llvm/lib/Transforms/Scalar/BDCE.cpp

   1 //===---- BDCE.cpp - Bit-tracking 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 the Bit-Tracking Dead Code Elimination pass. Some
  10 // instructions (shifts, some ands, ors, etc.) kill some of their input bits.
  11 // We track these dead bits and remove instructions that compute only these
  12 // dead bits. We also simplify sext that generates unused extension bits,
  13 // converting it to a zext.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/Transforms/Scalar/BDCE.h"
  18 #include "llvm/ADT/SmallPtrSet.h"
  19 #include "llvm/ADT/SmallVector.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/Analysis/DemandedBits.h"
  22 #include "llvm/Analysis/GlobalsModRef.h"
  23 #include "llvm/IR/IRBuilder.h"
  24 #include "llvm/IR/InstIterator.h"
  25 #include "llvm/IR/Instructions.h"
  26 #include "llvm/InitializePasses.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"
  31 #include "llvm/Transforms/Utils/Local.h"
  32 using namespace llvm;
  33
  34 #define DEBUG_TYPE "bdce"
  35
  36 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
  37 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
  38 STATISTIC(NumSExt2ZExt,
  39           "Number of sign extension instructions converted to zero extension");
  40
  41 /// If an instruction is trivialized (dead), then the chain of users of that
  42 /// instruction may need to be cleared of assumptions that can no longer be
  43 /// guaranteed correct.
  44 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
  45   assert(I->getType()->isIntOrIntVectorTy() &&
  46          "Trivializing a non-integer value?");
  47
  48   // Initialize the worklist with eligible direct users.
  49   SmallPtrSet<Instruction *, 16> Visited;
  50   SmallVector<Instruction *, 16> WorkList;
  51   for (User *JU : I->users()) {
  52     // If all bits of a user are demanded, then we know that nothing below that
  53     // in the def-use chain needs to be changed.
  54     auto *J = dyn_cast<Instruction>(JU);
  55     if (J && J->getType()->isIntOrIntVectorTy() &&
  56         !DB.getDemandedBits(J).isAllOnesValue()) {
  57       Visited.insert(J);
  58       WorkList.push_back(J);
  59     }
  60
  61     // Note that we need to check for non-int types above before asking for
  62     // demanded bits. Normally, the only way to reach an instruction with an
  63     // non-int type is via an instruction that has side effects (or otherwise
  64     // will demand its input bits). However, if we have a readnone function
  65     // that returns an unsized type (e.g., void), we must avoid asking for the
  66     // demanded bits of the function call's return value. A void-returning
  67     // readnone function is always dead (and so we can stop walking the use/def
  68     // chain here), but the check is necessary to avoid asserting.
  69   }
  70
  71   // DFS through subsequent users while tracking visits to avoid cycles.
  72   while (!WorkList.empty()) {
  73     Instruction *J = WorkList.pop_back_val();
  74
  75     // NSW, NUW, and exact are based on operands that might have changed.
  76     J->dropPoisonGeneratingFlags();
  77
  78     // We do not have to worry about llvm.assume or range metadata:
  79     // 1. llvm.assume demands its operand, so trivializing can't change it.
  80     // 2. range metadata only applies to memory accesses which demand all bits.
  81
  82     for (User *KU : J->users()) {
  83       // If all bits of a user are demanded, then we know that nothing below
  84       // that in the def-use chain needs to be changed.
  85       auto *K = dyn_cast<Instruction>(KU);
  86       if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
  87           !DB.getDemandedBits(K).isAllOnesValue())
  88         WorkList.push_back(K);
  89     }
  90   }
  91 }
  92
  93 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
  94   SmallVector<Instruction*, 128> Worklist;
  95   bool Changed = false;
  96   for (Instruction &I : instructions(F)) {
  97     // If the instruction has side effects and no non-dbg uses,
  98     // skip it. This way we avoid computing known bits on an instruction
  99     // that will not help us.
 100     if (I.mayHaveSideEffects() && I.use_empty())
 101       continue;
 102
 103     // Remove instructions that are dead, either because they were not reached
 104     // during analysis or have no demanded bits.
 105     if (DB.isInstructionDead(&I) ||
 106         (I.getType()->isIntOrIntVectorTy() &&
 107          DB.getDemandedBits(&I).isNullValue() &&
 108          wouldInstructionBeTriviallyDead(&I))) {
 109       salvageDebugInfo(I);
 110       Worklist.push_back(&I);
 111       I.dropAllReferences();
 112       Changed = true;
 113       continue;
 114     }
 115
 116     // Convert SExt into ZExt if none of the extension bits is required
 117     if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
 118       APInt Demanded = DB.getDemandedBits(SE);
 119       const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
 120       auto *const DstTy = SE->getDestTy();
 121       const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
 122       if (Demanded.countLeadingZeros() >= (DestBitSize - SrcBitSize)) {
 123         clearAssumptionsOfUsers(SE, DB);
 124         IRBuilder<> Builder(SE);
 125         I.replaceAllUsesWith(
 126             Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
 127         Worklist.push_back(SE);
 128         Changed = true;
 129         NumSExt2ZExt++;
 130         continue;
 131       }
 132     }
 133
 134     for (Use &U : I.operands()) {
 135       // DemandedBits only detects dead integer uses.
 136       if (!U->getType()->isIntOrIntVectorTy())
 137         continue;
 138
 139       if (!isa<Instruction>(U) && !isa<Argument>(U))
 140         continue;
 141
 142       if (!DB.isUseDead(&U))
 143         continue;
 144
 145       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
 146
 147       clearAssumptionsOfUsers(&I, DB);
 148
 149       // FIXME: In theory we could substitute undef here instead of zero.
 150       // This should be reconsidered once we settle on the semantics of
 151       // undef, poison, etc.
 152       U.set(ConstantInt::get(U->getType(), 0));
 153       ++NumSimplified;
 154       Changed = true;
 155     }
 156   }
 157
 158   for (Instruction *&I : Worklist) {
 159     ++NumRemoved;
 160     I->eraseFromParent();
 161   }
 162
 163   return Changed;
 164 }
 165
 166 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
 167   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
 168   if (!bitTrackingDCE(F, DB))
 169     return PreservedAnalyses::all();
 170
 171   PreservedAnalyses PA;
 172   PA.preserveSet<CFGAnalyses>();
 173   return PA;
 174 }
 175
 176 namespace {
 177 struct BDCELegacyPass : public FunctionPass {
 178   static char ID; // Pass identification, replacement for typeid
 179   BDCELegacyPass() : FunctionPass(ID) {
 180     initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
 181   }
 182
 183   bool runOnFunction(Function &F) override {
 184     if (skipFunction(F))
 185       return false;
 186     auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
 187     return bitTrackingDCE(F, DB);
 188   }
 189
 190   void getAnalysisUsage(AnalysisUsage &AU) const override {
 191     AU.setPreservesCFG();
 192     AU.addRequired<DemandedBitsWrapperPass>();
 193     AU.addPreserved<GlobalsAAWrapperPass>();
 194   }
 195 };
 196 }
 197
 198 char BDCELegacyPass::ID = 0;
 199 INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
 200                       "Bit-Tracking Dead Code Elimination", false, false)
 201 INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
 202 INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
 203                     "Bit-Tracking Dead Code Elimination", false, false)
 204
 205 FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }