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external/llvm/lib/Transforms/Scalar/BDCE.cpp
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external/llvm/lib/Transforms/Scalar/BDCE.cpp
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//===---- 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/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/Analysis/DemandedBits.h"
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#include "llvm/Analysis/GlobalsModRef.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/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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/// If an instruction is trivialized (dead), then the chain of users of that
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/// instruction may need to be cleared of assumptions that can no longer be
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/// guaranteed correct.
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static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
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assert(I->getType()->isIntegerTy() && "Trivializing a non-integer value?");
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// Initialize the worklist with eligible direct users.
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SmallVector<Instruction *, 16> WorkList;
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for (User *JU : I->users()) {
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// If all bits of a user are demanded, then we know that nothing below that
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// in the def-use chain needs to be changed.
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auto *J = dyn_cast<Instruction>(JU);
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if (J && J->getType()->isSized() &&
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!DB.getDemandedBits(J).isAllOnesValue())
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WorkList.push_back(J);
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// Note that we need to check for unsized types above before asking for
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// demanded bits. Normally, the only way to reach an instruction with an
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// unsized type is via an instruction that has side effects (or otherwise
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// will demand its input bits). However, if we have a readnone function
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// that returns an unsized type (e.g., void), we must avoid asking for the
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// demanded bits of the function call's return value. A void-returning
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// readnone function is always dead (and so we can stop walking the use/def
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// chain here), but the check is necessary to avoid asserting.
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}
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// DFS through subsequent users while tracking visits to avoid cycles.
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SmallPtrSet<Instruction *, 16> Visited;
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while (!WorkList.empty()) {
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Instruction *J = WorkList.pop_back_val();
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// NSW, NUW, and exact are based on operands that might have changed.
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J->dropPoisonGeneratingFlags();
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// We do not have to worry about llvm.assume or range metadata:
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// 1. llvm.assume demands its operand, so trivializing can't change it.
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// 2. range metadata only applies to memory accesses which demand all bits.
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Visited.insert(J);
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for (User *KU : J->users()) {
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// If all bits of a user are demanded, then we know that nothing below
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// that in the def-use chain needs to be changed.
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auto *K = dyn_cast<Instruction>(KU);
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if (K && !Visited.count(K) && K->getType()->isSized() &&
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!DB.getDemandedBits(K).isAllOnesValue())
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WorkList.push_back(K);
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}
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}
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}
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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 the instruction has side effects and no non-dbg uses,
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// skip it. This way we avoid computing known bits on an instruction
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// that will not help us.
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if (I.mayHaveSideEffects() && I.use_empty())
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continue;
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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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clearAssumptionsOfUsers(&I, DB);
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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.replaceNonMetadataUsesWith(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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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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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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