//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/DepthFirstIterator.h"
-#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
-#include "llvm/Analysis/AssumptionCache.h"
-#include "llvm/Analysis/ValueTracking.h"
-#include "llvm/IR/BasicBlock.h"
+#include "llvm/Analysis/GlobalsModRef.h"
+#include "llvm/Analysis/DemandedBits.h"
#include "llvm/IR/CFG.h"
-#include "llvm/IR/DataLayout.h"
-#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
-#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
-
using namespace llvm;
#define DEBUG_TYPE "bdce"
void getAnalysisUsage(AnalysisUsage& AU) const override {
AU.setPreservesCFG();
- AU.addRequired<AssumptionCacheTracker>();
- AU.addRequired<DominatorTreeWrapperPass>();
+ AU.addRequired<DemandedBits>();
+ AU.addPreserved<GlobalsAAWrapperPass>();
}
-
- void determineLiveOperandBits(const Instruction *UserI,
- const Instruction *I, unsigned OperandNo,
- const APInt &AOut, APInt &AB,
- APInt &KnownZero, APInt &KnownOne,
- APInt &KnownZero2, APInt &KnownOne2);
-
- AssumptionCache *AC;
- DominatorTree *DT;
};
}
char BDCE::ID = 0;
INITIALIZE_PASS_BEGIN(BDCE, "bdce", "Bit-Tracking Dead Code Elimination",
false, false)
-INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
-INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(DemandedBits)
INITIALIZE_PASS_END(BDCE, "bdce", "Bit-Tracking Dead Code Elimination",
false, false)
-static bool isAlwaysLive(Instruction *I) {
- return isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) ||
- isa<LandingPadInst>(I) || I->mayHaveSideEffects();
-}
-
-void BDCE::determineLiveOperandBits(const Instruction *UserI,
- const Instruction *I, unsigned OperandNo,
- const APInt &AOut, APInt &AB,
- APInt &KnownZero, APInt &KnownOne,
- APInt &KnownZero2, APInt &KnownOne2) {
- unsigned BitWidth = AB.getBitWidth();
-
- // We're called once per operand, but for some instructions, we need to
- // compute known bits of both operands in order to determine the live bits of
- // either (when both operands are instructions themselves). We don't,
- // however, want to do this twice, so we cache the result in APInts that live
- // in the caller. For the two-relevant-operands case, both operand values are
- // provided here.
- auto ComputeKnownBits =
- [&](unsigned BitWidth, const Value *V1, const Value *V2) {
- const DataLayout &DL = I->getModule()->getDataLayout();
- KnownZero = APInt(BitWidth, 0);
- KnownOne = APInt(BitWidth, 0);
- computeKnownBits(const_cast<Value *>(V1), KnownZero, KnownOne, DL, 0,
- AC, UserI, DT);
-
- if (V2) {
- KnownZero2 = APInt(BitWidth, 0);
- KnownOne2 = APInt(BitWidth, 0);
- computeKnownBits(const_cast<Value *>(V2), KnownZero2, KnownOne2, DL,
- 0, AC, UserI, DT);
- }
- };
-
- switch (UserI->getOpcode()) {
- default: break;
- case Instruction::Call:
- case Instruction::Invoke:
- if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
- switch (II->getIntrinsicID()) {
- default: break;
- case Intrinsic::bswap:
- // The alive bits of the input are the swapped alive bits of
- // the output.
- AB = AOut.byteSwap();
- break;
- case Intrinsic::ctlz:
- if (OperandNo == 0) {
- // We need some output bits, so we need all bits of the
- // input to the left of, and including, the leftmost bit
- // known to be one.
- ComputeKnownBits(BitWidth, I, nullptr);
- AB = APInt::getHighBitsSet(BitWidth,
- std::min(BitWidth, KnownOne.countLeadingZeros()+1));
- }
- break;
- case Intrinsic::cttz:
- if (OperandNo == 0) {
- // We need some output bits, so we need all bits of the
- // input to the right of, and including, the rightmost bit
- // known to be one.
- ComputeKnownBits(BitWidth, I, nullptr);
- AB = APInt::getLowBitsSet(BitWidth,
- std::min(BitWidth, KnownOne.countTrailingZeros()+1));
- }
- break;
- }
- break;
- case Instruction::Add:
- case Instruction::Sub:
- // Find the highest live output bit. We don't need any more input
- // bits than that (adds, and thus subtracts, ripple only to the
- // left).
- AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
- break;
- case Instruction::Shl:
- if (OperandNo == 0)
- if (ConstantInt *CI =
- dyn_cast<ConstantInt>(UserI->getOperand(1))) {
- uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
- AB = AOut.lshr(ShiftAmt);
-
- // If the shift is nuw/nsw, then the high bits are not dead
- // (because we've promised that they *must* be zero).
- const ShlOperator *S = cast<ShlOperator>(UserI);
- if (S->hasNoSignedWrap())
- AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
- else if (S->hasNoUnsignedWrap())
- AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
- }
- break;
- case Instruction::LShr:
- if (OperandNo == 0)
- if (ConstantInt *CI =
- dyn_cast<ConstantInt>(UserI->getOperand(1))) {
- uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
- AB = AOut.shl(ShiftAmt);
-
- // If the shift is exact, then the low bits are not dead
- // (they must be zero).
- if (cast<LShrOperator>(UserI)->isExact())
- AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
- }
- break;
- case Instruction::AShr:
- if (OperandNo == 0)
- if (ConstantInt *CI =
- dyn_cast<ConstantInt>(UserI->getOperand(1))) {
- uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
- AB = AOut.shl(ShiftAmt);
- // Because the high input bit is replicated into the
- // high-order bits of the result, if we need any of those
- // bits, then we must keep the highest input bit.
- if ((AOut & APInt::getHighBitsSet(BitWidth, ShiftAmt))
- .getBoolValue())
- AB.setBit(BitWidth-1);
-
- // If the shift is exact, then the low bits are not dead
- // (they must be zero).
- if (cast<AShrOperator>(UserI)->isExact())
- AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
- }
- break;
- case Instruction::And:
- AB = AOut;
-
- // For bits that are known zero, the corresponding bits in the
- // other operand are dead (unless they're both zero, in which
- // case they can't both be dead, so just mark the LHS bits as
- // dead).
- if (OperandNo == 0) {
- ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
- AB &= ~KnownZero2;
- } else {
- if (!isa<Instruction>(UserI->getOperand(0)))
- ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
- AB &= ~(KnownZero & ~KnownZero2);
- }
- break;
- case Instruction::Or:
- AB = AOut;
-
- // For bits that are known one, the corresponding bits in the
- // other operand are dead (unless they're both one, in which
- // case they can't both be dead, so just mark the LHS bits as
- // dead).
- if (OperandNo == 0) {
- ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
- AB &= ~KnownOne2;
- } else {
- if (!isa<Instruction>(UserI->getOperand(0)))
- ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
- AB &= ~(KnownOne & ~KnownOne2);
- }
- break;
- case Instruction::Xor:
- case Instruction::PHI:
- AB = AOut;
- break;
- case Instruction::Trunc:
- AB = AOut.zext(BitWidth);
- break;
- case Instruction::ZExt:
- AB = AOut.trunc(BitWidth);
- break;
- case Instruction::SExt:
- AB = AOut.trunc(BitWidth);
- // Because the high input bit is replicated into the
- // high-order bits of the result, if we need any of those
- // bits, then we must keep the highest input bit.
- if ((AOut & APInt::getHighBitsSet(AOut.getBitWidth(),
- AOut.getBitWidth() - BitWidth))
- .getBoolValue())
- AB.setBit(BitWidth-1);
- break;
- case Instruction::Select:
- if (OperandNo != 0)
- AB = AOut;
- break;
- }
-}
-
bool BDCE::runOnFunction(Function& F) {
if (skipOptnoneFunction(F))
return false;
+ DemandedBits &DB = getAnalysis<DemandedBits>();
- AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
- DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
-
- DenseMap<Instruction *, APInt> AliveBits;
SmallVector<Instruction*, 128> Worklist;
-
- // The set of visited instructions (non-integer-typed only).
- SmallPtrSet<Instruction*, 128> Visited;
-
- // Collect the set of "root" instructions that are known live.
- for (Instruction &I : inst_range(F)) {
- if (!isAlwaysLive(&I))
- continue;
-
- DEBUG(dbgs() << "BDCE: Root: " << I << "\n");
- // For integer-valued instructions, set up an initial empty set of alive
- // bits and add the instruction to the work list. For other instructions
- // add their operands to the work list (for integer values operands, mark
- // all bits as live).
- if (IntegerType *IT = dyn_cast<IntegerType>(I.getType())) {
- if (!AliveBits.count(&I)) {
- AliveBits[&I] = APInt(IT->getBitWidth(), 0);
- Worklist.push_back(&I);
- }
-
- continue;
- }
-
- // Non-integer-typed instructions...
- for (Use &OI : I.operands()) {
- if (Instruction *J = dyn_cast<Instruction>(OI)) {
- if (IntegerType *IT = dyn_cast<IntegerType>(J->getType()))
- AliveBits[J] = APInt::getAllOnesValue(IT->getBitWidth());
- Worklist.push_back(J);
- }
- }
- // To save memory, we don't add I to the Visited set here. Instead, we
- // check isAlwaysLive on every instruction when searching for dead
- // instructions later (we need to check isAlwaysLive for the
- // integer-typed instructions anyway).
- }
-
- // Propagate liveness backwards to operands.
- while (!Worklist.empty()) {
- Instruction *UserI = Worklist.pop_back_val();
-
- DEBUG(dbgs() << "BDCE: Visiting: " << *UserI);
- APInt AOut;
- if (UserI->getType()->isIntegerTy()) {
- AOut = AliveBits[UserI];
- DEBUG(dbgs() << " Alive Out: " << AOut);
- }
- DEBUG(dbgs() << "\n");
-
- if (!UserI->getType()->isIntegerTy())
- Visited.insert(UserI);
-
- APInt KnownZero, KnownOne, KnownZero2, KnownOne2;
- // Compute the set of alive bits for each operand. These are anded into the
- // existing set, if any, and if that changes the set of alive bits, the
- // operand is added to the work-list.
- for (Use &OI : UserI->operands()) {
- if (Instruction *I = dyn_cast<Instruction>(OI)) {
- if (IntegerType *IT = dyn_cast<IntegerType>(I->getType())) {
- unsigned BitWidth = IT->getBitWidth();
- APInt AB = APInt::getAllOnesValue(BitWidth);
- if (UserI->getType()->isIntegerTy() && !AOut &&
- !isAlwaysLive(UserI)) {
- AB = APInt(BitWidth, 0);
- } else {
- // If all bits of the output are dead, then all bits of the input
- // Bits of each operand that are used to compute alive bits of the
- // output are alive, all others are dead.
- determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
- KnownZero, KnownOne,
- KnownZero2, KnownOne2);
- }
-
- // If we've added to the set of alive bits (or the operand has not
- // been previously visited), then re-queue the operand to be visited
- // again.
- APInt ABPrev(BitWidth, 0);
- auto ABI = AliveBits.find(I);
- if (ABI != AliveBits.end())
- ABPrev = ABI->second;
-
- APInt ABNew = AB | ABPrev;
- if (ABNew != ABPrev || ABI == AliveBits.end()) {
- AliveBits[I] = std::move(ABNew);
- Worklist.push_back(I);
- }
- } else if (!Visited.count(I)) {
- Worklist.push_back(I);
- }
- }
- }
- }
-
bool Changed = false;
- // The inverse of the live set is the dead set. These are those instructions
- // which have no side effects and do not influence the control flow or return
- // value of the function, and may therefore be deleted safely.
- // NOTE: We reuse the Worklist vector here for memory efficiency.
- for (Instruction &I : inst_range(F)) {
- // For live instructions that have all dead bits, first make them dead by
- // replacing all uses with something else. Then, if they don't need to
- // remain live (because they have side effects, etc.) we can remove them.
- if (I.getType()->isIntegerTy()) {
- auto ABI = AliveBits.find(&I);
- if (ABI != AliveBits.end()) {
- if (ABI->second.getBoolValue())
- continue;
-
- DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
- // 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.
- Value *Zero = ConstantInt::get(I.getType(), 0);
- ++NumSimplified;
- I.replaceAllUsesWith(Zero);
- Changed = true;
- }
- } else if (Visited.count(&I)) {
- continue;
+ for (Instruction &I : instructions(F)) {
+ if (I.getType()->isIntegerTy() &&
+ !DB.getDemandedBits(&I).getBoolValue()) {
+ // For live instructions that have all dead bits, first make them dead by
+ // replacing all uses with something else. Then, if they don't need to
+ // remain live (because they have side effects, etc.) we can remove them.
+ DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
+ // 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.
+ Value *Zero = ConstantInt::get(I.getType(), 0);
+ ++NumSimplified;
+ I.replaceAllUsesWith(Zero);
+ Changed = true;
}
-
- if (isAlwaysLive(&I))
+ if (!DB.isInstructionDead(&I))
continue;
Worklist.push_back(&I);
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