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2ccf761faa
Instead of bailing out when we see an icmp, we can instead at least say that if the upper bits of both operands are known zero, they are not demanded. This doesn't help with signed comparisons, but it's at least better than bailing out. llvm-svn: 249687
393 lines
14 KiB
C++
393 lines
14 KiB
C++
//===---- DemandedBits.cpp - Determine demanded bits ----------------------===//
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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 pass implements a demanded bits analysis. A demanded bit is one that
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// contributes to a result; bits that are not demanded can be either zero or
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// one without affecting control or data flow. For example in this sequence:
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//
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// %1 = add i32 %x, %y
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// %2 = trunc i32 %1 to i16
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//
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// Only the lowest 16 bits of %1 are demanded; the rest are removed by the
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// trunc.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/DemandedBits.h"
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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/StringExtras.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 "demanded-bits"
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char DemandedBits::ID = 0;
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INITIALIZE_PASS_BEGIN(DemandedBits, "demanded-bits", "Demanded bits analysis",
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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(DemandedBits, "demanded-bits", "Demanded bits analysis",
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false, false)
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DemandedBits::DemandedBits() : FunctionPass(ID), F(nullptr), Analyzed(false) {
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initializeDemandedBitsPass(*PassRegistry::getPassRegistry());
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}
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void DemandedBits::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<AssumptionCacheTracker>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.setPreservesAll();
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}
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static bool isAlwaysLive(Instruction *I) {
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return isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) ||
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I->isEHPad() || I->mayHaveSideEffects();
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}
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void DemandedBits::determineLiveOperandBits(
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const Instruction *UserI, const Instruction *I, unsigned OperandNo,
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const APInt &AOut, APInt &AB, 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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case Instruction::Mul:
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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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case Instruction::ICmp:
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// Count the number of leading zeroes in each operand.
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ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
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auto NumLeadingZeroes = std::min(KnownZero.countLeadingOnes(),
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KnownZero2.countLeadingOnes());
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AB = ~APInt::getHighBitsSet(BitWidth, NumLeadingZeroes);
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break;
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}
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}
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bool DemandedBits::runOnFunction(Function& Fn) {
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F = &Fn;
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Analyzed = false;
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return false;
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}
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void DemandedBits::performAnalysis() {
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if (Analyzed)
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// Analysis already completed for this function.
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return;
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Analyzed = true;
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AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(*F);
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DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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Visited.clear();
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AliveBits.clear();
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SmallVector<Instruction*, 128> Worklist;
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// Collect the set of "root" instructions that are known live.
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for (Instruction &I : instructions(*F)) {
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if (!isAlwaysLive(&I))
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continue;
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DEBUG(dbgs() << "DemandedBits: 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() << "DemandedBits: 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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}
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APInt DemandedBits::getDemandedBits(Instruction *I) {
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performAnalysis();
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const DataLayout &DL = I->getParent()->getModule()->getDataLayout();
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if (AliveBits.count(I))
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return AliveBits[I];
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return APInt::getAllOnesValue(DL.getTypeSizeInBits(I->getType()));
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}
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bool DemandedBits::isInstructionDead(Instruction *I) {
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performAnalysis();
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return !Visited.count(I) && AliveBits.find(I) == AliveBits.end() &&
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!isAlwaysLive(I);
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}
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void DemandedBits::print(raw_ostream &OS, const Module *M) const {
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// This is gross. But the alternative is making all the state mutable
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// just because of this one debugging method.
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const_cast<DemandedBits*>(this)->performAnalysis();
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for (auto &KV : AliveBits) {
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OS << "DemandedBits: 0x" << utohexstr(KV.second.getLimitedValue()) << " for "
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<< *KV.first << "\n";
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}
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}
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FunctionPass *llvm::createDemandedBitsPass() {
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return new DemandedBits();
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}
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