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llvm-project/llvm/lib/Analysis/ValueTracking.cpp

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//===- ValueTracking.cpp - Walk computations to compute properties --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains routines that help analyze properties that chains of
// computations have.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/GlobalVariable.h"
#include "llvm/GlobalAlias.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include <cstring>
using namespace llvm;
/// ComputeMaskedBits - Determine which of the bits specified in Mask are
/// known to be either zero or one and return them in the KnownZero/KnownOne
/// bit sets. This code only analyzes bits in Mask, in order to short-circuit
/// processing.
/// NOTE: we cannot consider 'undef' to be "IsZero" here. The problem is that
/// we cannot optimize based on the assumption that it is zero without changing
/// it to be an explicit zero. If we don't change it to zero, other code could
/// optimized based on the contradictory assumption that it is non-zero.
/// Because instcombine aggressively folds operations with undef args anyway,
/// this won't lose us code quality.
///
/// This function is defined on values with integer type, values with pointer
/// type (but only if TD is non-null), and vectors of integers. In the case
/// where V is a vector, the mask, known zero, and known one values are the
/// same width as the vector element, and the bit is set only if it is true
/// for all of the elements in the vector.
void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
APInt &KnownZero, APInt &KnownOne,
const TargetData *TD, unsigned Depth) {
const unsigned MaxDepth = 6;
assert(V && "No Value?");
assert(Depth <= MaxDepth && "Limit Search Depth");
unsigned BitWidth = Mask.getBitWidth();
assert((V->getType()->isIntOrIntVectorTy() || V->getType()->isPointerTy())
&& "Not integer or pointer type!");
assert((!TD ||
TD->getTypeSizeInBits(V->getType()->getScalarType()) == BitWidth) &&
(!V->getType()->isIntOrIntVectorTy() ||
V->getType()->getScalarSizeInBits() == BitWidth) &&
KnownZero.getBitWidth() == BitWidth &&
KnownOne.getBitWidth() == BitWidth &&
"V, Mask, KnownOne and KnownZero should have same BitWidth");
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
// We know all of the bits for a constant!
KnownOne = CI->getValue() & Mask;
KnownZero = ~KnownOne & Mask;
return;
}
// Null and aggregate-zero are all-zeros.
if (isa<ConstantPointerNull>(V) ||
isa<ConstantAggregateZero>(V)) {
KnownOne.clear();
KnownZero = Mask;
return;
}
// Handle a constant vector by taking the intersection of the known bits of
// each element.
if (ConstantVector *CV = dyn_cast<ConstantVector>(V)) {
KnownZero.set(); KnownOne.set();
for (unsigned i = 0, e = CV->getNumOperands(); i != e; ++i) {
APInt KnownZero2(BitWidth, 0), KnownOne2(BitWidth, 0);
ComputeMaskedBits(CV->getOperand(i), Mask, KnownZero2, KnownOne2,
TD, Depth);
KnownZero &= KnownZero2;
KnownOne &= KnownOne2;
}
return;
}
// The address of an aligned GlobalValue has trailing zeros.
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
unsigned Align = GV->getAlignment();
if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) {
const Type *ObjectType = GV->getType()->getElementType();
// If the object is defined in the current Module, we'll be giving
// it the preferred alignment. Otherwise, we have to assume that it
// may only have the minimum ABI alignment.
if (!GV->isDeclaration() && !GV->mayBeOverridden())
Align = TD->getPrefTypeAlignment(ObjectType);
else
Align = TD->getABITypeAlignment(ObjectType);
}
if (Align > 0)
KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
CountTrailingZeros_32(Align));
else
KnownZero.clear();
KnownOne.clear();
return;
}
// A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
// the bits of its aliasee.
if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (GA->mayBeOverridden()) {
KnownZero.clear(); KnownOne.clear();
} else {
ComputeMaskedBits(GA->getAliasee(), Mask, KnownZero, KnownOne,
TD, Depth+1);
}
return;
}
KnownZero.clear(); KnownOne.clear(); // Start out not knowing anything.
if (Depth == MaxDepth || Mask == 0)
return; // Limit search depth.
Operator *I = dyn_cast<Operator>(V);
if (!I) return;
APInt KnownZero2(KnownZero), KnownOne2(KnownOne);
switch (I->getOpcode()) {
default: break;
case Instruction::And: {
// If either the LHS or the RHS are Zero, the result is zero.
ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
APInt Mask2(Mask & ~KnownZero);
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
// Output known-1 bits are only known if set in both the LHS & RHS.
KnownOne &= KnownOne2;
// Output known-0 are known to be clear if zero in either the LHS | RHS.
KnownZero |= KnownZero2;
return;
}
case Instruction::Or: {
ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
APInt Mask2(Mask & ~KnownOne);
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
// Output known-0 bits are only known if clear in both the LHS & RHS.
KnownZero &= KnownZero2;
// Output known-1 are known to be set if set in either the LHS | RHS.
KnownOne |= KnownOne2;
return;
}
case Instruction::Xor: {
ComputeMaskedBits(I->getOperand(1), Mask, KnownZero, KnownOne, TD, Depth+1);
ComputeMaskedBits(I->getOperand(0), Mask, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
// Output known-0 bits are known if clear or set in both the LHS & RHS.
APInt KnownZeroOut = (KnownZero & KnownZero2) | (KnownOne & KnownOne2);
// Output known-1 are known to be set if set in only one of the LHS, RHS.
KnownOne = (KnownZero & KnownOne2) | (KnownOne & KnownZero2);
KnownZero = KnownZeroOut;
return;
}
case Instruction::Mul: {
APInt Mask2 = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero, KnownOne, TD,Depth+1);
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
// If low bits are zero in either operand, output low known-0 bits.
// Also compute a conserative estimate for high known-0 bits.
// More trickiness is possible, but this is sufficient for the
// interesting case of alignment computation.
