forked from OSchip/llvm-project
100 lines
3.0 KiB
C++
100 lines
3.0 KiB
C++
//===- DCE.cpp - Code to perform dead code 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 the Aggressive Dead Code Elimination pass. This pass
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// optimistically assumes that all instructions are dead until proven otherwise,
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// allowing it to eliminate dead computations that other DCE passes do not
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// catch, particularly involving loop computations.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/InstIterator.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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using namespace llvm;
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#define DEBUG_TYPE "adce"
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STATISTIC(NumRemoved, "Number of instructions removed");
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namespace {
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struct ADCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass(ID) {
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initializeADCEPass(*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 ADCE::ID = 0;
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INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
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bool ADCE::runOnFunction(Function& F) {
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if (skipOptnoneFunction(F))
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return false;
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SmallPtrSet<Instruction*, 128> Alive;
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SmallVector<Instruction*, 128> Worklist;
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// Collect the set of "root" instructions that are known live.
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for (Instruction &I : inst_range(F)) {
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if (isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) ||
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isa<LandingPadInst>(I) || I.mayHaveSideEffects()) {
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Alive.insert(&I);
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Worklist.push_back(&I);
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}
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}
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// Propagate liveness backwards to operands.
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while (!Worklist.empty()) {
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Instruction *Curr = Worklist.pop_back_val();
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for (Use &OI : Curr->operands()) {
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if (Instruction *Inst = dyn_cast<Instruction>(OI))
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if (Alive.insert(Inst).second)
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Worklist.push_back(Inst);
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}
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}
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// The inverse of the live set is the dead set. These are those instructions
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// which have no side effects and do not influence the control flow or return
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// value of the function, and may therefore be deleted safely.
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// NOTE: We reuse the Worklist vector here for memory efficiency.
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for (Instruction &I : inst_range(F)) {
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if (!Alive.count(&I)) {
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Worklist.push_back(&I);
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I.dropAllReferences();
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}
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}
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for (Instruction *&I : Worklist) {
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++NumRemoved;
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I->eraseFromParent();
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}
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return !Worklist.empty();
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}
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FunctionPass *llvm::createAggressiveDCEPass() {
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return new ADCE();
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}
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