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Factor out the analysis of addition and subtraction in ComputeMaskedBits. Reuse 

it to analyze extractvalue(llvm.[us](add|sub).with.overflow.*) intrinsics!

llvm-svn: 152398
This commit is contained in:
Nick Lewycky 2012-03-09 09:23:50 +00:00
parent c04bbd27af
commit fea3e00e09
1 changed files with 123 additions and 83 deletions
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206
llvm/lib/Analysis/ValueTracking.cpp
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@ -41,6 +41,95 @@ static unsigned getBitWidth(Type *Ty, const TargetData *TD) {
return TD ? TD->getPointerSizeInBits() : 0; return TD ? TD->getPointerSizeInBits() : 0;
} }
static void ComputeMaskedBitsAddSub(bool Add, Value *Op0, Value *Op1, bool NSW,
const APInt &Mask,
APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2,
const TargetData *TD, unsigned Depth) {
if (!Add) {
if (ConstantInt *CLHS = dyn_cast<ConstantInt>(Op0)) {
// 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 BitWidth = Mask.getBitWidth();
unsigned NLZ = (CLHS->getValue()+1).countLeadingZeros();
// NLZ can't be BitWidth with no sign bit
APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1);
llvm::ComputeMaskedBits(Op1, 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;
}
}
}
}
unsigned BitWidth = Mask.getBitWidth();
// 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());
llvm::ComputeMaskedBits(Op0, Mask2, LHSKnownZero, LHSKnownOne, TD, Depth+1);
assert((LHSKnownZero & LHSKnownOne) == 0 &&
"Bits known to be one AND zero?");
unsigned LHSKnownZeroOut = LHSKnownZero.countTrailingOnes();
llvm::ComputeMaskedBits(Op1, 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 (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;
}
// Are we still trying to solve for the sign bit?
if (Mask.isNegative() && !KnownZero.isNegative() && !KnownOne.isNegative()) {
if (NSW) {
if (Add) {
// Adding two positive numbers can't wrap into negative
if (LHSKnownZero.isNegative() && KnownZero2.isNegative())
KnownZero |= APInt::getSignBit(BitWidth);
// and adding two negative numbers can't wrap into positive.
else if (LHSKnownOne.isNegative() && KnownOne2.isNegative())
KnownOne |= APInt::getSignBit(BitWidth);
} else {
// Subtracting a negative number from a positive one can't wrap
if (LHSKnownZero.isNegative() && KnownOne2.isNegative())
KnownZero |= APInt::getSignBit(BitWidth);
// neither can subtracting a positive number from a negative one.
else if (LHSKnownOne.isNegative() && KnownZero2.isNegative())
KnownOne |= APInt::getSignBit(BitWidth);
}
}
}
}
/// ComputeMaskedBits - Determine which of the bits specified in Mask are /// ComputeMaskedBits - Determine which of the bits specified in Mask are
/// known to be either zero or one and return them in the KnownZero/KnownOne /// 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 /// bit sets. This code only analyzes bits in Mask, in order to short-circuit
@ -424,91 +513,18 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
} }
break; break;
case Instruction::Sub: { case Instruction::Sub: {
if (ConstantInt *CLHS = dyn_cast<ConstantInt>(I->getOperand(0))) { bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
// We know that the top bits of C-X are clear if X contains less bits ComputeMaskedBitsAddSub(false, I->getOperand(0), I->getOperand(1), NSW,
// than C (i.e. no wrap-around can happen). For example, 20-X is Mask, KnownZero, KnownOne, KnownZero2, KnownOne2,
// positive if we can prove that X is >= 0 and < 16. TD, Depth);
if (!CLHS->getValue().isNegative()) { break;
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: { case Instruction::Add: {
// If one of the operands has trailing zeros, then the bits that the bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
// other operand has in those bit positions will be preserved in the ComputeMaskedBitsAddSub(true, I->getOperand(0), I->getOperand(1), NSW,
// result. For an add, this works with either operand. For a subtract, Mask, KnownZero, KnownOne, KnownZero2, KnownOne2,
// this only works if the known zeros are in the right operand. TD, Depth);
APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0); break;
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;
}
// Are we still trying to solve for the sign bit?
if (Mask.isNegative() && !KnownZero.isNegative() && !KnownOne.isNegative()){
OverflowingBinaryOperator *OBO = cast<OverflowingBinaryOperator>(I);
if (OBO->hasNoSignedWrap()) {
if (I->getOpcode() == Instruction::Add) {
// Adding two positive numbers can't wrap into negative
if (LHSKnownZero.isNegative() && KnownZero2.isNegative())
KnownZero |= APInt::getSignBit(BitWidth);
// and adding two negative numbers can't wrap into positive.
else if (LHSKnownOne.isNegative() && KnownOne2.isNegative())
KnownOne |= APInt::getSignBit(BitWidth);
} else {
// Subtracting a negative number from a positive one can't wrap
if (LHSKnownZero.isNegative() && KnownOne2.isNegative())
KnownZero |= APInt::getSignBit(BitWidth);
// neither can subtracting a positive number from a negative one.
else if (LHSKnownOne.isNegative() && KnownZero2.isNegative())
KnownOne |= APInt::getSignBit(BitWidth);
}
}
}
return;
} }
case Instruction::SRem: case Instruction::SRem:
if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) { if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
@ -740,6 +756,30 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
} }
} }
break; break;
case Instruction::ExtractValue:
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I->getOperand(0))) {
ExtractValueInst *EVI = cast<ExtractValueInst>(I);
if (EVI->getNumIndices() != 1) break;
if (EVI->getIndices()[0] == 0) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::uadd_with_overflow:
case Intrinsic::sadd_with_overflow:
ComputeMaskedBitsAddSub(true, II->getArgOperand(0),
II->getArgOperand(1), false, Mask,
KnownZero, KnownOne, KnownZero2, KnownOne2,
TD, Depth);
break;
case Intrinsic::usub_with_overflow:
case Intrinsic::ssub_with_overflow:
ComputeMaskedBitsAddSub(false, II->getArgOperand(0),
II->getArgOperand(1), false, Mask,
KnownZero, KnownOne, KnownZero2, KnownOne2,
TD, Depth);
break;
}
}
}
} }
} }