry());
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}
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virtual bool runOnFunction(Function &F) {
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AA = &getAnalysis<AliasAnalysis>();
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MD = &getAnalysis<MemoryDependenceAnalysis>();
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DT = &getAnalysis<DominatorTree>();
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TLI = AA->getTargetLibraryInfo();
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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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// Only check non-dead blocks. Dead blocks may have strange pointer
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// cycles that will confuse alias analysis.
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if (DT->isReachableFromEntry(I))
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Changed |= runOnBasicBlock(*I);
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AA = 0; MD = 0; DT = 0;
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return Changed;
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}
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bool runOnBasicBlock(BasicBlock &BB);
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bool HandleFree(CallInst *F);
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bool handleEndBlock(BasicBlock &BB);
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void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
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SmallSetVector<Value*, 16> &DeadStackObjects);
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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<AliasAnalysis>();
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AU.addPreserved<DominatorTree>();
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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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INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTree)
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INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
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INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
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INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
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FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
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//===----------------------------------------------------------------------===//
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// Helper functions
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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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static void DeleteDeadInstruction(Instruction *I,
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MemoryDependenceAnalysis &MD,
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const TargetLibraryInfo *TLI,
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SmallSetVector<Value*, 16> *ValueSet = 0) {
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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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do {
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Instruction *DeadInst = NowDeadInsts.pop_back_val();
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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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MD.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, TLI))
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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->remove(DeadInst);
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} while (!NowDeadInsts.empty());
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}
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/// hasMemoryWrite - Does this instruction write some memory? This only returns
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/// true for things that we can analyze with other helpers below.
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static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI) {
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if (isa<StoreInst>(I))
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return true;
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default:
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return false;
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case Intrinsic::memset:
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case Intrinsic::memmove:
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case Intrinsic::memcpy:
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case Intrinsic::init_trampoline:
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case Intrinsic::lifetime_end:
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return true;
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}
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}
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if (CallSite CS = I) {
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if (Function *F = CS.getCalledFunction()) {
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if (TLI && TLI->has(LibFunc::strcpy) &&
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F->getName() == TLI->getName(LibFunc::strcpy)) {
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return true;
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}
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if (TLI && TLI->has(LibFunc::strncpy) &&
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F->getName() == TLI->getName(LibFunc::strncpy)) {
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return true;
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}
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if (TLI && TLI->has(LibFunc::strcat) &&
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F->getName() == TLI->getName(LibFunc::strcat)) {
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return true;
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}
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if (TLI && TLI->has(LibFunc::strncat) &&
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F->getName() == TLI->getName(LibFunc::strncat)) {
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return true;
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}
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}
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}
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return false;
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}
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/// getLocForWrite - Return a Location stored to by the specified instruction.
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/// If isRemovable returns true, this function and getLocForRead completely
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/// describe the memory operations for this instruction.
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static AliasAnalysis::Location
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getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
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if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
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return AA.getLocation(SI);
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if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
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// memcpy/memmove/memset.
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AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
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// If we don't have target data around, an unknown size in Location means
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// that we should use the size of the pointee type. This isn't valid for
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// memset/memcpy, which writes more than an i8.
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if (Loc.Size == AliasAnalysis::UnknownSize && AA.getDataLayout() == 0)
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return AliasAnalysis::Location();
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return Loc;
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}
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IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
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if (II == 0) return AliasAnalysis::Location();
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switch (II->getIntrinsicID()) {
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default: return AliasAnalysis::Location(); // Unhandled intrinsic.
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case Intrinsic::init_trampoline:
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// If we don't have target data around, an unknown size in Location means
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// that we should use the size of the pointee type. This isn't valid for
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// init.trampoline, which writes more than an i8.
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if (AA.getDataLayout() == 0) return AliasAnalysis::Location();
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// FIXME: We don't know the size of the trampoline, so we can't really
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// handle it here.
