142 строки
5.1 KiB
C++
142 строки
5.1 KiB
C++
//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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///////////////////////////////////////////////////////////////////////////////
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// //
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// DCE.cpp //
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// Copyright (C) Microsoft Corporation. All rights reserved. //
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// Licensed under the MIT license. See COPYRIGHT in the project root for //
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// full license information. //
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// //
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// This file implements dead inst elimination and dead code elimination. //
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// //
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// Dead Inst Elimination performs a single pass over the function removing //
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// instructions that are obviously dead. Dead Code Elimination is similar, but//
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// it rechecks instructions that were used by removed instructions to see if //
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// they are newly dead. //
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// //
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///////////////////////////////////////////////////////////////////////////////
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/IR/InstIterator.h"
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#include "llvm/IR/Instruction.h"
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#include "llvm/Pass.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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#define DEBUG_TYPE "dce"
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STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
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STATISTIC(DCEEliminated, "Number of insts removed");
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namespace {
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//===--------------------------------------------------------------------===//
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// DeadInstElimination pass implementation
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//
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struct DeadInstElimination : public BasicBlockPass {
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static char ID; // Pass identification, replacement for typeid
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DeadInstElimination() : BasicBlockPass(ID) {
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initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
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}
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bool runOnBasicBlock(BasicBlock &BB) override {
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if (skipOptnoneFunction(BB))
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return false;
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auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
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TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
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bool Changed = false;
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for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
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Instruction *Inst = DI++;
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if (isInstructionTriviallyDead(Inst, TLI)) {
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Inst->eraseFromParent();
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Changed = true;
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++DIEEliminated;
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}
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}
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return Changed;
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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}
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};
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}
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char DeadInstElimination::ID = 0;
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INITIALIZE_PASS(DeadInstElimination, "die",
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"Dead Instruction Elimination", false, false)
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Pass *llvm::createDeadInstEliminationPass() {
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return new DeadInstElimination();
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}
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namespace {
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//===--------------------------------------------------------------------===//
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// DeadCodeElimination pass implementation
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//
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struct DCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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DCE() : FunctionPass(ID) {
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initializeDCEPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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}
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};
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}
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char DCE::ID = 0;
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INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
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bool DCE::runOnFunction(Function &F) {
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if (skipOptnoneFunction(F))
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return false;
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auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
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TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
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// Start out with all of the instructions in the worklist...
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std::vector<Instruction*> WorkList;
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for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
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WorkList.push_back(&*i);
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// Loop over the worklist finding instructions that are dead. If they are
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// dead make them drop all of their uses, making other instructions
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// potentially dead, and work until the worklist is empty.
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//
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bool MadeChange = false;
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back();
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WorkList.pop_back();
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if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
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// Loop over all of the values that the instruction uses, if there are
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// instructions being used, add them to the worklist, because they might
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// go dead after this one is removed.
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//
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for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
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if (Instruction *Used = dyn_cast<Instruction>(*OI))
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WorkList.push_back(Used);
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// Remove the instruction.
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I->eraseFromParent();
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// Remove the instruction from the worklist if it still exists in it.
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WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
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WorkList.end());
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MadeChange = true;
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++DCEEliminated;
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}
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}
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return MadeChange;
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}
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FunctionPass *llvm::createDeadCodeEliminationPass() {
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return new DCE();
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}
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