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[SCEV] Infer known bits from known sign bits

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This was suggested by lebedev.ri over on D96534.  You'll note lack of tests.  During review, we weren't actually able to find a case which exercises it, but both I and lebedev.ri feel it's a reasonable change, straight forward, and near free.

Differential Revision: https://reviews.llvm.org/D97064
This commit is contained in:
Philip Reames 2021-03-09 12:35:16 -08:00
parent c4e8be9da1
commit 54b4ece1ff
1 changed files with 14 additions and 7 deletions
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21
lib/Analysis/ScalarEvolution.cpp
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@ -5848,24 +5848,31 @@ ScalarEvolution::getRangeRef(const SCEV *S,
KnownBits Known = computeKnownBits(U->getValue(), DL, 0, &AC, nullptr, &DT);
if (Known.getBitWidth() != BitWidth)
Known = Known.zextOrTrunc(BitWidth);
// If Known does not result in full-set, intersect with it.
if (Known.getMinValue() != Known.getMaxValue() + 1)
ConservativeResult = ConservativeResult.intersectWith(
ConstantRange(Known.getMinValue(), Known.getMaxValue() + 1),
RangeType);
// ValueTracking may be able to compute a tighter result for the number of
// sign bits than for the value of those sign bits.
unsigned NS = ComputeNumSignBits(U->getValue(), DL, 0, &AC, nullptr, &DT);
// If the pointer size is larger than the index size type, this can cause
// NS to be larger than BitWidth. So compensate for this.
if (U->getType()->isPointerTy()) {
// If the pointer size is larger than the index size type, this can cause
// NS to be larger than BitWidth. So compensate for this.
unsigned ptrSize = DL.getPointerTypeSizeInBits(U->getType());
int ptrIdxDiff = ptrSize - BitWidth;
if (ptrIdxDiff > 0 && ptrSize > BitWidth && NS > (unsigned)ptrIdxDiff)
NS -= ptrIdxDiff;
}
if (NS > 1) {
// If we know any of the sign bits, we know all of the sign bits.
if (!Known.Zero.getHiBits(NS).isNullValue())
Known.Zero.setHighBits(NS);
if (!Known.One.getHiBits(NS).isNullValue())
Known.One.setHighBits(NS);
}
if (Known.getMinValue() != Known.getMaxValue() + 1)
ConservativeResult = ConservativeResult.intersectWith(
ConstantRange(Known.getMinValue(), Known.getMaxValue() + 1),
RangeType);
if (NS > 1)
ConservativeResult = ConservativeResult.intersectWith(
ConstantRange(APInt::getSignedMinValue(BitWidth).ashr(NS - 1),