KnownOne.clear();
unsigned TrailZ = KnownZero.countTrailingOnes() +
KnownZero2.countTrailingOnes();
unsigned LeadZ = std::max(KnownZero.countLeadingOnes() +
KnownZero2.countLeadingOnes(),
BitWidth) - BitWidth;
TrailZ = std::min(TrailZ, BitWidth);
LeadZ = std::min(LeadZ, BitWidth);
KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) |
APInt::getHighBitsSet(BitWidth, LeadZ);
KnownZero &= Mask;
return;
}
case Instruction::UDiv: {
// For the purposes of computing leading zeros we can conservatively
// treat a udiv as a logical right shift by the power of 2 known to
// be less than the denominator.
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(I->getOperand(0),
AllOnes, KnownZero2, KnownOne2, TD, Depth+1);
unsigned LeadZ = KnownZero2.countLeadingOnes();
KnownOne2.clear();
KnownZero2.clear();
ComputeMaskedBits(I->getOperand(1),
AllOnes, KnownZero2, KnownOne2, TD, Depth+1);
unsigned RHSUnknownLeadingOnes = KnownOne2.countLeadingZeros();
if (RHSUnknownLeadingOnes != BitWidth)
LeadZ = std::min(BitWidth,
LeadZ + BitWidth - RHSUnknownLeadingOnes - 1);
KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ) & Mask;
return;
}
case Instruction::Select:
ComputeMaskedBits(I->getOperand(2), Mask, KnownZero, KnownOne, TD, Depth+1);
ComputeMaskedBits(I->getOperand(1), Mask, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
// Only known if known in both the LHS and RHS.
KnownOne &= KnownOne2;
KnownZero &= KnownZero2;
return;
case Instruction::FPTrunc:
case Instruction::FPExt:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::SIToFP:
case Instruction::UIToFP:
return; // Can't work with floating point.
case Instruction::PtrToInt:
case Instruction::IntToPtr:
// We can't handle these if we don't know the pointer size.
if (!TD) return;
// FALL THROUGH and handle them the same as zext/trunc.
case Instruction::ZExt:
case Instruction::Trunc: {
const Type *SrcTy = I->getOperand(0)->getType();
unsigned SrcBitWidth;
// Note that we handle pointer operands here because of inttoptr/ptrtoint
// which fall through here.
if (SrcTy->isPointerTy())
SrcBitWidth = TD->getTypeSizeInBits(SrcTy);
else
SrcBitWidth = SrcTy->getScalarSizeInBits();
APInt MaskIn(Mask);
MaskIn.zextOrTrunc(SrcBitWidth);
KnownZero.zextOrTrunc(SrcBitWidth);
KnownOne.zextOrTrunc(SrcBitWidth);
ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD,
Depth+1);
KnownZero.zextOrTrunc(BitWidth);
KnownOne.zextOrTrunc(BitWidth);
// Any top bits are known to be zero.
if (BitWidth > SrcBitWidth)
KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
return;
}
case Instruction::BitCast: {
const Type *SrcTy = I->getOperand(0)->getType();
if ((SrcTy->isIntegerTy() || SrcTy->isPointerTy()) &&
// TODO: For now, not handling conversions like:
// (bitcast i64 %x to <2 x i32>)
!I->getType()->isVectorTy()) {
ComputeMaskedBits(I->getOperand(0), Mask, KnownZero, KnownOne, TD,
Depth+1);
return;
}
break;
}
case Instruction::SExt: {
// Compute the bits in the result that are not present in the input.
unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();
APInt MaskIn(Mask);
MaskIn.trunc(SrcBitWidth);
KnownZero.trunc(SrcBitWidth);
KnownOne.trunc(SrcBitWidth);
ComputeMaskedBits(I->getOperand(0), MaskIn, KnownZero, KnownOne, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
KnownZero.zext(BitWidth);
KnownOne.zext(BitWidth);
// If the sign bit of the input is known set or clear, then we know the
// top bits of the result.
if (KnownZero[SrcBitWidth-1]) // Input sign bit known zero
KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
else if (KnownOne[SrcBitWidth-1]) // Input sign bit known set
KnownOne |= APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth);
return;
}
case Instruction::Shl:
// (shl X, C1) & C2 == 0 iff (X & C2 >>u C1) == 0
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
APInt Mask2(Mask.lshr(ShiftAmt));
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
KnownZero <<= ShiftAmt;
KnownOne <<= ShiftAmt;
KnownZero |= APInt::getLowBitsSet(BitWidth, ShiftAmt); // low bits known 0
return;
}
break;
case Instruction::LShr:
// (ushr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
// Compute the new bits that are at the top now.
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
// Unsigned shift right.
APInt Mask2(Mask.shl(ShiftAmt));
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero,KnownOne, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
KnownOne = APIntOps::lshr(KnownOne, ShiftAmt);
// high bits known zero.
KnownZero |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
return;
}
break;
case Instruction::AShr:
// (ashr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
// Compute the new bits that are at the top now.
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth);
// Signed shift right.
APInt Mask2(Mask.shl(ShiftAmt));
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
KnownOne = APIntOps::lshr(KnownOne, ShiftAmt);
APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
if (KnownZero[BitWidth-ShiftAmt-1]) // New bits are known zero.
KnownZero |= HighBits;
else if (KnownOne[BitWidth-ShiftAmt-1]) // New bits are known one.
KnownOne |= HighBits;
return;
}
break;
case Instruction::Sub: {
if (ConstantInt *CLHS = dyn_cast<ConstantInt>(I->getOperand(0))) {
// We know that the top bits of C-X are clear if X contains less bits
// than C (i.e. no wrap-around can happen). For example, 20-X is
// positive if we can prove that X is >= 0 and < 16.
if (!CLHS->getValue().isNegative()) {
unsigned NLZ = (CLHS->getValue()+1).countLeadingZeros();
// NLZ can't be BitWidth with no sign bit
APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1);
ComputeMaskedBits(I->getOperand(1), MaskV, KnownZero2, KnownOne2,
TD, Depth+1);
// If all of the MaskV bits are known to be zero, then we know the
// output top bits are zero, because we now know that the output is
// from [0-C].
if ((KnownZero2 & MaskV) == MaskV) {
unsigned NLZ2 = CLHS->getValue().countLeadingZeros();
// Top bits known zero.
