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When the low bits of one operand of an add are zero, that number

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of low bits of the other operand are preserved in the output.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72365 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-05-24 18:02:35 +00:00
parent b679de2a21
commit 3925043af0
1 changed files with 31 additions and 10 deletions
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41
lib/Analysis/ValueTracking.cpp
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@ -343,22 +343,43 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
}
// fall through
case Instruction::Add: {
// Output known-0 bits are known if clear or set in both the low clear bits
// common to both LHS & RHS. For example, 8+(X<<3) is known to have the
// low 3 bits clear.
APInt Mask2 = APInt::getLowBitsSet(BitWidth, Mask.countTrailingOnes());
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
// If one of the operands has trailing zeros, than 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((KnownZero2 & KnownOne2) == 0 && "Bits known to be one AND zero?");
unsigned KnownZeroOut = KnownZero2.countTrailingOnes();
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?");
KnownZeroOut = std::min(KnownZeroOut,
KnownZero2.countTrailingOnes());
unsigned RHSKnownZeroOut = KnownZero2.countTrailingOnes();
KnownZero |= APInt::getLowBitsSet(BitWidth, KnownZeroOut);
// Determine which operand has more trailing zeros, and use that
// many bits from the other operand.
if (LHSKnownZeroOut > RHSKnownZeroOut) {
if (getOpcode(I) == 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: