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Fix a bogus srem rule - a negative value srem'd by a power-of-2

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can have a non-negative result; for example, -16%16 is 0. Also,
clarify the related comments. This fixes PR2670.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54767 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
2008-08-13 23:12:35 +00:00
parent 289983123b
commit a60832b018
4 changed files with 24 additions and 13 deletions
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8
lib/Analysis/ValueTracking.cpp
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@@ -369,15 +369,13 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD,
Depth+1);
// The sign of a remainder is equal to the sign of the first
// operand (zero being positive).
// If the sign bit of the first operand is zero, the sign bit of
// the result is zero. If the first operand has no one bits below
// the second operand's single 1 bit, its sign will be zero.
if (KnownZero2[BitWidth-1] || ((KnownZero2 & LowBits) == LowBits))
KnownZero2 |= ~LowBits;
else if (KnownOne2[BitWidth-1])
KnownOne2 |= ~LowBits;
KnownZero |= KnownZero2 & Mask;
KnownOne |= KnownOne2 & Mask;
assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND zero?");
}