KnownZero = APInt::getHighBitsSet(BitWidth, NLZ2) & Mask;
}
}
}
}
// fall through
case Instruction::Add: {
// If one of the operands has trailing zeros, then the bits that the
// other operand has in those bit positions will be preserved in the
// result. For an add, this works with either operand. For a subtract,
// this only works if the known zeros are in the right operand.
APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
APInt Mask2 = APInt::getLowBitsSet(BitWidth,
BitWidth - Mask.countLeadingZeros());
ComputeMaskedBits(I->getOperand(0), Mask2, LHSKnownZero, LHSKnownOne, TD,
Depth+1);
assert((LHSKnownZero & LHSKnownOne) == 0 &&
"Bits known to be one AND zero?");
unsigned LHSKnownZeroOut = LHSKnownZero.countTrailingOnes();
ComputeMaskedBits(I->getOperand(1), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
assert((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
unsigned RHSKnownZeroOut = KnownZero2.countTrailingOnes();
// Determine which operand has more trailing zeros, and use that
// many bits from the other operand.
if (LHSKnownZeroOut > RHSKnownZeroOut) {
if (I->getOpcode() == Instruction::Add) {
APInt Mask = APInt::getLowBitsSet(BitWidth, LHSKnownZeroOut);
KnownZero |= KnownZero2 & Mask;
KnownOne |= KnownOne2 & Mask;
} else {
// If the known zeros are in the left operand for a subtract,
// fall back to the minimum known zeros in both operands.
KnownZero |= APInt::getLowBitsSet(BitWidth,
std::min(LHSKnownZeroOut,
RHSKnownZeroOut));
}
} else if (RHSKnownZeroOut >= LHSKnownZeroOut) {
APInt Mask = APInt::getLowBitsSet(BitWidth, RHSKnownZeroOut);
KnownZero |= LHSKnownZero & Mask;
KnownOne |= LHSKnownOne & Mask;
}
return;
}
case Instruction::SRem:
if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
APInt RA = Rem->getValue().abs();
if (RA.isPowerOf2()) {
APInt LowBits = RA - 1;
APInt Mask2 = LowBits | APInt::getSignBit(BitWidth);
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
// The low bits of the first operand are unchanged by the srem.
KnownZero = KnownZero2 & LowBits;
KnownOne = KnownOne2 & LowBits;
// If the first operand is non-negative or has all low bits zero, then
// the upper bits are all zero.
if (KnownZero2[BitWidth-1] || ((KnownZero2 & LowBits) == LowBits))
KnownZero |= ~LowBits;
// If the first operand is negative and not all low bits are zero, then
// the upper bits are all one.
if (KnownOne2[BitWidth-1] && ((KnownOne2 & LowBits) != 0))
KnownOne |= ~LowBits;
KnownZero &= Mask;
KnownOne &= Mask;
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
}
}
break;
case Instruction::URem: {
if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
APInt RA = Rem->getValue();
if (RA.isPowerOf2()) {
APInt LowBits = (RA - 1);
APInt Mask2 = LowBits & Mask;
KnownZero |= ~LowBits & Mask;
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero, KnownOne, TD,
Depth+1);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
break;
}
}
// Since the result is less than or equal to either operand, any leading
// zero bits in either operand must also exist in the result.
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(I->getOperand(0), AllOnes, KnownZero, KnownOne,
TD, Depth+1);
ComputeMaskedBits(I->getOperand(1), AllOnes, KnownZero2, KnownOne2,
TD, Depth+1);
unsigned Leaders = std::max(KnownZero.countLeadingOnes(),
KnownZero2.countLeadingOnes());
KnownOne.clear();
KnownZero = APInt::getHighBitsSet(BitWidth, Leaders) & Mask;
break;
}
case Instruction::Alloca: {
AllocaInst *AI = cast<AllocaInst>(V);
unsigned Align = AI->getAlignment();
if (Align == 0 && TD)
Align = TD->getABITypeAlignment(AI->getType()->getElementType());
if (Align > 0)
KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
CountTrailingZeros_32(Align));
break;
}
case Instruction::GetElementPtr: {
// Analyze all of the subscripts of this getelementptr instruction
// to determine if we can prove known low zero bits.
APInt LocalMask = APInt::getAllOnesValue(BitWidth);
APInt LocalKnownZero(BitWidth, 0), LocalKnownOne(BitWidth, 0);
ComputeMaskedBits(I->getOperand(0), LocalMask,
LocalKnownZero, LocalKnownOne, TD, Depth+1);
unsigned TrailZ = LocalKnownZero.countTrailingOnes();
gep_type_iterator GTI = gep_type_begin(I);
for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i, ++GTI) {
Value *Index = I->getOperand(i);
if (const StructType *STy = dyn_cast<StructType>(*GTI)) {
// Handle struct member offset arithmetic.
if (!TD) return;
const StructLayout *SL = TD->getStructLayout(STy);
unsigned Idx = cast<ConstantInt>(Index)->getZExtValue();
uint64_t Offset = SL->getElementOffset(Idx);
TrailZ = std::min(TrailZ,
CountTrailingZeros_64(Offset));
} else {
// Handle array index arithmetic.
const Type *IndexedTy = GTI.getIndexedType();
if (!IndexedTy->isSized()) return;
unsigned GEPOpiBits = Index->getType()->getScalarSizeInBits();
uint64_t TypeSize = TD ? TD->getTypeAllocSize(IndexedTy) : 1;
LocalMask = APInt::getAllOnesValue(GEPOpiBits);
LocalKnownZero = LocalKnownOne = APInt(GEPOpiBits, 0);
ComputeMaskedBits(Index, LocalMask,
LocalKnownZero, LocalKnownOne, TD, Depth+1);
TrailZ = std::min(TrailZ,
unsigned(CountTrailingZeros_64(TypeSize) +
LocalKnownZero.countTrailingOnes()));
}
}
KnownZero = APInt::getLowBitsSet(BitWidth, TrailZ) & Mask;
break;
}
case Instruction::PHI: {
PHINode *P = cast<PHINode>(I);
// Handle the case of a simple two-predecessor recurrence PHI.
// There's a lot more that could theoretically be done here, but
// this is sufficient to catch some interesting cases.
if (P->getNumIncomingValues() == 2) {
for (unsigned i = 0; i != 2; ++i) {
Value *L = P->getIncomingValue(i);
Value *R = P->getIncomingValue(!i);
Operator *LU = dyn_cast<Operator>(L);
if (!LU)
continue;
unsigned Opcode = LU->getOpcode();
// Check for operations that have the property that if
// both their operands have low zero bits, the result
// will have low zero bits.
if (Opcode == Instruction::Add ||
Opcode == Instruction::Sub ||
Opcode == Instruction::And ||
Opcode == Instruction::Or ||
Opcode == Instruction::Mul) {
Value *LL = LU->getOperand(0);
Value *LR = LU->getOperand(1);
// Find a recurrence.
if (LL == I)
L = LR;
else if (LR == I)
L = LL;
else
break;
// Ok, we have a PHI of the form L op= R. Check for low
// zero bits.
APInt Mask2 = APInt::getAllOnesValue(BitWidth);
ComputeMaskedBits(R, Mask2, KnownZero2, KnownOne2, TD, Depth+1);
Mask2 = APInt::getLowBitsSet(BitWidth,
KnownZero2.countTrailingOnes());
// We need to take the minimum number of known bits
APInt KnownZero3(KnownZero), KnownOne3(KnownOne);
ComputeMaskedBits(L, Mask2, KnownZero3, KnownOne3, TD, Depth+1);
KnownZero = Mask &
APInt::getLowBitsSet(BitWidth,
std::min(KnownZero2.countTrailingOnes(),
KnownZero3.countTrailingOnes()));
break;
}
}
}
// Otherwise take the unions of the known bit sets of the operands,
// taking conservative care to avoid excessive recursion.
if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) {
KnownZero = APInt::getAllOnesValue(BitWidth);
KnownOne = APInt::getAllOnesValue(BitWidth);
for (unsigned i = 0, e = P->getNumIncomingValues(); i != e; ++i) {
// Skip direct self references.
if (P->getIncomingValue(i) == P) continue;
KnownZero2 = APInt(BitWidth, 0);
KnownOne2 = APInt(BitWidth, 0);
// Recurse, but cap the recursion to one level, because we don't
// want to waste time spinning around in loops.
ComputeMaskedBits(P->getIncomingValue(i), KnownZero | KnownOne,
KnownZero2, KnownOne2, TD, MaxDepth-1);
KnownZero &= KnownZero2;
KnownOne &= KnownOne2;
// If all bits have been ruled out, there's no need to check
// more operands.
if (!KnownZero && !KnownOne)
break;
}
}
break;
}
case Instruction::Call:
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::ctpop:
case Intrinsic::ctlz:
case Intrinsic::cttz: {
unsigned LowBits = Log2_32(BitWidth)+1;
KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
break;
}
}
}
break;
}
}
/// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero. We use
/// this predicate to simplify operations downstream. Mask is known to be zero
/// for bits that V cannot have.
///
/// This function is defined on values with integer type, values with pointer
/// type (but only if TD is non-null), and vectors of integers. In the case
/// where V is a vector, the mask, known zero, and known one values are the
/// same width as the vector element, and the bit is set only if it is true
/// for all of the elements in the vector.
bool llvm::MaskedValueIsZero(Value *V, const APInt &Mask,
const TargetData *TD, unsigned Depth) {
APInt KnownZero(Mask.getBitWidth(), 0), KnownOne(Mask.getBitWidth(), 0);
ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth);
assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
return (KnownZero & Mask) == Mask;
}
/// ComputeNumSignBits - Return the number of times the sign bit of the
/// register is replicated into the other bits. We know that at least 1 bit
/// is always equal to the sign bit (itself), but other cases can give us
/// information. For example, immediately after an "ashr X, 2", we know that
/// the top 3 bits are all equal to each other, so we return 3.
///
/// 'Op' must have a scalar integer type.
///
unsigned llvm::ComputeNumSignBits(Value *V, const TargetData *TD,
unsigned Depth) {
assert((TD || V->getType()->isIntOrIntVectorTy()) &&
"ComputeNumSignBits requires a TargetData object to operate "
"on non-integer values!");
const Type *Ty = V->getType();
unsigned TyBits = TD ? TD->getTypeSizeInBits(V->getType()->getScalarType()) :
Ty->getScalarSizeInBits();
unsigned Tmp, Tmp2;
unsigned FirstAnswer = 1;
// Note that ConstantInt is handled by the general ComputeMaskedBits case
// below.
if (Depth == 6)
return 1; // Limit search depth.
Operator *U = dyn_cast<Operator>(V);
switch (Operator::getOpcode(V)) {
default: break;
case Instruction::SExt:
Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
return ComputeNumSignBits(U->getOperand(0), TD, Depth+1) + Tmp;
case Instruction::AShr:
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
// ashr X, C -> adds C sign bits.
if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
Tmp += C->getZExtValue();
if (Tmp > TyBits) Tmp = TyBits;
}
return Tmp;
case Instruction::Shl:
if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
// shl destroys sign bits.
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
if (C->getZExtValue() >= TyBits || // Bad shift.
C->getZExtValue() >= Tmp) break; // Shifted all sign bits out.
return Tmp - C->getZExtValue();
}
break;
case Instruction::And:
case Instruction::Or:
case Instruction::Xor: // NOT is handled here.
// Logical binary ops preserve the number of sign bits at the worst.
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
if (Tmp != 1) {
Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
FirstAnswer = std::min(Tmp, Tmp2);
// We computed what we know about the sign bits as our first
// answer. Now proceed to the generic code that uses
// ComputeMaskedBits, and pick whichever answer is better.
}
break;
case Instruction::Select:
Tmp = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
if (Tmp == 1) return 1; // Early out.
Tmp2 = ComputeNumSignBits(U->getOperand(2), TD, Depth+1);
return std::min(Tmp, Tmp2);
case Instruction::Add:
// Add can have at most one carry bit. Thus we know that the output
// is, at worst, one more bit than the inputs.
