1 //===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Bit-Tracking Dead Code Elimination pass. Some
11 // instructions (shifts, some ands, ors, etc.) kill some of their input bits.
12 // We track these dead bits and remove instructions that compute only these
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar/BDCE.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/DemandedBits.h"
21 #include "llvm/Analysis/GlobalsModRef.h"
22 #include "llvm/IR/CFG.h"
23 #include "llvm/IR/InstIterator.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/IntrinsicInst.h"
26 #include "llvm/IR/Operator.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Transforms/Scalar.h"
33 #define DEBUG_TYPE "bdce"
35 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
36 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
38 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
39 SmallVector<Instruction*, 128> Worklist;
41 for (Instruction &I : instructions(F)) {
42 // If the instruction has side effects and no non-dbg uses,
43 // skip it. This way we avoid computing known bits on an instruction
44 // that will not help us.
45 if (I.mayHaveSideEffects() && I.use_empty())
48 if (I.getType()->isIntegerTy() &&
49 !DB.getDemandedBits(&I).getBoolValue()) {
50 // For live instructions that have all dead bits, first make them dead by
51 // replacing all uses with something else. Then, if they don't need to
52 // remain live (because they have side effects, etc.) we can remove them.
53 DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
54 // FIXME: In theory we could substitute undef here instead of zero.
55 // This should be reconsidered once we settle on the semantics of
56 // undef, poison, etc.
57 Value *Zero = ConstantInt::get(I.getType(), 0);
59 I.replaceNonMetadataUsesWith(Zero);
62 if (!DB.isInstructionDead(&I))
65 Worklist.push_back(&I);
66 I.dropAllReferences();
70 for (Instruction *&I : Worklist) {
78 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
79 auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
80 if (!bitTrackingDCE(F, DB))
81 return PreservedAnalyses::all();
84 PA.preserveSet<CFGAnalyses>();
85 PA.preserve<GlobalsAA>();
90 struct BDCELegacyPass : public FunctionPass {
91 static char ID; // Pass identification, replacement for typeid
92 BDCELegacyPass() : FunctionPass(ID) {
93 initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
96 bool runOnFunction(Function &F) override {
99 auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
100 return bitTrackingDCE(F, DB);
103 void getAnalysisUsage(AnalysisUsage &AU) const override {
104 AU.setPreservesCFG();
105 AU.addRequired<DemandedBitsWrapperPass>();
106 AU.addPreserved<GlobalsAAWrapperPass>();
111 char BDCELegacyPass::ID = 0;
112 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
113 "Bit-Tracking Dead Code Elimination", false, false)
114 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
115 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
116 "Bit-Tracking Dead Code Elimination", false, false)
118 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }