From 5a5ac124e1efaf208671f01c46edb15f29ed2a0b Mon Sep 17 00:00:00 2001 From: Dimitry Andric Date: Wed, 27 May 2015 18:44:32 +0000 Subject: Vendor import of llvm trunk r238337: https://llvm.org/svn/llvm-project/llvm/trunk@238337 --- lib/Transforms/Scalar/BDCE.cpp | 410 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 410 insertions(+) create mode 100644 lib/Transforms/Scalar/BDCE.cpp (limited to 'lib/Transforms/Scalar/BDCE.cpp') diff --git a/lib/Transforms/Scalar/BDCE.cpp b/lib/Transforms/Scalar/BDCE.cpp new file mode 100644 index 000000000000..09c605e76737 --- /dev/null +++ b/lib/Transforms/Scalar/BDCE.cpp @@ -0,0 +1,410 @@ +//===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// 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. +// +//===----------------------------------------------------------------------===// + +#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/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" + +STATISTIC(NumRemoved, "Number of instructions removed (unused)"); +STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)"); + +namespace { +struct BDCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + BDCE() : FunctionPass(ID) { + initializeBDCEPass(*PassRegistry::getPassRegistry()); + } + + bool runOnFunction(Function& F) override; + + void getAnalysisUsage(AnalysisUsage& AU) const override { + AU.setPreservesCFG(); + AU.addRequired(); + AU.addRequired(); + } + + 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_END(BDCE, "bdce", "Bit-Tracking Dead Code Elimination", + false, false) + +static bool isAlwaysLive(Instruction *I) { + return isa(I) || isa(I) || + isa(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(V1), KnownZero, KnownOne, DL, 0, + AC, UserI, DT); + + if (V2) { + KnownZero2 = APInt(BitWidth, 0); + KnownOne2 = APInt(BitWidth, 0); + computeKnownBits(const_cast(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(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(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(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(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(UserI)->isExact()) + AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt); + } + break; + case Instruction::AShr: + if (OperandNo == 0) + if (ConstantInt *CI = + dyn_cast(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(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(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(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; + + AC = &getAnalysis().getAssumptionCache(F); + DT = &getAnalysis().getDomTree(); + + DenseMap AliveBits; + SmallVector Worklist; + + // The set of visited instructions (non-integer-typed only). + SmallPtrSet 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(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(OI)) { + if (IntegerType *IT = dyn_cast(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(OI)) { + if (IntegerType *IT = dyn_cast(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; + } + + if (isAlwaysLive(&I)) + continue; + + Worklist.push_back(&I); + I.dropAllReferences(); + Changed = true; + } + + for (Instruction *&I : Worklist) { + ++NumRemoved; + I->eraseFromParent(); + } + + return Changed; +} + +FunctionPass *llvm::createBitTrackingDCEPass() { + return new BDCE(); +} + -- cgit v1.2.3