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/IR/PatternMatch.h"
  27 #include "llvm/Support/Debug.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include "llvm/Transforms/Utils/Local.h"
  30
  31 using namespace llvm;
  32 using namespace PatternMatch;
  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   // If all bits of a user are demanded, then we know that nothing below that
  49   // in the def-use chain needs to be changed.
  50   if (DB.getDemandedBits(I).isAllOnes())
  51     return;
  52
  53   // Initialize the worklist with eligible direct users.
  54   SmallPtrSet<Instruction *, 16> Visited;
  55   SmallVector<Instruction *, 16> WorkList;
  56   for (User *JU : I->users()) {
  57     auto *J = cast<Instruction>(JU);
  58     if (J->getType()->isIntOrIntVectorTy()) {
  59       Visited.insert(J);
  60       WorkList.push_back(J);
  61     }
  62
  63     // Note that we need to check for non-int types above before asking for
  64     // demanded bits. Normally, the only way to reach an instruction with an
  65     // non-int type is via an instruction that has side effects (or otherwise
  66     // will demand its input bits). However, if we have a readnone function
  67     // that returns an unsized type (e.g., void), we must avoid asking for the
  68     // demanded bits of the function call's return value. A void-returning
  69     // readnone function is always dead (and so we can stop walking the use/def
  70     // chain here), but the check is necessary to avoid asserting.
  71   }
  72
  73   // DFS through subsequent users while tracking visits to avoid cycles.
  74   while (!WorkList.empty()) {
  75     Instruction *J = WorkList.pop_back_val();
  76
  77     // NSW, NUW, and exact are based on operands that might have changed.
  78     J->dropPoisonGeneratingAnnotations();
  79
  80     // We do not have to worry about llvm.assume, because it demands its
  81     // operand, so trivializing can't change it.
  82
  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     if (DB.getDemandedBits(J).isAllOnes())
  86       continue;
  87
  88     for (User *KU : J->users()) {
  89       auto *K = cast<Instruction>(KU);
  90       if (Visited.insert(K).second && K->getType()->isIntOrIntVectorTy())
  91         WorkList.push_back(K);
  92     }
  93   }
  94 }
  95
  96 static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
  97   SmallVector<Instruction*, 128> Worklist;
  98   bool Changed = false;
  99   for (Instruction &I : instructions(F)) {
 100     // If the instruction has side effects and no non-dbg uses,
 101     // skip it. This way we avoid computing known bits on an instruction
 102     // that will not help us.
 103     if (I.mayHaveSideEffects() && I.use_empty())
 104       continue;
 105
 106     // Remove instructions that are dead, either because they were not reached
 107     // during analysis or have no demanded bits.
 108     if (DB.isInstructionDead(&I) ||
 109         (I.getType()->isIntOrIntVectorTy() && DB.getDemandedBits(&I).isZero() &&
 110          wouldInstructionBeTriviallyDead(&I))) {
 111       Worklist.push_back(&I);
 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.countl_zero() >= (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     // Simplify and, or, xor when their mask does not affect the demanded bits.
 135     if (auto *BO = dyn_cast<BinaryOperator>(&I)) {
 136       APInt Demanded = DB.getDemandedBits(BO);
 137       if (!Demanded.isAllOnes()) {
 138         const APInt *Mask;
 139         if (match(BO->getOperand(1), m_APInt(Mask))) {
 140           bool CanBeSimplified = false;
 141           switch (BO->getOpcode()) {
 142           case Instruction::Or:
 143           case Instruction::Xor:
 144             CanBeSimplified = !Demanded.intersects(*Mask);
 145             break;
 146           case Instruction::And:
 147             CanBeSimplified = Demanded.isSubsetOf(*Mask);
 148             break;
 149           default:
 150             // TODO: Handle more cases here.
 151             break;
 152           }
 153
 154           if (CanBeSimplified) {
 155             clearAssumptionsOfUsers(BO, DB);
 156             BO->replaceAllUsesWith(BO->getOperand(0));
 157             Worklist.push_back(BO);
 158             ++NumSimplified;
 159             Changed = true;
 160             continue;
 161           }
 162         }
 163       }
 164     }
 165
 166     for (Use &U : I.operands()) {
 167       // DemandedBits only detects dead integer uses.
 168       if (!U->getType()->isIntOrIntVectorTy())
 169         continue;
 170
 171       if (!isa<Instruction>(U) && !isa<Argument>(U))
 172         continue;
 173
 174       if (!DB.isUseDead(&U))
 175         continue;
 176
 177       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
 178
 179       clearAssumptionsOfUsers(&I, DB);
 180
 181       // Substitute all uses with zero. In theory we could use `freeze poison`
 182       // instead, but that seems unlikely to be profitable.
 183       U.set(ConstantInt::get(U->getType(), 0));
 184       ++NumSimplified;
 185       Changed = true;
 186     }
 187   }
 188
 189   for (Instruction *&I : llvm::reverse(Worklist)) {
 190     salvageDebugInfo(*I);
 191     I->dropAllReferences();
 192   }
 193
 194   for (Instruction *&I : Worklist) {
 195     ++NumRemoved;
 196     I->eraseFromParent();
 197   }
 198
 199   return Changed;
 200 }
 201
 202 PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
 203   auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
 204   if (!bitTrackingDCE(F, DB))
 205     return PreservedAnalyses::all();
 206
 207   PreservedAnalyses PA;
 208   PA.preserveSet<CFGAnalyses>();
 209   return PA;
 210 }