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/Support/Debug.h"
  27 #include "llvm/Support/raw_ostream.h"
  28 #include "llvm/Transforms/Utils/Local.h"
  29 using namespace llvm;
  30
  31 #define DEBUG_TYPE "bdce"
  32
  33 STATISTIC(NumRemoved, "Number of instructions removed (unused)");
  34 STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
  35 STATISTIC(NumSExt2ZExt,
  36           "Number of sign extension instructions converted to zero extension");
  37
  38 /// If an instruction is trivialized (dead), then the chain of users of that
  39 /// instruction may need to be cleared of assumptions that can no longer be
  40 /// guaranteed correct.
  41 static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
  42   assert(I->getType()->isIntOrIntVectorTy() &&
  43          "Trivializing a non-integer value?");
  44
  45   // Initialize the worklist with eligible direct users.
  46   SmallPtrSet<Instruction *, 16> Visited;
  47   SmallVector<Instruction *, 16> WorkList;
  48   for (User *JU : I->users()) {
  49     // If all bits of a user are demanded, then we know that nothing below that
  50     // in the def-use chain needs to be changed.
  51     auto *J = dyn_cast<Instruction>(JU);
  52     if (J && J->getType()->isIntOrIntVectorTy() &&
  53         !DB.getDemandedBits(J).isAllOnes()) {
  54       Visited.insert(J);
  55       WorkList.push_back(J);
  56     }
  57
  58     // Note that we need to check for non-int types above before asking for
  59     // demanded bits. Normally, the only way to reach an instruction with an
  60     // non-int type is via an instruction that has side effects (or otherwise
  61     // will demand its input bits). However, if we have a readnone function
  62     // that returns an unsized type (e.g., void), we must avoid asking for the
  63     // demanded bits of the function call's return value. A void-returning
  64     // readnone function is always dead (and so we can stop walking the use/def
  65     // chain here), but the check is necessary to avoid asserting.
  66   }
  67
  68   // DFS through subsequent users while tracking visits to avoid cycles.
  69   while (!WorkList.empty()) {
  70     Instruction *J = WorkList.pop_back_val();
  71
  72     // NSW, NUW, and exact are based on operands that might have changed.
  73     J->dropPoisonGeneratingFlags();
  74
  75     // We do not have to worry about llvm.assume or range metadata:
  76     // 1. llvm.assume demands its operand, so trivializing can't change it.
  77     // 2. range metadata only applies to memory accesses which demand all bits.
  78
  79     for (User *KU : J->users()) {
  80       // If all bits of a user are demanded, then we know that nothing below
  81       // that in the def-use chain needs to be changed.
  82       auto *K = dyn_cast<Instruction>(KU);
  83       if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
  84           !DB.getDemandedBits(K).isAllOnes())
  85         WorkList.push_back(K);
  86     }
  87   }
  88 }
  89
  90 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
  91   SmallVector<Instruction*, 128> Worklist;
  92   bool Changed = false;
  93   for (Instruction &I : instructions(F)) {
  94     // If the instruction has side effects and no non-dbg uses,
  95     // skip it. This way we avoid computing known bits on an instruction
  96     // that will not help us.
  97     if (I.mayHaveSideEffects() && I.use_empty())
  98       continue;
  99
 100     // Remove instructions that are dead, either because they were not reached
 101     // during analysis or have no demanded bits.
 102     if (DB.isInstructionDead(&I) ||
 103         (I.getType()->isIntOrIntVectorTy() && DB.getDemandedBits(&I).isZero() &&
 104          wouldInstructionBeTriviallyDead(&I))) {
 105       Worklist.push_back(&I);
 106       Changed = true;
 107       continue;
 108     }
 109
 110     // Convert SExt into ZExt if none of the extension bits is required
 111     if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
 112       APInt Demanded = DB.getDemandedBits(SE);
 113       const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
 114       auto *const DstTy = SE->getDestTy();
 115       const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
 116       if (Demanded.countl_zero() >= (DestBitSize - SrcBitSize)) {
 117         clearAssumptionsOfUsers(SE, DB);
 118         IRBuilder<> Builder(SE);
 119         I.replaceAllUsesWith(
 120             Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
 121         Worklist.push_back(SE);
 122         Changed = true;
 123         NumSExt2ZExt++;
 124         continue;
 125       }
 126     }
 127
 128     for (Use &U : I.operands()) {
 129       // DemandedBits only detects dead integer uses.
 130       if (!U->getType()->isIntOrIntVectorTy())
 131         continue;
 132
 133       if (!isa<Instruction>(U) && !isa<Argument>(U))
 134         continue;
 135
 136       if (!DB.isUseDead(&U))
 137         continue;
 138
 139       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
 140
 141       clearAssumptionsOfUsers(&I, DB);
 142
 143       // Substitute all uses with zero. In theory we could use `freeze poison`
 144       // instead, but that seems unlikely to be profitable.
 145       U.set(ConstantInt::get(U->getType(), 0));
 146       ++NumSimplified;
 147       Changed = true;
 148     }
 149   }
 150
 151   for (Instruction *&I : llvm::reverse(Worklist)) {
 152     salvageDebugInfo(*I);
 153     I->dropAllReferences();
 154   }
 155
 156   for (Instruction *&I : Worklist) {
 157     ++NumRemoved;
 158     I->eraseFromParent();
 159   }
 160
 161   return Changed;
 162 }
 163
 164 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
 165   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
 166   if (!bitTrackingDCE(F, DB))
 167     return PreservedAnalyses::all();
 168
 169   PreservedAnalyses PA;
 170   PA.preserveSet<CFGAnalyses>();
 171   return PA;
 172 }