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Separate out BDCE's analysis into a separate DemandedBits analysis. 

This allows other areas of the compiler to use BDCE's bit-tracking.
NFCI.

llvm-svn: 245039
This commit is contained in:
James Molloy 2015-08-14 11:09:09 +00:00
parent 1fdc177ccf
commit 87405c7f66
6 changed files with 456 additions and 328 deletions
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71
llvm/include/llvm/Analysis/DemandedBits.h Normal file
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@ -0,0 +1,71 @@
//===-- llvm/Analysis/DemandedBits.h - Determine demanded bits --*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a demanded bits analysis. A demanded bit is one that
// contributes to a result; bits that are not demanded can be either zero or
// one without affecting control or data flow. For example in this sequence:
//
// %1 = add i32 %x, %y
// %2 = trunc i32 %1 to i16
//
// Only the lowest 16 bits of %1 are demanded; the rest are removed by the
// trunc.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_DEMANDED_BITS_H
#define LLVM_ANALYSIS_DEMANDED_BITS_H
#include "llvm/Pass.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
namespace llvm {
class FunctionPass;
class Function;
class Instruction;
class DominatorTree;
class AssumptionCache;
struct DemandedBits : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DemandedBits();
bool runOnFunction(Function& F) override;
void getAnalysisUsage(AnalysisUsage& AU) const override;
/// Return the bits demanded from instruction I.
APInt getDemandedBits(Instruction *I);
/// Return true if, during analysis, I could not be reached.
bool isInstructionDead(Instruction *I);
private:
void determineLiveOperandBits(const Instruction *UserI,
const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB,
APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2);
AssumptionCache *AC;
DominatorTree *DT;
// The set of visited instructions (non-integer-typed only).
SmallPtrSet<Instruction*, 128> Visited;
DenseMap<Instruction *, APInt> AliveBits;
};
/// Create a demanded bits analysis pass.
FunctionPass *createDemandedBitsPass();
} // End llvm namespace
#endif

1
llvm/include/llvm/InitializePasses.h
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@ -303,6 +303,7 @@ void initializeDwarfEHPreparePass(PassRegistry&);
void initializeFloat2IntPass(PassRegistry&);
void initializeLoopDistributePass(PassRegistry&);
void initializeSjLjEHPreparePass(PassRegistry&);
void initializeDemandedBitsPass(PassRegistry&);
}
#endif

1
llvm/lib/Analysis/Analysis.cpp
View File

@ -36,6 +36,7 @@ void llvm::initializeAnalysis(PassRegistry &Registry) {
initializeCFLAliasAnalysisPass(Registry);
initializeDependenceAnalysisPass(Registry);
initializeDelinearizationPass(Registry);
initializeDemandedBitsPass(Registry);
initializeDivergenceAnalysisPass(Registry);
initializeDominanceFrontierPass(Registry);
initializeDomViewerPass(Registry);

1
llvm/lib/Analysis/CMakeLists.txt
View File

@ -18,6 +18,7 @@ add_llvm_library(LLVMAnalysis
CodeMetrics.cpp
ConstantFolding.cpp
Delinearization.cpp
DemandedBits.cpp
DependenceAnalysis.cpp
DivergenceAnalysis.cpp
DomPrinter.cpp

