forked from OSchip/llvm-project
113 lines
3.7 KiB
C++
113 lines
3.7 KiB
C++
//===---- BDCE.cpp - Bit-tracking 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 Bit-Tracking Dead Code Elimination pass. Some
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// instructions (shifts, some ands, ors, etc.) kill some of their input bits.
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// We track these dead bits and remove instructions that compute only these
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// dead bits.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/BDCE.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/Analysis/DemandedBits.h"
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#include "llvm/Analysis/GlobalsModRef.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/IR/Operator.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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using namespace llvm;
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#define DEBUG_TYPE "bdce"
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STATISTIC(NumRemoved, "Number of instructions removed (unused)");
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STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
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static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
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SmallVector<Instruction*, 128> Worklist;
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bool Changed = false;
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for (Instruction &I : instructions(F)) {
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if (I.getType()->isIntegerTy() &&
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!DB.getDemandedBits(&I).getBoolValue()) {
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// For live instructions that have all dead bits, first make them dead by
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// replacing all uses with something else. Then, if they don't need to
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// remain live (because they have side effects, etc.) we can remove them.
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DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
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// FIXME: In theory we could substitute undef here instead of zero.
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// This should be reconsidered once we settle on the semantics of
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// undef, poison, etc.
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Value *Zero = ConstantInt::get(I.getType(), 0);
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++NumSimplified;
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I.replaceAllUsesWith(Zero);
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Changed = true;
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}
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if (!DB.isInstructionDead(&I))
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continue;
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Worklist.push_back(&I);
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I.dropAllReferences();
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Changed = true;
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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 Changed;
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}
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PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
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auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
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if (!bitTrackingDCE(F, DB))
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return PreservedAnalyses::all();
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// FIXME: This should also 'preserve the CFG'.
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auto PA = PreservedAnalyses();
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PA.preserve<GlobalsAA>();
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return PA;
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}
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namespace {
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struct BDCELegacyPass : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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BDCELegacyPass() : FunctionPass(ID) {
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initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override {
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if (skipFunction(F))
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return false;
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auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
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return bitTrackingDCE(F, DB);
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<DemandedBitsWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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}
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};
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}
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char BDCELegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
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"Bit-Tracking Dead Code Elimination", false, false)
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INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
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INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
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"Bit-Tracking Dead Code Elimination", false, false)
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FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }
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