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return AliasAnalysis::Location(II->getArgOperand(0));
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case Intrinsic::lifetime_end: {
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uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
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return AliasAnalysis::Location(II->getArgOperand(1), Len);
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}
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}
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}
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/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
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/// instruction if any.
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static AliasAnalysis::Location
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getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
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assert(hasMemoryWrite(Inst, AA.getTargetLibraryInfo()) &&
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"Unknown instruction case");
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// The only instructions that both read and write are the mem transfer
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// instructions (memcpy/memmove).
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if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
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return AA.getLocationForSource(MTI);
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return AliasAnalysis::Location();
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}
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/// isRemovable - If the value of this instruction and the memory it writes to
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/// is unused, may we delete this instruction?
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static bool isRemovable(Instruction *I) {
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// Don't remove volatile/atomic stores.
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if (StoreInst *SI = dyn_cast<StoreInst>(I))
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return SI->isUnordered();
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
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case Intrinsic::lifetime_end:
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// Never remove dead lifetime_end's, e.g. because it is followed by a
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// free.
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return false;
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case Intrinsic::init_trampoline:
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// Always safe to remove init_trampoline.
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return true;
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case Intrinsic::memset:
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case Intrinsic::memmove:
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case Intrinsic::memcpy:
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// Don't remove volatile memory intrinsics.
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return !cast<MemIntrinsic>(II)->isVolatile();
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}
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}
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if (CallSite CS = I)
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return CS.getInstruction()->use_empty();
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return false;
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}
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/// isShortenable - Returns true if this instruction can be safely shortened in
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/// length.
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static bool isShortenable(Instruction *I) {
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// Don't shorten stores for now
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if (isa<StoreInst>(I))
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return false;
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: return false;
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case Intrinsic::memset:
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case Intrinsic::memcpy:
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// Do shorten memory intrinsics.
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return true;
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}
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}
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// Don't shorten libcalls calls for now.
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return false;
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}
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/// getStoredPointerOperand - Return the pointer that is being written to.
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static Value *getStoredPointerOperand(Instruction *I) {
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if (StoreInst *SI = dyn_cast<StoreInst>(I))
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return SI->getPointerOperand();
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if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
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return MI->getDest();
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: llvm_unreachable("Unexpected intrinsic!");
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case Intrinsic::init_trampoline:
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return II->getArgOperand(0);
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}
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}
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CallSite CS = I;
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// All the supported functions so far happen to have dest as their first
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// argument.
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return CS.getArgument(0);
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}
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static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
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uint64_t Size;
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if (getObjectSize(V, Size, AA.getDataLayout(), AA.getTargetLibraryInfo()))
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return Size;
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return AliasAnalysis::UnknownSize;
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}
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namespace {
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enum OverwriteResult
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{
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OverwriteComplete,
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OverwriteEnd,
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OverwriteUnknown
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};
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}
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/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
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/// completely overwrites a store to the 'Earlier' location.
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/// 'OverwriteEnd' if the end of the 'Earlier' location is completely
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/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
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static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
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const AliasAnalysis::Location &Earlier,
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AliasAnalysis &AA,
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int64_t &EarlierOff,
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int64_t &LaterOff) {
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const Value *P1 = Earlier.Ptr->stripPointerCasts();
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const Value *P2 = Later.Ptr->stripPointerCasts();
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// If the start pointers are the same, we just have to compare sizes to see if
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// the later store was larger than the earlier store.
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if (P1 == P2) {
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// If we don't know the sizes of either access, then we can't do a
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// comparison.
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if (Later.Size == AliasAnalysis::UnknownSize ||
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Earlier.Size == AliasAnalysis::UnknownSize) {
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// If we have no DataLayout information around, then the size of the store
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// is inferrable from the pointee type. If they are the same type, then
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// we know that the store is safe.
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if (AA.getDataLayout() == 0 &&
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Later.Ptr->getType() == Earlier.Ptr->getType())
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return OverwriteComplete;
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return OverwriteUnknown;
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}
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// Make sure that the Later size is >= the Earlier size.
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if (Later.Size >= Earlier.Size)
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return OverwriteComplete;
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}
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// Otherwise, we have to have size information, and the later store has to be
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// larger than the earlier one.
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if (Later.Size == AliasAnalysis::UnknownSize ||
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Earlier.Size == AliasAnalysis::UnknownSize ||
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AA.getDataLayout() == 0)
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return OverwriteUnknown;
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// Check to see if the later store is to the entire object (either a global,
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// an alloca, or a byval argument). If so, then it clearly overwrites any
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// other store to the same object.
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const DataLayout *TD = AA.getDataLayout();
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const Value *UO1 = GetUnderlyingObject(P1, TD),
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*UO2 = GetUnderlyingObject(P2, TD);
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// If we can't resolve the same pointers to the same object, then we can't
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// analyze them at all.
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if (UO1 != UO2)
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return OverwriteUnknown;
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// If the "Later" store is to a recognizable object, get its size.
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uint64_t ObjectSize = getPointerSize(UO2, AA);
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if (ObjectSize != AliasAnalysis::UnknownSize)
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if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
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return OverwriteComplete;
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// Okay, we have stores to two completely different pointers. Try to
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// decompose the pointer into a "base + constant_offset" form. If the base
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// pointers are equal, then we can reason about the two stores.
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EarlierOff = 0;
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LaterOff = 0;
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const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
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const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
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// If the base pointers still differ, we have two completely different stores.
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if (BP1 != BP2)
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return OverwriteUnknown;
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// The later store completely overlaps the earlier store if:
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//
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// 1. Both start at the same offset and the later one's size is greater than
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// or equal to the earlier one's, or
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//
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// |--earlier--|
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// |-- later --|
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//
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// 2. The earlier store has an offset greater than the later offset, but which
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// still lies completely within the later store.
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//
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// |--earlier--|
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// |----- later ------|
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//
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// We have to be careful here as *Off is signed while *.Size is unsigned.
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if (EarlierOff >= LaterOff &&
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Later.Size >= Earlier.Size &&
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uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
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return OverwriteComplete;
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// The other interesting case is if the later store overwrites the end of
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// the earlier store
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//
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// |--earlier--|
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// |-- later --|
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//
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// In this case we may want to trim the size of earlier to avoid generating
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// writes to addresses which will definitely be overwritten later
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if (LaterOff > EarlierOff &&
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LaterOff < int64_t(EarlierOff + Earlier.Size) &&
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int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
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return OverwriteEnd;
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// Otherwise, they don't completely overlap.
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return OverwriteUnknown;
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}
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/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
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/// memory region into an identical pointer) then it doesn't actually make its
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/// input dead in the traditional sense. Consider this case:
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///
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/// memcpy(A <- B)
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/// memcpy(A <- A)
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///
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/// In this case, the second store to A does not make the first store to A dead.
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/// The usual situation isn't an explicit A<-A store like this (which can be
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/// trivially removed) but a case where two pointers may alias.
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///
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/// This function detects when it is unsafe to remove a dependent instruction
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/// because the DSE inducing instruction may be a self-read.
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static bool isPossibleSelfRead(Instruction *Inst,
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const AliasAnalysis::Location &InstStoreLoc,
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Instruction *DepWrite, AliasAnalysis &AA) {
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// Self reads can only happen for instructions that read memory. Get the
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// location read.
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AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
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if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
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// If the read and written loc obviously don't alias, it isn't a read.
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if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
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// Okay, 'Inst' may copy over itself. However, we can still remove a the
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// DepWrite instruction if we can prove that it reads from the same location
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// as Inst. This handles useful cases like:
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// memcpy(A <- B)
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// memcpy(A <- B)
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// Here we don't know if A/B may alias, but we do know that B/B are must
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// aliases, so removing the first memcpy is safe (assuming it writes <= #
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// bytes as the second one.
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AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
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if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
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return false;
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// If DepWrite doesn't read memory or if we can't prove it is a must alias,
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// then it can't be considered dead.