364
llvm/lib/Analysis/DemandedBits.cpp Normal file
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@ -0,0 +1,364 @@
//===---- DemandedBits.cpp - Determine demanded bits -----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a demanded bits analysis. A demanded bit is one that
// contributes to a result; bits that are not demanded can be either zero or
// one without affecting control or data flow. For example in this sequence:
//
// %1 = add i32 %x, %y
// %2 = trunc i32 %1 to i16
//
// Only the lowest 16 bits of %1 are demanded; the rest are removed by the
// trunc.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/DemandedBits.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "demanded-bits"
char DemandedBits::ID = 0;
INITIALIZE_PASS_BEGIN(DemandedBits, "demanded-bits", "Demanded bits analysis",
false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(DemandedBits, "demanded-bits", "Demanded bits analysis",
false, false)
DemandedBits::DemandedBits() : FunctionPass(ID) {
initializeDemandedBitsPass(*PassRegistry::getPassRegistry());
}
void DemandedBits::getAnalysisUsage(AnalysisUsage& AU) const {
AU.setPreservesCFG();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.setPreservesAll();
}
static bool isAlwaysLive(Instruction *I) {
return isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) ||
I->isEHPad() || I->mayHaveSideEffects();
}
void
DemandedBits::determineLiveOperandBits(const Instruction *UserI,
const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB,
APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2) {
unsigned BitWidth = AB.getBitWidth();
// We're called once per operand, but for some instructions, we need to
// compute known bits of both operands in order to determine the live bits of
// either (when both operands are instructions themselves). We don't,
// however, want to do this twice, so we cache the result in APInts that live
// in the caller. For the two-relevant-operands case, both operand values are
// provided here.
auto ComputeKnownBits =
[&](unsigned BitWidth, const Value *V1, const Value *V2) {
const DataLayout &DL = I->getModule()->getDataLayout();
KnownZero = APInt(BitWidth, 0);
KnownOne = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V1), KnownZero, KnownOne, DL, 0,
AC, UserI, DT);
if (V2) {
KnownZero2 = APInt(BitWidth, 0);
KnownOne2 = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V2), KnownZero2, KnownOne2, DL,
0, AC, UserI, DT);
}
};
switch (UserI->getOpcode()) {
default: break;
case Instruction::Call:
case Instruction::Invoke:
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::bswap:
// The alive bits of the input are the swapped alive bits of
// the output.
AB = AOut.byteSwap();
break;
case Intrinsic::ctlz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the left of, and including, the leftmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getHighBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countLeadingZeros()+1));
}
break;
case Intrinsic::cttz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the right of, and including, the rightmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getLowBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countTrailingZeros()+1));
}
break;
}
break;
case Instruction::Add:
case Instruction::Sub:
// Find the highest live output bit. We don't need any more input
// bits than that (adds, and thus subtracts, ripple only to the
// left).
AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
break;
case Instruction::Shl:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.lshr(ShiftAmt);
// If the shift is nuw/nsw, then the high bits are not dead
// (because we've promised that they *must* be zero).
const ShlOperator *S = cast<ShlOperator>(UserI);
if (S->hasNoSignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
else if (S->hasNoUnsignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::LShr:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.shl(ShiftAmt);
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<LShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::AShr:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.shl(ShiftAmt);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(BitWidth, ShiftAmt))
.getBoolValue())
AB.setBit(BitWidth-1);
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<AShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::And:
AB = AOut;
// For bits that are known zero, the corresponding bits in the
// other operand are dead (unless they're both zero, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownZero2;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownZero & ~KnownZero2);
}
break;
case Instruction::Or:
AB = AOut;
// For bits that are known one, the corresponding bits in the
// other operand are dead (unless they're both one, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownOne2;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownOne & ~KnownOne2);
}
break;
case Instruction::Xor:
case Instruction::PHI:
AB = AOut;
break;
case Instruction::Trunc:
AB = AOut.zext(BitWidth);
break;
case Instruction::ZExt:
AB = AOut.trunc(BitWidth);
break;
case Instruction::SExt:
AB = AOut.trunc(BitWidth);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(AOut.getBitWidth(),
AOut.getBitWidth() - BitWidth))
.getBoolValue())
AB.setBit(BitWidth-1);
break;
case Instruction::Select:
if (OperandNo != 0)
AB = AOut;
break;
}
}
bool DemandedBits::runOnFunction(Function& F) {
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
Visited.clear();
AliveBits.clear();
SmallVector<Instruction*, 128> Worklist;
// Collect the set of "root" instructions that are known live.
for (Instruction &I : instructions(F)) {
if (!isAlwaysLive(&I))
continue;
DEBUG(dbgs() << "DemandedBits: Root: " << I << "\n");
// For integer-valued instructions, set up an initial empty set of alive
// bits and add the instruction to the work list. For other instructions
// add their operands to the work list (for integer values operands, mark
// all bits as live).
if (IntegerType *IT = dyn_cast<IntegerType>(I.getType())) {
if (!AliveBits.count(&I)) {
AliveBits[&I] = APInt(IT->getBitWidth(), 0);
Worklist.push_back(&I);
}
continue;
}
// Non-integer-typed instructions...
for (Use &OI : I.operands()) {
if (Instruction *J = dyn_cast<Instruction>(OI)) {
if (IntegerType *IT = dyn_cast<IntegerType>(J->getType()))
AliveBits[J] = APInt::getAllOnesValue(IT->getBitWidth());
Worklist.push_back(J);
}
}
// To save memory, we don't add I to the Visited set here. Instead, we
// check isAlwaysLive on every instruction when searching for dead
// instructions later (we need to check isAlwaysLive for the
// integer-typed instructions anyway).
}
// Propagate liveness backwards to operands.
while (!Worklist.empty()) {
Instruction *UserI = Worklist.pop_back_val();
DEBUG(dbgs() << "DemandedBits: Visiting: " << *UserI);
APInt AOut;
if (UserI->getType()->isIntegerTy()) {
AOut = AliveBits[UserI];
DEBUG(dbgs() << " Alive Out: " << AOut);
}
DEBUG(dbgs() << "\n");
if (!UserI->getType()->isIntegerTy())
Visited.insert(UserI);
APInt KnownZero, KnownOne, KnownZero2, KnownOne2;
// Compute the set of alive bits for each operand. These are anded into the
// existing set, if any, and if that changes the set of alive bits, the
// operand is added to the work-list.
for (Use &OI : UserI->operands()) {
if (Instruction *I = dyn_cast<Instruction>(OI)) {
if (IntegerType *IT = dyn_cast<IntegerType>(I->getType())) {
unsigned BitWidth = IT->getBitWidth();
APInt AB = APInt::getAllOnesValue(BitWidth);
if (UserI->getType()->isIntegerTy() && !AOut &&
!isAlwaysLive(UserI)) {
AB = APInt(BitWidth, 0);
} else {
// If all bits of the output are dead, then all bits of the input
// Bits of each operand that are used to compute alive bits of the
// output are alive, all others are dead.
determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
KnownZero, KnownOne,
KnownZero2, KnownOne2);
}
// If we've added to the set of alive bits (or the operand has not
// been previously visited), then re-queue the operand to be visited
// again.
APInt ABPrev(BitWidth, 0);
auto ABI = AliveBits.find(I);
if (ABI != AliveBits.end())
ABPrev = ABI->second;
APInt ABNew = AB | ABPrev;
if (ABNew != ABPrev || ABI == AliveBits.end()) {
AliveBits[I] = std::move(ABNew);
Worklist.push_back(I);
}
} else if (!Visited.count(I)) {
Worklist.push_back(I);
}
}
}
}
return false;
}
APInt DemandedBits::getDemandedBits(Instruction *I) {
const DataLayout &DL = I->getParent()->getModule()->getDataLayout();
if (AliveBits.count(I))
return AliveBits[I];
return APInt::getAllOnesValue(DL.getTypeSizeInBits(I->getType()));
}
bool DemandedBits::isInstructionDead(Instruction *I) {
return !Visited.count(I) && AliveBits.find(I) == AliveBits.end() &&
!isAlwaysLive(I);
}
FunctionPass *llvm::createDemandedBitsPass() {
return new DemandedBits();
}

