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Summary: Now that the DataLayout is a mandatory part of the module, let's start cleaning the codebase. This patch is a first attempt at doing that. This patch is not exactly NFC as for instance some places were passing a nullptr instead of the DataLayout, possibly just because there was a default value on the DataLayout argument to many functions in the API. Even though it is not purely NFC, there is no change in the validation. I turned as many pointer to DataLayout to references, this helped figuring out all the places where a nullptr could come up. I had initially a local version of this patch broken into over 30 independant, commits but some later commit were cleaning the API and touching part of the code modified in the previous commits, so it seemed cleaner without the intermediate state. Test Plan: Reviewers: echristo Subscribers: llvm-commits From: Mehdi Amini <mehdi.amini@apple.com> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231740 91177308-0d34-0410-b5e6-96231b3b80d8
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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