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
if (Tmp == 1) return 1; // Early out.
// Special case decrementing a value (ADD X, -1):
if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(1)))
if (CRHS->isAllOnesValue()) {
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(U->getOperand(0), Mask, KnownZero, KnownOne, TD,
Depth+1);
// If the input is known to be 0 or 1, the output is 0/-1, which is all
// sign bits set.
if ((KnownZero | APInt(TyBits, 1)) == Mask)
return TyBits;
// If we are subtracting one from a positive number, there is no carry
// out of the result.
if (KnownZero.isNegative())
return Tmp;
}
Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
if (Tmp2 == 1) return 1;
return std::min(Tmp, Tmp2)-1;
case Instruction::Sub:
Tmp2 = ComputeNumSignBits(U->getOperand(1), TD, Depth+1);
if (Tmp2 == 1) return 1;
// Handle NEG.
if (ConstantInt *CLHS = dyn_cast<ConstantInt>(U->getOperand(0)))
if (CLHS->isNullValue()) {
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(U->getOperand(1), Mask, KnownZero, KnownOne,
TD, Depth+1);
// If the input is known to be 0 or 1, the output is 0/-1, which is all
// sign bits set.
if ((KnownZero | APInt(TyBits, 1)) == Mask)
return TyBits;
// If the input is known to be positive (the sign bit is known clear),
// the output of the NEG has the same number of sign bits as the input.
if (KnownZero.isNegative())
return Tmp2;
// Otherwise, we treat this like a SUB.
}
// Sub can have at most one carry bit. Thus we know that the output
// is, at worst, one more bit than the inputs.
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1);
if (Tmp == 1) return 1; // Early out.
return std::min(Tmp, Tmp2)-1;
case Instruction::PHI: {
PHINode *PN = cast<PHINode>(U);
// Don't analyze large in-degree PHIs.
if (PN->getNumIncomingValues() > 4) break;
// Take the minimum of all incoming values. This can't infinitely loop
// because of our depth threshold.
Tmp = ComputeNumSignBits(PN->getIncomingValue(0), TD, Depth+1);
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
if (Tmp == 1) return Tmp;
Tmp = std::min(Tmp,
ComputeNumSignBits(PN->getIncomingValue(i), TD, Depth+1));
}
return Tmp;
}
case Instruction::Trunc:
// FIXME: it's tricky to do anything useful for this, but it is an important
// case for targets like X86.
break;
}
// Finally, if we can prove that the top bits of the result are 0's or 1's,
// use this information.
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD, Depth);
if (KnownZero.isNegative()) { // sign bit is 0
Mask = KnownZero;
} else if (KnownOne.isNegative()) { // sign bit is 1;
Mask = KnownOne;
} else {
// Nothing known.
return FirstAnswer;
}
// Okay, we know that the sign bit in Mask is set. Use CLZ to determine
// the number of identical bits in the top of the input value.
Mask = ~Mask;
Mask <<= Mask.getBitWidth()-TyBits;
// Return # leading zeros. We use 'min' here in case Val was zero before
// shifting. We don't want to return '64' as for an i32 "0".
return std::max(FirstAnswer, std::min(TyBits, Mask.countLeadingZeros()));
}
/// ComputeMultiple - This function computes the integer multiple of Base that
/// equals V. If successful, it returns true and returns the multiple in
/// Multiple. If unsuccessful, it returns false. It looks
/// through SExt instructions only if LookThroughSExt is true.
bool llvm::ComputeMultiple(Value *V, unsigned Base, Value *&Multiple,
bool LookThroughSExt, unsigned Depth) {
const unsigned MaxDepth = 6;
assert(V && "No Value?");
assert(Depth <= MaxDepth && "Limit Search Depth");
assert(V->getType()->isIntegerTy() && "Not integer or pointer type!");
const Type *T = V->getType();
ConstantInt *CI = dyn_cast<ConstantInt>(V);
if (Base == 0)
return false;
if (Base == 1) {
Multiple = V;
return true;
}
ConstantExpr *CO = dyn_cast<ConstantExpr>(V);
Constant *BaseVal = ConstantInt::get(T, Base);
if (CO && CO == BaseVal) {
// Multiple is 1.
Multiple = ConstantInt::get(T, 1);
return true;
}
if (CI && CI->getZExtValue() % Base == 0) {
Multiple = ConstantInt::get(T, CI->getZExtValue() / Base);
return true;
}
if (Depth == MaxDepth) return false; // Limit search depth.
Operator *I = dyn_cast<Operator>(V);
if (!I) return false;
switch (I->getOpcode()) {
default: break;
case Instruction::SExt:
if (!LookThroughSExt) return false;
// otherwise fall through to ZExt
case Instruction::ZExt:
return ComputeMultiple(I->getOperand(0), Base, Multiple,
LookThroughSExt, Depth+1);
case Instruction::Shl:
case Instruction::Mul: {
Value *Op0 = I->getOperand(0);
Value *Op1 = I->getOperand(1);
if (I->getOpcode() == Instruction::Shl) {
ConstantInt *Op1CI = dyn_cast<ConstantInt>(Op1);
if (!Op1CI) return false;
// Turn Op0 << Op1 into Op0 * 2^Op1
APInt Op1Int = Op1CI->getValue();
uint64_t BitToSet = Op1Int.getLimitedValue(Op1Int.getBitWidth() - 1);
Op1 = ConstantInt::get(V->getContext(),
APInt(Op1Int.getBitWidth(), 0).set(BitToSet));
}
Value *Mul0 = NULL;
Value *Mul1 = NULL;
bool M0 = ComputeMultiple(Op0, Base, Mul0,
LookThroughSExt, Depth+1);
bool M1 = ComputeMultiple(Op1, Base, Mul1,
LookThroughSExt, Depth+1);
if (M0) {
if (isa<Constant>(Op1) && isa<Constant>(Mul0)) {
// V == Base * (Mul0 * Op1), so return (Mul0 * Op1)
Multiple = ConstantExpr::getMul(cast<Constant>(Mul0),
cast<Constant>(Op1));
return true;
}
if (ConstantInt *Mul0CI = dyn_cast<ConstantInt>(Mul0))
if (Mul0CI->getValue() == 1) {
// V == Base * Op1, so return Op1
Multiple = Op1;
return true;
}
}
if (M1) {
if (isa<Constant>(Op0) && isa<Constant>(Mul1)) {
// V == Base * (Mul1 * Op0), so return (Mul1 * Op0)
Multiple = ConstantExpr::getMul(cast<Constant>(Mul1),
cast<Constant>(Op0));
return true;
}
if (ConstantInt *Mul1CI = dyn_cast<ConstantInt>(Mul1))
if (Mul1CI->getValue() == 1) {
// V == Base * Op0, so return Op0
Multiple = Op0;
return true;
}
}
}
}
// We could not determine if V is a multiple of Base.