346
llvm/lib/Transforms/Scalar/BDCE.cpp
View File

@ -15,26 +15,17 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/Analysis/DemandedBits.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "bdce"
@ -53,342 +44,41 @@ struct BDCE : public FunctionPass {
void getAnalysisUsage(AnalysisUsage& AU) const override {
AU.setPreservesCFG();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<DemandedBits>();
}
void determineLiveOperandBits(const Instruction *UserI,
const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB,
APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2);
AssumptionCache *AC;
DominatorTree *DT;
};
}
char BDCE::ID = 0;
INITIALIZE_PASS_BEGIN(BDCE, "bdce", "Bit-Tracking Dead Code Elimination",
false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DemandedBits)
INITIALIZE_PASS_END(BDCE, "bdce", "Bit-Tracking Dead Code Elimination",
false, false)
static bool isAlwaysLive(Instruction *I) {
return isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) || I->isEHPad() ||
I->mayHaveSideEffects();
}
void BDCE::determineLiveOperandBits(const Instruction *UserI,
const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB,
APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2) {
unsigned BitWidth = AB.getBitWidth();
// We're called once per operand, but for some instructions, we need to
// compute known bits of both operands in order to determine the live bits of
// either (when both operands are instructions themselves). We don't,
// however, want to do this twice, so we cache the result in APInts that live
// in the caller. For the two-relevant-operands case, both operand values are
// provided here.
auto ComputeKnownBits =
[&](unsigned BitWidth, const Value *V1, const Value *V2) {
const DataLayout &DL = I->getModule()->getDataLayout();
KnownZero = APInt(BitWidth, 0);
KnownOne = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V1), KnownZero, KnownOne, DL, 0,
AC, UserI, DT);
if (V2) {
KnownZero2 = APInt(BitWidth, 0);
KnownOne2 = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V2), KnownZero2, KnownOne2, DL,
0, AC, UserI, DT);
}
};
switch (UserI->getOpcode()) {
default: break;
case Instruction::Call:
case Instruction::Invoke:
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::bswap:
// The alive bits of the input are the swapped alive bits of
// the output.
AB = AOut.byteSwap();
break;
case Intrinsic::ctlz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the left of, and including, the leftmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getHighBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countLeadingZeros()+1));
}
break;
case Intrinsic::cttz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the right of, and including, the rightmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getLowBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countTrailingZeros()+1));
}
break;
}
break;
case Instruction::Add:
case Instruction::Sub:
// Find the highest live output bit. We don't need any more input
// bits than that (adds, and thus subtracts, ripple only to the
// left).
AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
break;
case Instruction::Shl:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.lshr(ShiftAmt);
// If the shift is nuw/nsw, then the high bits are not dead
// (because we've promised that they *must* be zero).
const ShlOperator *S = cast<ShlOperator>(UserI);
if (S->hasNoSignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
else if (S->hasNoUnsignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::LShr:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.shl(ShiftAmt);
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<LShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::AShr:
if (OperandNo == 0)
if (ConstantInt *CI =
dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = CI->getLimitedValue(BitWidth-1);
AB = AOut.shl(ShiftAmt);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(BitWidth, ShiftAmt))
.getBoolValue())
AB.setBit(BitWidth-1);
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<AShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::And:
AB = AOut;
// For bits that are known zero, the corresponding bits in the
// other operand are dead (unless they're both zero, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownZero2;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownZero & ~KnownZero2);
}
break;
case Instruction::Or:
AB = AOut;
// For bits that are known one, the corresponding bits in the
// other operand are dead (unless they're both one, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownOne2;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownOne & ~KnownOne2);
}
break;
case Instruction::Xor:
case Instruction::PHI:
AB = AOut;
break;
case Instruction::Trunc:
AB = AOut.zext(BitWidth);
break;
case Instruction::ZExt:
AB = AOut.trunc(BitWidth);
break;
case Instruction::SExt:
AB = AOut.trunc(BitWidth);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(AOut.getBitWidth(),
AOut.getBitWidth() - BitWidth))
.getBoolValue())
AB.setBit(BitWidth-1);
break;
case Instruction::Select:
if (OperandNo != 0)
AB = AOut;
break;
}
}
bool BDCE::runOnFunction(Function& F) {
if (skipOptnoneFunction(F))
return false;
DemandedBits &DB = getAnalysis<DemandedBits>();
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
DenseMap<Instruction *, APInt> AliveBits;
SmallVector<Instruction*, 128> Worklist;
// The set of visited instructions (non-integer-typed only).
SmallPtrSet<Instruction*, 128> Visited;
// Collect the set of "root" instructions that are known live.
for (Instruction &I : instructions(F)) {
if (!isAlwaysLive(&I))
continue;
DEBUG(dbgs() << "BDCE: Root: " << I << "\n");
// For integer-valued instructions, set up an initial empty set of alive
// bits and add the instruction to the work list. For other instructions
// add their operands to the work list (for integer values operands, mark
// all bits as live).
if (IntegerType *IT = dyn_cast<IntegerType>(I.getType())) {
if (!AliveBits.count(&I)) {
AliveBits[&I] = APInt(IT->getBitWidth(), 0);
Worklist.push_back(&I);
}
continue;
}
// Non-integer-typed instructions...
for (Use &OI : I.operands()) {
if (Instruction *J = dyn_cast<Instruction>(OI)) {
if (IntegerType *IT = dyn_cast<IntegerType>(J->getType()))
AliveBits[J] = APInt::getAllOnesValue(IT->getBitWidth());
Worklist.push_back(J);
}
}
// To save memory, we don't add I to the Visited set here. Instead, we
// check isAlwaysLive on every instruction when searching for dead
// instructions later (we need to check isAlwaysLive for the
// integer-typed instructions anyway).
}
// Propagate liveness backwards to operands.
while (!Worklist.empty()) {
Instruction *UserI = Worklist.pop_back_val();
DEBUG(dbgs() << "BDCE: Visiting: " << *UserI);
APInt AOut;
if (UserI->getType()->isIntegerTy()) {
AOut = AliveBits[UserI];
DEBUG(dbgs() << " Alive Out: " << AOut);
}
DEBUG(dbgs() << "\n");
if (!UserI->getType()->isIntegerTy())
Visited.insert(UserI);
APInt KnownZero, KnownOne, KnownZero2, KnownOne2;
// Compute the set of alive bits for each operand. These are anded into the
// existing set, if any, and if that changes the set of alive bits, the
// operand is added to the work-list.
for (Use &OI : UserI->operands()) {
if (Instruction *I = dyn_cast<Instruction>(OI)) {
if (IntegerType *IT = dyn_cast<IntegerType>(I->getType())) {
unsigned BitWidth = IT->getBitWidth();
APInt AB = APInt::getAllOnesValue(BitWidth);
if (UserI->getType()->isIntegerTy() && !AOut &&
!isAlwaysLive(UserI)) {
AB = APInt(BitWidth, 0);
} else {
// If all bits of the output are dead, then all bits of the input
// Bits of each operand that are used to compute alive bits of the
// output are alive, all others are dead.
determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
KnownZero, KnownOne,
KnownZero2, KnownOne2);
}
// If we've added to the set of alive bits (or the operand has not
// been previously visited), then re-queue the operand to be visited
// again.
APInt ABPrev(BitWidth, 0);
auto ABI = AliveBits.find(I);
if (ABI != AliveBits.end())
ABPrev = ABI->second;
APInt ABNew = AB | ABPrev;
if (ABNew != ABPrev || ABI == AliveBits.end()) {
AliveBits[I] = std::move(ABNew);
Worklist.push_back(I);
}
} else if (!Visited.count(I)) {
Worklist.push_back(I);
}
}
}
}
bool Changed = false;
// The inverse of the live set is the dead set. These are those instructions
// which have no side effects and do not influence the control flow or return
// value of the function, and may therefore be deleted safely.
// NOTE: We reuse the Worklist vector here for memory efficiency.
for (Instruction &I : instructions(F)) {
// For live instructions that have all dead bits, first make them dead by
// replacing all uses with something else. Then, if they don't need to
// remain live (because they have side effects, etc.) we can remove them.
if (I.getType()->isIntegerTy()) {
auto ABI = AliveBits.find(&I);
if (ABI != AliveBits.end()) {
if (ABI->second.getBoolValue())
continue;
DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
// FIXME: In theory we could substitute undef here instead of zero.
// This should be reconsidered once we settle on the semantics of
// undef, poison, etc.
Value *Zero = ConstantInt::get(I.getType(), 0);
++NumSimplified;
I.replaceAllUsesWith(Zero);
Changed = true;
}
} else if (Visited.count(&I)) {
continue;
if (I.getType()->isIntegerTy() &&
!DB.getDemandedBits(&I).getBoolValue()) {
// For live instructions that have all dead bits, first make them dead by
// replacing all uses with something else. Then, if they don't need to
// remain live (because they have side effects, etc.) we can remove them.
DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
// FIXME: In theory we could substitute undef here instead of zero.
// This should be reconsidered once we settle on the semantics of
// undef, poison, etc.
Value *Zero = ConstantInt::get(I.getType(), 0);
++NumSimplified;
I.replaceAllUsesWith(Zero);
Changed = true;
}
if (isAlwaysLive(&I))
if (!DB.isInstructionDead(&I))
continue;
Worklist.push_back(&I);