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📄 ADCE.cpp(24.71 KB)
📄 AlignmentFromAssumptions.cpp(15.9 KB)
📄 BDCE.cpp(7.42 KB)
📄 CallSiteSplitting.cpp(21.43 KB)
📄 ConstantHoisting.cpp(38.95 KB)
📄 ConstantProp.cpp(4.13 KB)
📄 CorrelatedValuePropagation.cpp(32.36 KB)
📄 DCE.cpp(6.84 KB)
📄 DeadStoreElimination.cpp(91.51 KB)
📄 DivRemPairs.cpp(15.01 KB)
📄 EarlyCSE.cpp(57.05 KB)
📄 FlattenCFGPass.cpp(2.74 KB)
📄 Float2Int.cpp(18.19 KB)
📄 GVN.cpp(100.11 KB)
📄 GVNHoist.cpp(44.85 KB)
📄 GVNSink.cpp(29.97 KB)
📄 GuardWidening.cpp(32.36 KB)
📄 IVUsersPrinter.cpp(839 B)
📄 IndVarSimplify.cpp(113.49 KB)
📄 InductiveRangeCheckElimination.cpp(72.96 KB)
📄 InferAddressSpaces.cpp(44.22 KB)
📄 InstSimplifyPass.cpp(5.39 KB)
📄 JumpThreading.cpp(115.06 KB)
📄 LICM.cpp(92.71 KB)
📄 LoopAccessAnalysisPrinter.cpp(977 B)
📄 LoopDataPrefetch.cpp(14.54 KB)
📄 LoopDeletion.cpp(11.37 KB)
📄 LoopDistribute.cpp(40.25 KB)
📄 LoopFuse.cpp(68.6 KB)
📄 LoopIdiomRecognize.cpp(69.09 KB)
📄 LoopInstSimplify.cpp(9.38 KB)
📄 LoopInterchange.cpp(60.65 KB)
📄 LoopLoadElimination.cpp(26.71 KB)
📄 LoopPassManager.cpp(4.02 KB)
📄 LoopPredication.cpp(49.21 KB)
📄 LoopRerollPass.cpp(58.54 KB)
📄 LoopRotation.cpp(4.79 KB)
📄 LoopSimplifyCFG.cpp(28.59 KB)
📄 LoopSink.cpp(14.93 KB)
📄 LoopStrengthReduce.cpp(216.88 KB)
📄 LoopUnrollAndJamPass.cpp(21.07 KB)
📄 LoopUnrollPass.cpp(60.73 KB)
📄 LoopUnswitch.cpp(64.17 KB)
📄 LoopVersioningLICM.cpp(23.65 KB)
📄 LowerAtomic.cpp(5.12 KB)
📄 LowerConstantIntrinsics.cpp(5.8 KB)
📄 LowerExpectIntrinsic.cpp(15 KB)
📄 LowerGuardIntrinsic.cpp(2.82 KB)
📄 LowerMatrixIntrinsics.cpp(73.36 KB)
📄 LowerWidenableCondition.cpp(2.75 KB)
📄 MakeGuardsExplicit.cpp(3.83 KB)
📄 MemCpyOptimizer.cpp(52.16 KB)
📄 MergeICmps.cpp(35.61 KB)
📄 MergedLoadStoreMotion.cpp(15.18 KB)
📄 NaryReassociate.cpp(19.86 KB)
📄 NewGVN.cpp(170.95 KB)
📄 PartiallyInlineLibCalls.cpp(6.22 KB)
📄 PlaceSafepoints.cpp(27.5 KB)
📄 Reassociate.cpp(95.61 KB)
📄 Reg2Mem.cpp(4.34 KB)
📄 RewriteStatepointsForGC.cpp(116.35 KB)
📄 SCCP.cpp(76.12 KB)
📄 SROA.cpp(183.83 KB)
📄 Scalar.cpp(10.08 KB)
📄 Scalarizer.cpp(32.93 KB)
📄 SeparateConstOffsetFromGEP.cpp(52.99 KB)
📄 SimpleLoopUnswitch.cpp(124.57 KB)
📄 SimplifyCFGPass.cpp(12.31 KB)
📄 Sink.cpp(10.81 KB)
📄 SpeculateAroundPHIs.cpp(35.69 KB)
📄 SpeculativeExecution.cpp(11.54 KB)
📄 StraightLineStrengthReduce.cpp(28.92 KB)
📄 StructurizeCFG.cpp(32.49 KB)
📄 TailRecursionElimination.cpp(34.9 KB)
📄 WarnMissedTransforms.cpp(6.14 KB)

Editing: BDCE.cpp

//===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the Bit-Tracking Dead Code Elimination pass. Some
// instructions (shifts, some ands, ors, etc.) kill some of their input bits.
// We track these dead bits and remove instructions that compute only these
// dead bits. We also simplify sext that generates unused extension bits,
// converting it to a zext.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/BDCE.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DemandedBits.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;

#define DEBUG_TYPE "bdce"

STATISTIC(NumRemoved, "Number of instructions removed (unused)");
STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
STATISTIC(NumSExt2ZExt,
          "Number of sign extension instructions converted to zero extension");

/// If an instruction is trivialized (dead), then the chain of users of that
/// instruction may need to be cleared of assumptions that can no longer be
/// guaranteed correct.
static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
  assert(I->getType()->isIntOrIntVectorTy() &&
         "Trivializing a non-integer value?");