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return true;
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}
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//===----------------------------------------------------------------------===//
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// DSE Pass
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//===----------------------------------------------------------------------===//
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bool DSE::runOnBasicBlock(BasicBlock &BB) {
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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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// Handle 'free' calls specially.
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if (CallInst *F = isFreeCall(Inst, TLI)) {
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MadeChange |= HandleFree(F);
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continue;
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}
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// If we find something that writes memory, get its memory dependence.
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if (!hasMemoryWrite(Inst, TLI))
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continue;
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MemDepResult InstDep = MD->getDependency(Inst);
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// Ignore any store where we can't find a local dependence.
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// FIXME: cross-block DSE would be fun. :)
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if (!InstDep.isDef() && !InstDep.isClobber())
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continue;
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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 (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
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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 && isRemovable(SI)) {
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DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
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<< "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
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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, *MD, TLI);
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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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// Figure out what location is being stored to.
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AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
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// If we didn't get a useful location, fail.
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if (Loc.Ptr == 0)
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continue;
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while (InstDep.isDef() || InstDep.isClobber()) {
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// Get the memory clobbered by the instruction we depend on. MemDep will
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// skip any instructions that 'Loc' clearly doesn't interact with. If we
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// end up depending on a may- or must-aliased load, then we can't optimize
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// away the store and we bail out. However, if we depend on on something
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// that overwrites the memory location we *can* potentially optimize it.
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//
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// Find out what memory location the dependent instruction stores.
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Instruction *DepWrite = InstDep.getInst();
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AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
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// If we didn't get a useful location, or if it isn't a size, bail out.
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if (DepLoc.Ptr == 0)
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break;
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// If we find a write that is a) removable (i.e., non-volatile), b) is
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// completely obliterated by the store to 'Loc', and c) which we know that
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// 'Inst' doesn't load from, then we can remove it.
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if (isRemovable(DepWrite) &&
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!isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
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int64_t InstWriteOffset, DepWriteOffset;
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OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
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DepWriteOffset, InstWriteOffset);
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if (OR == OverwriteComplete) {
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DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
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<< *DepWrite << "\n KILLER: " << *Inst << '\n');
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// Delete the store and now-dead instructions that feed it.
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DeleteDeadInstruction(DepWrite, *MD, TLI);
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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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break;
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} else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
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// TODO: base this on the target vector size so that if the earlier
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// store was too small to get vector writes anyway then its likely
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// a good idea to shorten it
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// Power of 2 vector writes are probably always a bad idea to optimize
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// as any store/memset/memcpy is likely using vector instructions so
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// shortening it to not vector size is likely to be slower
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MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
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unsigned DepWriteAlign = DepIntrinsic->getAlignment();
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if (llvm::isPowerOf2_64(InstWriteOffset) ||
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((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
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DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
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<< *DepWrite << "\n KILLER (offset "
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<< InstWriteOffset << ", "
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<< DepLoc.Size << ")"
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<< *Inst << '\n');
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Value* DepWriteLength = DepIntrinsic->getLength();
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Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
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InstWriteOffset -
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DepWriteOffset);
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DepIntrinsic->setLength(TrimmedLength);
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MadeChange = true;
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}
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}
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}
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// If this is a may-aliased store that is clobbering the store value, we
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// can keep searching past it for another must-aliased pointer that stores
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// to the same location. For example, in:
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// store -> P
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// store -> Q
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// store -> P
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// we can remove the first store to P even though we don't know if P and Q
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// alias.
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if (DepWrite == &BB.front()) break;
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// Can't look past this instruction if it might read 'Loc'.
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if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
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break;
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InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
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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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/// Find all blocks that will unconditionally lead to the block BB and append
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/// them to F.
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static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
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BasicBlock *BB, DominatorTree *DT) {
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for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
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BasicBlock *Pred = *I;
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if (Pred == BB) continue;
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TerminatorInst *PredTI = Pred->getTerminator();
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if (PredTI->getNumSuccessors() != 1)
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continue;
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if (DT->isReachableFromEntry(Pred))
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Blocks.push_back(Pred);
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}
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}
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/// HandleFree - Handle frees of entire structures whose dependency is a store
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/// to a field of that structure.
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bool DSE::HandleFree(CallInst *F) {
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bool MadeChange = false;
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AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
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SmallVector<BasicBlock *, 16> Blocks;
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Blocks.push_back(F->getParent());
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while (!Blocks.empty()) {
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BasicBlock *BB = Blocks.pop_back_val();
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Instruction *InstPt = BB->getTerminator();
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if (BB == F->getParent()) InstPt = F;
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MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
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while (Dep.isDef() || Dep.isClobber()) {
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Instruction *Dependency = Dep.getInst();
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if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
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break;
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Value *DepPointer =
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GetUnderlyingObject(getStoredPointerOperand(Dependency));
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// Check for aliasing.
|
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if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
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break;
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Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
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// DCE instructions only used to calculate that store
|
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DeleteDeadInstruction(Dependency, *MD, TLI);
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++NumFastStores;
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MadeChange = true;
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|
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// Inst's old Dependency is now deleted. Compute the next dependency,
|
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// which may also be dead, as in
|
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// s[0] = 0;
|
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// s[1] = 0; // This has just been deleted.
|
|
// free(s);
|
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Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
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}
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|
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if (Dep.isNonLocal())
|
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FindUnconditionalPreds(Blocks, BB, DT);
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|
}
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|
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return MadeChange;
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}
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|
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namespace {
|
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struct CouldRef {
|
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typedef Value *argument_type;
|
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const CallSite CS;
|
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AliasAnalysis *AA;
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bool operator()(Value *I) {
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// See if the call site touches the value.
|
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AliasAnalysis::ModRefResult A =
|
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AA->getModRefInfo(CS, I, getPointerSize(I, *AA));
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return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
|
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}
|
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};
|
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}
|
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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
|
|
/// ...
|
|
/// store i32 1, i32* %A
|
|
/// ret void
|
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bool DSE::handleEndBlock(BasicBlock &BB) {
|
|
bool MadeChange = false;
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|
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// Keep track of all of the stack objects that are dead at the end of the
|
|
// function.
|
|
SmallSetVector<Value*, 16> DeadStackObjects;
|
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|
|
// 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 (isa<AllocaInst>(I))
|
|
DeadStackObjects.insert(I);
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|
|
// Okay, so these are dead heap objects, but if the pointer never escapes
|
|
// then it's leaked by this function anyways.
|
|
else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
|
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DeadStackObjects.insert(I);
|
|
}
|
|
|
|
// 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())
|
|
DeadStackObjects.insert(AI);
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|
|
// Scan the basic block backwards
|
|
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
|
|
--BBI;
|
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|
|
// If we find a store, check to see if it points into a dead stack value.
|
|
if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
|
|
// See through pointer-to-pointer bitcasts
|
|
SmallVector<Value *, 4> Pointers;
|
|
GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);
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|
|
// Stores to stack values are valid candidates for removal.
|
|
bool AllDead = true;
|
|
for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
|
|
E = Pointers.end(); I != E; ++I)
|
|
if (!DeadStackObjects.count(*I)) {
|
|
AllDead = false;
|
|
break;
|
|
}
|
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|
|
if (AllDead) {
|
|
Instruction *Dead = BBI++;
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|
|
DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
|
|
<< *Dead << "\n Objects: ";
|
|
for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
|
|
E = Pointers.end(); I != E; ++I) {
|
|
dbgs() << **I;
|
|
if (llvm::next(I) != E)
|
|
dbgs() << ", ";
|
|
}
|
|
dbgs() << '\n');
|
|
|
|
// DCE instructions only used to calculate that store.
|
|
DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
|
|
++NumFastStores;
|
|
MadeChange = true;
|
|
continue;
|
|
}
|
|
}
|
|
|
|
// Remove any dead non-memory-mutating instructions.
