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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