return false;
}
/// CannotBeNegativeZero - Return true if we can prove that the specified FP
/// value is never equal to -0.0.
///
/// NOTE: this function will need to be revisited when we support non-default
/// rounding modes!
///
bool llvm::CannotBeNegativeZero(const Value *V, unsigned Depth) {
if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V))
return !CFP->getValueAPF().isNegZero();
if (Depth == 6)
return 1; // Limit search depth.
const Operator *I = dyn_cast<Operator>(V);
if (I == 0) return false;
// (add x, 0.0) is guaranteed to return +0.0, not -0.0.
if (I->getOpcode() == Instruction::FAdd &&
isa<ConstantFP>(I->getOperand(1)) &&
cast<ConstantFP>(I->getOperand(1))->isNullValue())
return true;
// sitofp and uitofp turn into +0.0 for zero.
if (isa<SIToFPInst>(I) || isa<UIToFPInst>(I))
return true;
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
// sqrt(-0.0) = -0.0, no other negative results are possible.
if (II->getIntrinsicID() == Intrinsic::sqrt)
return CannotBeNegativeZero(II->getArgOperand(0), Depth+1);
if (const CallInst *CI = dyn_cast<CallInst>(I))
if (const Function *F = CI->getCalledFunction()) {
if (F->isDeclaration()) {
// abs(x) != -0.0
if (F->getName() == "abs") return true;
// fabs[lf](x) != -0.0
if (F->getName() == "fabs") return true;
if (F->getName() == "fabsf") return true;
if (F->getName() == "fabsl") return true;
if (F->getName() == "sqrt" || F->getName() == "sqrtf" ||
F->getName() == "sqrtl")
return CannotBeNegativeZero(CI->getArgOperand(0), Depth+1);
}
}
return false;
}
// This is the recursive version of BuildSubAggregate. It takes a few different
// arguments. Idxs is the index within the nested struct From that we are
// looking at now (which is of type IndexedType). IdxSkip is the number of
// indices from Idxs that should be left out when inserting into the resulting
// struct. To is the result struct built so far, new insertvalue instructions
// build on that.
static Value *BuildSubAggregate(Value *From, Value* To, const Type *IndexedType,
SmallVector<unsigned, 10> &Idxs,
unsigned IdxSkip,
Instruction *InsertBefore) {
const llvm::StructType *STy = llvm::dyn_cast<llvm::StructType>(IndexedType);
if (STy) {
// Save the original To argument so we can modify it
Value *OrigTo = To;
// General case, the type indexed by Idxs is a struct
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
// Process each struct element recursively
Idxs.push_back(i);
Value *PrevTo = To;
To = BuildSubAggregate(From, To, STy->getElementType(i), Idxs, IdxSkip,
InsertBefore);
Idxs.pop_back();
if (!To) {
// Couldn't find any inserted value for this index? Cleanup
while (PrevTo != OrigTo) {
InsertValueInst* Del = cast<InsertValueInst>(PrevTo);
PrevTo = Del->getAggregateOperand();
Del->eraseFromParent();
}
// Stop processing elements
break;
}
}
// If we succesfully found a value for each of our subaggregates
if (To)
return To;
}
// Base case, the type indexed by SourceIdxs is not a struct, or not all of
// the struct's elements had a value that was inserted directly. In the latter
// case, perhaps we can't determine each of the subelements individually, but
// we might be able to find the complete struct somewhere.
// Find the value that is at that particular spot
Value *V = FindInsertedValue(From, Idxs.begin(), Idxs.end());
if (!V)
return NULL;
// Insert the value in the new (sub) aggregrate
return llvm::InsertValueInst::Create(To, V, Idxs.begin() + IdxSkip,
Idxs.end(), "tmp", InsertBefore);
}
// This helper takes a nested struct and extracts a part of it (which is again a
// struct) into a new value. For example, given the struct:
// { a, { b, { c, d }, e } }
// and the indices "1, 1" this returns
// { c, d }.
//
// It does this by inserting an insertvalue for each element in the resulting
// struct, as opposed to just inserting a single struct. This will only work if
// each of the elements of the substruct are known (ie, inserted into From by an
// insertvalue instruction somewhere).
//
// All inserted insertvalue instructions are inserted before InsertBefore
static Value *BuildSubAggregate(Value *From, const unsigned *idx_begin,
const unsigned *idx_end,
Instruction *InsertBefore) {
assert(InsertBefore && "Must have someplace to insert!");
const Type *IndexedType = ExtractValueInst::getIndexedType(From->getType(),
idx_begin,
idx_end);
Value *To = UndefValue::get(IndexedType);
SmallVector<unsigned, 10> Idxs(idx_begin, idx_end);
unsigned IdxSkip = Idxs.size();
return BuildSubAggregate(From, To, IndexedType, Idxs, IdxSkip, InsertBefore);
}
/// FindInsertedValue - Given an aggregrate and an sequence of indices, see if
/// the scalar value indexed is already around as a register, for example if it
/// were inserted directly into the aggregrate.
///
/// If InsertBefore is not null, this function will duplicate (modified)
/// insertvalues when a part of a nested struct is extracted.