|
|
if (isInstructionTriviallyDead(BBI, TLI)) {
|
|
Instruction *Inst = BBI++;
|
|
DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
|
|
++NumFastOther;
|
|
MadeChange = true;
|
|
continue;
|
|
}
|
|
|
|
if (isa<AllocaInst>(BBI)) {
|
|
// Remove allocas from the list of dead stack objects; there can't be
|
|
// any references before the definition.
|
|
DeadStackObjects.remove(BBI);
|
|
continue;
|
|
}
|
|
|
|
if (CallSite CS = cast<Value>(BBI)) {
|
|
// Remove allocation function calls from the list of dead stack objects;
|
|
// there can't be any references before the definition.
|
|
if (isAllocLikeFn(BBI, TLI))
|
|
DeadStackObjects.remove(BBI);
|
|
|
|
// If this call does not access memory, it can't be loading any of our
|
|
// pointers.
|
|
if (AA->doesNotAccessMemory(CS))
|
|
continue;
|
|
|
|
// If the call might load from any of our allocas, then any store above
|
|
// the call is live.
|
|
CouldRef Pred = { CS, AA };
|
|
DeadStackObjects.remove_if(Pred);
|
|
|
|
// If all of the allocas were clobbered by the call then we're not going
|
|
// to find anything else to process.
|
|
if (DeadStackObjects.empty())
|
|
break;
|
|
|
|
continue;
|
|
}
|
|
|
|
AliasAnalysis::Location LoadedLoc;
|
|
|
|
// If we encounter a use of the pointer, it is no longer considered dead
|
|
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
|
|
if (!L->isUnordered()) // Be conservative with atomic/volatile load
|
|
break;
|
|
LoadedLoc = AA->getLocation(L);
|
|
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
|
|
LoadedLoc = AA->getLocation(V);
|
|
} else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
|
|
LoadedLoc = AA->getLocationForSource(MTI);
|
|
} else if (!BBI->mayReadFromMemory()) {
|
|
// Instruction doesn't read memory. Note that stores that weren't removed
|
|
// above will hit this case.
|
|
continue;
|
|
} else {
|
|
// Unknown inst; assume it clobbers everything.
|
|
break;
|
|
}
|
|
|
|
// Remove any allocas from the DeadPointer set that are loaded, as this
|
|
// makes any stores above the access live.
|
|
RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
|
|
|
|
// If all of the allocas were clobbered by the access then we're not going
|
|
// to find anything else to process.
|
|
if (DeadStackObjects.empty())
|
|
break;
|
|
}
|
|
|
|
return MadeChange;
|
|
}
|
|
|
|
namespace {
|
|
struct CouldAlias {
|
|
typedef Value *argument_type;
|
|
const AliasAnalysis::Location &LoadedLoc;
|
|
AliasAnalysis *AA;
|
|
|
|
bool operator()(Value *I) {
|
|
// See if the loaded location could alias the stack location.
|
|
AliasAnalysis::Location StackLoc(I, getPointerSize(I, *AA));
|
|
return !AA->isNoAlias(StackLoc, LoadedLoc);
|
|
}
|
|
};
|
|
}
|
|
|
|
/// RemoveAccessedObjects - Check to see if the specified location may alias any
|
|
/// of the stack objects in the DeadStackObjects set. If so, they become live
|
|
/// because the location is being loaded.
|
|
void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
|
|
SmallSetVector<Value*, 16> &DeadStackObjects) {
|
|
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
|
|
|
|
// A constant can't be in the dead pointer set.
|
|
if (isa<Constant>(UnderlyingPointer))
|
|
return;
|
|
|
|
// If the kill pointer can be easily reduced to an alloca, don't bother doing
|
|
// extraneous AA queries.
|
|
if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
|
|
DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
|
|
return;
|
|
}
|
|
|
|
// Remove objects that could alias LoadedLoc.
|
|
CouldAlias Pred = { LoadedLoc, AA };
|
|
DeadStackObjects.remove_if(Pred);
|
|
}
|