// Initialize the worklist with eligible direct users.
  SmallPtrSet<Instruction *, 16> Visited;
  SmallVector<Instruction *, 16> WorkList;
  for (User *JU : I->users()) {
    // If all bits of a user are demanded, then we know that nothing below that
    // in the def-use chain needs to be changed.
    auto *J = dyn_cast<Instruction>(JU);
    if (J && J->getType()->isIntOrIntVectorTy() &&
        !DB.getDemandedBits(J).isAllOnesValue()) {
      Visited.insert(J);
      WorkList.push_back(J);
    }

// Note that we need to check for non-int types above before asking for
    // demanded bits. Normally, the only way to reach an instruction with an
    // non-int type is via an instruction that has side effects (or otherwise
    // will demand its input bits). However, if we have a readnone function
    // that returns an unsized type (e.g., void), we must avoid asking for the
    // demanded bits of the function call's return value. A void-returning
    // readnone function is always dead (and so we can stop walking the use/def
    // chain here), but the check is necessary to avoid asserting.
  }

// DFS through subsequent users while tracking visits to avoid cycles.
  while (!WorkList.empty()) {
    Instruction *J = WorkList.pop_back_val();

// NSW, NUW, and exact are based on operands that might have changed.
    J->dropPoisonGeneratingFlags();

// We do not have to worry about llvm.assume or range metadata:
    // 1. llvm.assume demands its operand, so trivializing can't change it.
    // 2. range metadata only applies to memory accesses which demand all bits.

for (User *KU : J->users()) {
      // If all bits of a user are demanded, then we know that nothing below
      // that in the def-use chain needs to be changed.
      auto *K = dyn_cast<Instruction>(KU);
      if (K && Visited.insert(K).second && K->getType()->isIntOrIntVectorTy() &&
          !DB.getDemandedBits(K).isAllOnesValue())
        WorkList.push_back(K);
    }
  }
}

static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
  SmallVector<Instruction*, 128> Worklist;
  bool Changed = false;
  for (Instruction &I : instructions(F)) {
    // If the instruction has side effects and no non-dbg uses,
    // skip it. This way we avoid computing known bits on an instruction
    // that will not help us.
    if (I.mayHaveSideEffects() && I.use_empty())
      continue;

// Remove instructions that are dead, either because they were not reached
    // during analysis or have no demanded bits.
    if (DB.isInstructionDead(&I) ||
        (I.getType()->isIntOrIntVectorTy() &&
         DB.getDemandedBits(&I).isNullValue() &&
         wouldInstructionBeTriviallyDead(&I))) {
      salvageDebugInfo(I);
      Worklist.push_back(&I);
      I.dropAllReferences();
      Changed = true;
      continue;
    }

// Convert SExt into ZExt if none of the extension bits is required
    if (SExtInst *SE = dyn_cast<SExtInst>(&I)) {
      APInt Demanded = DB.getDemandedBits(SE);
      const uint32_t SrcBitSize = SE->getSrcTy()->getScalarSizeInBits();
      auto *const DstTy = SE->getDestTy();
      const uint32_t DestBitSize = DstTy->getScalarSizeInBits();
      if (Demanded.countLeadingZeros() >= (DestBitSize - SrcBitSize)) {
        clearAssumptionsOfUsers(SE, DB);
        IRBuilder<> Builder(SE);
        I.replaceAllUsesWith(
            Builder.CreateZExt(SE->getOperand(0), DstTy, SE->getName()));
        Worklist.push_back(SE);
        Changed = true;
        NumSExt2ZExt++;
        continue;
      }
    }

for (Use &U : I.operands()) {
      // DemandedBits only detects dead integer uses.
      if (!U->getType()->isIntOrIntVectorTy())
        continue;

if (!isa<Instruction>(U) && !isa<Argument>(U))
        continue;

if (!DB.isUseDead(&U))
        continue;

LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");

clearAssumptionsOfUsers(&I, DB);

// FIXME: In theory we could substitute undef here instead of zero.
      // This should be reconsidered once we settle on the semantics of
      // undef, poison, etc.
      U.set(ConstantInt::get(U->getType(), 0));
      ++NumSimplified;
      Changed = true;
    }
  }

for (Instruction *&I : Worklist) {
    ++NumRemoved;
    I->eraseFromParent();
  }

return Changed;
}

PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
  auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
  if (!bitTrackingDCE(F, DB))
    return PreservedAnalyses::all();

PreservedAnalyses PA;
  PA.preserveSet<CFGAnalyses>();
  PA.preserve<GlobalsAA>();
  return PA;
}

namespace {
struct BDCELegacyPass : public FunctionPass {
  static char ID; // Pass identification, replacement for typeid
  BDCELegacyPass() : FunctionPass(ID) {
    initializeBDCELegacyPassPass(*PassRegistry::getPassRegistry());
  }

bool runOnFunction(Function &F) override {
    if (skipFunction(F))
      return false;
    auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
    return bitTrackingDCE(F, DB);
  }

void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesCFG();
    AU.addRequired<DemandedBitsWrapperPass>();
    AU.addPreserved<GlobalsAAWrapperPass>();
  }
};
}

char BDCELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(BDCELegacyPass, "bdce",
                      "Bit-Tracking Dead Code Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DemandedBitsWrapperPass)
INITIALIZE_PASS_END(BDCELegacyPass, "bdce",
                    "Bit-Tracking Dead Code Elimination", false, false)

FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }

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Editing: BDCE.cpp

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