Value *llvm::FindInsertedValue(Value *V, const unsigned *idx_begin,
const unsigned *idx_end, Instruction *InsertBefore) {
// Nothing to index? Just return V then (this is useful at the end of our
// recursion)
if (idx_begin == idx_end)
return V;
// We have indices, so V should have an indexable type
assert((V->getType()->isStructTy() || V->getType()->isArrayTy())
&& "Not looking at a struct or array?");
assert(ExtractValueInst::getIndexedType(V->getType(), idx_begin, idx_end)
&& "Invalid indices for type?");
const CompositeType *PTy = cast<CompositeType>(V->getType());
if (isa<UndefValue>(V))
return UndefValue::get(ExtractValueInst::getIndexedType(PTy,
idx_begin,
idx_end));
else if (isa<ConstantAggregateZero>(V))
return Constant::getNullValue(ExtractValueInst::getIndexedType(PTy,
idx_begin,
idx_end));
else if (Constant *C = dyn_cast<Constant>(V)) {
if (isa<ConstantArray>(C) || isa<ConstantStruct>(C))
// Recursively process this constant
return FindInsertedValue(C->getOperand(*idx_begin), idx_begin + 1,
idx_end, InsertBefore);
} else if (InsertValueInst *I = dyn_cast<InsertValueInst>(V)) {
// Loop the indices for the insertvalue instruction in parallel with the
// requested indices
const unsigned *req_idx = idx_begin;
for (const unsigned *i = I->idx_begin(), *e = I->idx_end();
i != e; ++i, ++req_idx) {
if (req_idx == idx_end) {
if (InsertBefore)
// The requested index identifies a part of a nested aggregate. Handle
// this specially. For example,
// %A = insertvalue { i32, {i32, i32 } } undef, i32 10, 1, 0
// %B = insertvalue { i32, {i32, i32 } } %A, i32 11, 1, 1
// %C = extractvalue {i32, { i32, i32 } } %B, 1
// This can be changed into
// %A = insertvalue {i32, i32 } undef, i32 10, 0
// %C = insertvalue {i32, i32 } %A, i32 11, 1
// which allows the unused 0,0 element from the nested struct to be
// removed.
return BuildSubAggregate(V, idx_begin, req_idx, InsertBefore);
else
// We can't handle this without inserting insertvalues
return 0;
}
// This insert value inserts something else than what we are looking for.
// See if the (aggregrate) value inserted into has the value we are
// looking for, then.
if (*req_idx != *i)
return FindInsertedValue(I->getAggregateOperand(), idx_begin, idx_end,
InsertBefore);
}
// If we end up here, the indices of the insertvalue match with those
// requested (though possibly only partially). Now we recursively look at
// the inserted value, passing any remaining indices.
return FindInsertedValue(I->getInsertedValueOperand(), req_idx, idx_end,
InsertBefore);
} else if (ExtractValueInst *I = dyn_cast<ExtractValueInst>(V)) {
// If we're extracting a value from an aggregrate that was extracted from
// something else, we can extract from that something else directly instead.
// However, we will need to chain I's indices with the requested indices.
// Calculate the number of indices required
unsigned size = I->getNumIndices() + (idx_end - idx_begin);
// Allocate some space to put the new indices in
SmallVector<unsigned, 5> Idxs;
Idxs.reserve(size);
// Add indices from the extract value instruction
for (const unsigned *i = I->idx_begin(), *e = I->idx_end();
i != e; ++i)
Idxs.push_back(*i);
// Add requested indices
for (const unsigned *i = idx_begin, *e = idx_end; i != e; ++i)
Idxs.push_back(*i);
assert(Idxs.size() == size
&& "Number of indices added not correct?");
return FindInsertedValue(I->getAggregateOperand(), Idxs.begin(), Idxs.end(),
InsertBefore);
}
// Otherwise, we don't know (such as, extracting from a function return value
// or load instruction)
return 0;
}
/// GetConstantStringInfo - This function computes the length of a
/// null-terminated C string pointed to by V. If successful, it returns true
/// and returns the string in Str. If unsuccessful, it returns false.
bool llvm::GetConstantStringInfo(const Value *V, std::string &Str,
uint64_t Offset,
bool StopAtNul) {
// If V is NULL then return false;
if (V == NULL) return false;
// Look through bitcast instructions.
if (const BitCastInst *BCI = dyn_cast<BitCastInst>(V))
return GetConstantStringInfo(BCI->getOperand(0), Str, Offset, StopAtNul);
// If the value is not a GEP instruction nor a constant expression with a
// GEP instruction, then return false because ConstantArray can't occur
// any other way
const User *GEP = 0;
if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(V)) {
GEP = GEPI;
} else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
if (CE->getOpcode() == Instruction::BitCast)
return GetConstantStringInfo(CE->getOperand(0), Str, Offset, StopAtNul);
if (CE->getOpcode() != Instruction::GetElementPtr)
return false;
GEP = CE;
}
if (GEP) {
// Make sure the GEP has exactly three arguments.
if (GEP->getNumOperands() != 3)
return false;
// Make sure the index-ee is a pointer to array of i8.
const PointerType *PT = cast<PointerType>(GEP->getOperand(0)->getType());
const ArrayType *AT = dyn_cast<ArrayType>(PT->getElementType());
if (AT == 0 || !AT->getElementType()->isIntegerTy(8))
return false;
// Check to make sure that the first operand of the GEP is an integer and
// has value 0 so that we are sure we're indexing into the initializer.
const ConstantInt *FirstIdx = dyn_cast<ConstantInt>(GEP->getOperand(1));
if (FirstIdx == 0 || !FirstIdx->isZero())
return false;
// If the second index isn't a ConstantInt, then this is a variable index
// into the array. If this occurs, we can't say anything meaningful about
// the string.
uint64_t StartIdx = 0;
if (const ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(2)))
StartIdx = CI->getZExtValue();
else
return false;
return GetConstantStringInfo(GEP->getOperand(0), Str, StartIdx+Offset,
StopAtNul);
}
// The GEP instruction, constant or instruction, must reference a global
// variable that is a constant and is initialized. The referenced constant
// initializer is the array that we'll use for optimization.
const GlobalVariable* GV = dyn_cast<GlobalVariable>(V);
if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer())
return false;
const Constant *GlobalInit = GV->getInitializer();
// Handle the ConstantAggregateZero case
if (isa<ConstantAggregateZero>(GlobalInit)) {
// This is a degenerate case. The initializer is constant zero so the
// length of the string must be zero.
Str.clear();
return true;
}
// Must be a Constant Array
const ConstantArray *Array = dyn_cast<ConstantArray>(GlobalInit);
if (Array == 0 || !Array->getType()->getElementType()->isIntegerTy(8))
return false;
// Get the number of elements in the array
uint64_t NumElts = Array->getType()->getNumElements();
if (Offset > NumElts)
return false;
// Traverse the constant array from 'Offset' which is the place the GEP refers
// to in the array.
Str.reserve(NumElts-Offset);
for (unsigned i = Offset; i != NumElts; ++i) {
const Constant *Elt = Array->getOperand(i);
const ConstantInt *CI = dyn_cast<ConstantInt>(Elt);
if (!CI) // This array isn't suitable, non-int initializer.
return false;
if (StopAtNul && CI->isZero())
return true; // we found end of string, success!
Str += (char)CI->getZExtValue();
}
// The array isn't null terminated, but maybe this is a memcpy, not a strcpy.
return true;
}
// These next two are very similar to the above, but also look through PHI
// nodes.
// TODO: See if we can integrate these two together.
/// GetStringLengthH - If we can compute the length of the string pointed to by
/// the specified pointer, return 'len+1'. If we can't, return 0.
static uint64_t GetStringLengthH(Value *V, SmallPtrSet<PHINode*, 32> &PHIs) {
// Look through noop bitcast instructions.
if (BitCastInst *BCI = dyn_cast<BitCastInst>(V))
return GetStringLengthH(BCI->getOperand(0), PHIs);
// If this is a PHI node, there are two cases: either we have already seen it
// or we haven't.
if (PHINode *PN = dyn_cast<PHINode>(V)) {
if (!PHIs.insert(PN))
return ~0ULL; // already in the set.
// If it was new, see if all the input strings are the same length.
uint64_t LenSoFar = ~0ULL;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
uint64_t Len = GetStringLengthH(PN->getIncomingValue(i), PHIs);
if (Len == 0) return 0; // Unknown length -> unknown.
if (Len == ~0ULL) continue;
if (Len != LenSoFar && LenSoFar != ~0ULL)
return 0; // Disagree -> unknown.
LenSoFar = Len;
}
// Success, all agree.
return LenSoFar;
}
// strlen(select(c,x,y)) -> strlen(x) ^ strlen(y)
if (SelectInst *SI = dyn_cast<SelectInst>(V)) {
uint64_t Len1 = GetStringLengthH(SI->getTrueValue(), PHIs);
if (Len1 == 0) return 0;
uint64_t Len2 = GetStringLengthH(SI->getFalseValue(), PHIs);
if (Len2 == 0) return 0;
if (Len1 == ~0ULL) return Len2;
if (Len2 == ~0ULL) return Len1;
if (Len1 != Len2) return 0;
return Len1;
}
// If the value is not a GEP instruction nor a constant expression with a
// GEP instruction, then return unknown.
User *GEP = 0;
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(V)) {
GEP = GEPI;
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
if (CE->getOpcode() != Instruction::GetElementPtr)
return 0;
GEP = CE;
} else {
return 0;
}
// Make sure the GEP has exactly three arguments.
if (GEP->getNumOperands() != 3)
return 0;
// Check to make sure that the first operand of the GEP is an integer and
// has value 0 so that we are sure we're indexing into the initializer.
if (ConstantInt *Idx = dyn_cast<ConstantInt>(GEP->getOperand(1))) {
if (!Idx->isZero())
return 0;
} else
return 0;
// If the second index isn't a ConstantInt, then this is a variable index
// into the array. If this occurs, we can't say anything meaningful about
// the string.
uint64_t StartIdx = 0;
if (ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(2)))
StartIdx = CI->getZExtValue();
else
return 0;
// The GEP instruction, constant or instruction, must reference a global
// variable that is a constant and is initialized. The referenced constant
// initializer is the array that we'll use for optimization.
GlobalVariable* GV = dyn_cast<GlobalVariable>(GEP->getOperand(0));
if (!GV || !GV->isConstant() || !GV->hasInitializer() ||
GV->mayBeOverridden())
return 0;
Constant *GlobalInit = GV->getInitializer();
// Handle the ConstantAggregateZero case, which is a degenerate case. The
// initializer is constant zero so the length of the string must be zero.
if (isa<ConstantAggregateZero>(GlobalInit))
return 1; // Len = 0 offset by 1.
// Must be a Constant Array
ConstantArray *Array = dyn_cast<ConstantArray>(GlobalInit);
if (!Array || !Array->getType()->getElementType()->isIntegerTy(8))
return false;
// Get the number of elements in the array
uint64_t NumElts = Array->getType()->getNumElements();
// Traverse the constant array from StartIdx (derived above) which is
// the place the GEP refers to in the array.
for (unsigned i = StartIdx; i != NumElts; ++i) {
Constant *Elt = Array->getOperand(i);
ConstantInt *CI = dyn_cast<ConstantInt>(Elt);
if (!CI) // This array isn't suitable, non-int initializer.
return 0;
if (CI->isZero())
return i-StartIdx+1; // We found end of string, success!
}
return 0; // The array isn't null terminated, conservatively return 'unknown'.
}
/// GetStringLength - If we can compute the length of the string pointed to by
/// the specified pointer, return 'len+1'. If we can't, return 0.
uint64_t llvm::GetStringLength(Value *V) {
if (!V->getType()->isPointerTy()) return 0;
SmallPtrSet<PHINode*, 32> PHIs;
uint64_t Len = GetStringLengthH(V, PHIs);
// If Len is ~0ULL, we had an infinite phi cycle: this is dead code, so return
// an empty string as a length.
return Len == ~0ULL ? 1 : Len;
}
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