lib/Transforms/Scalar/DCE.cpp

   1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements dead inst elimination and dead code elimination.
  10 //
  11 // Dead Inst Elimination performs a single pass over the function removing
  12 // instructions that are obviously dead.  Dead Code Elimination is similar, but
  13 // it rechecks instructions that were used by removed instructions to see if
  14 // they are newly dead.
  15 //
  16 //===----------------------------------------------------------------------===//
  17
  18 #include "llvm/Transforms/Scalar/DCE.h"
  19 #include "llvm/ADT/SetVector.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/Analysis/TargetLibraryInfo.h"
  22 #include "llvm/Transforms/Utils/Local.h"
  23 #include "llvm/IR/InstIterator.h"
  24 #include "llvm/IR/Instruction.h"
  25 #include "llvm/Pass.h"
  26 #include "llvm/Support/DebugCounter.h"
  27 #include "llvm/Transforms/Scalar.h"
  28 using namespace llvm;
  29
  30 #define DEBUG_TYPE "dce"
  31
  32 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
  33 STATISTIC(DCEEliminated, "Number of insts removed");
  34 DEBUG_COUNTER(DCECounter, "dce-transform",
  35               "Controls which instructions are eliminated");
  36
  37 namespace {
  38   //===--------------------------------------------------------------------===//
  39   // DeadInstElimination pass implementation
  40   //
  41   struct DeadInstElimination : public BasicBlockPass {
  42     static char ID; // Pass identification, replacement for typeid
  43     DeadInstElimination() : BasicBlockPass(ID) {
  44       initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
  45     }
  46     bool runOnBasicBlock(BasicBlock &BB) override {
  47       if (skipBasicBlock(BB))
  48         return false;
  49       auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
  50       TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
  51       bool Changed = false;
  52       for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
  53         Instruction *Inst = &*DI++;
  54         if (isInstructionTriviallyDead(Inst, TLI)) {
  55           if (!DebugCounter::shouldExecute(DCECounter))
  56             continue;
  57           salvageDebugInfo(*Inst);
  58           Inst->eraseFromParent();
  59           Changed = true;
  60           ++DIEEliminated;
  61         }
  62       }
  63       return Changed;
  64     }
  65
  66     void getAnalysisUsage(AnalysisUsage &AU) const override {
  67       AU.setPreservesCFG();
  68     }
  69   };
  70 }
  71
  72 char DeadInstElimination::ID = 0;
  73 INITIALIZE_PASS(DeadInstElimination, "die",
  74                 "Dead Instruction Elimination", false, false)
  75
  76 Pass *llvm::createDeadInstEliminationPass() {
  77   return new DeadInstElimination();
  78 }
  79
  80 static bool DCEInstruction(Instruction *I,
  81                            SmallSetVector<Instruction *, 16> &WorkList,
  82                            const TargetLibraryInfo *TLI) {
  83   if (isInstructionTriviallyDead(I, TLI)) {
  84     if (!DebugCounter::shouldExecute(DCECounter))
  85       return false;
  86
  87     salvageDebugInfo(*I);
  88
  89     // Null out all of the instruction's operands to see if any operand becomes
  90     // dead as we go.
  91     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
  92       Value *OpV = I->getOperand(i);
  93       I->setOperand(i, nullptr);
  94
  95       if (!OpV->use_empty() || I == OpV)
  96         continue;
  97
  98       // If the operand is an instruction that became dead as we nulled out the
  99       // operand, and if it is 'trivially' dead, delete it in a future loop
 100       // iteration.
 101       if (Instruction *OpI = dyn_cast<Instruction>(OpV))
 102         if (isInstructionTriviallyDead(OpI, TLI))
 103           WorkList.insert(OpI);
 104     }
 105
 106     I->eraseFromParent();
 107     ++DCEEliminated;
 108     return true;
 109   }
 110   return false;
 111 }
 112
 113 static bool eliminateDeadCode(Function &F, TargetLibraryInfo *TLI) {
 114   bool MadeChange = false;
 115   SmallSetVector<Instruction *, 16> WorkList;
 116   // Iterate over the original function, only adding insts to the worklist
 117   // if they actually need to be revisited. This avoids having to pre-init
 118   // the worklist with the entire function's worth of instructions.
 119   for (inst_iterator FI = inst_begin(F), FE = inst_end(F); FI != FE;) {
 120     Instruction *I = &*FI;
 121     ++FI;
 122
 123     // We're visiting this instruction now, so make sure it's not in the
 124     // worklist from an earlier visit.
 125     if (!WorkList.count(I))
 126       MadeChange |= DCEInstruction(I, WorkList, TLI);
 127   }
 128
 129   while (!WorkList.empty()) {
 130     Instruction *I = WorkList.pop_back_val();
 131     MadeChange |= DCEInstruction(I, WorkList, TLI);
 132   }
 133   return MadeChange;
 134 }
 135
 136 PreservedAnalyses DCEPass::run(Function &F, FunctionAnalysisManager &AM) {
 137   if (!eliminateDeadCode(F, AM.getCachedResult<TargetLibraryAnalysis>(F)))
 138     return PreservedAnalyses::all();
 139
 140   PreservedAnalyses PA;
 141   PA.preserveSet<CFGAnalyses>();
 142   return PA;
 143 }
 144
 145 namespace {
 146 struct DCELegacyPass : public FunctionPass {
 147   static char ID; // Pass identification, replacement for typeid
 148   DCELegacyPass() : FunctionPass(ID) {
 149     initializeDCELegacyPassPass(*PassRegistry::getPassRegistry());
 150   }
 151
 152   bool runOnFunction(Function &F) override {
 153     if (skipFunction(F))
 154       return false;
 155
 156     auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
 157     TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
 158
 159     return eliminateDeadCode(F, TLI);
 160   }
 161
 162   void getAnalysisUsage(AnalysisUsage &AU) const override {
 163     AU.setPreservesCFG();
 164   }
 165 };
 166 }
 167
 168 char DCELegacyPass::ID = 0;
 169 INITIALIZE_PASS(DCELegacyPass, "dce", "Dead Code Elimination", false, false)
 170
 171 FunctionPass *llvm::createDeadCodeEliminationPass() {
 172   return new DCELegacyPass();
 173 }