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