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[ConstantRange] Remove costly udivrem from ConstantRange::truncate

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Truncate currently uses a udivrem call which is going to be slow particularly for larger than 64-bit widths.

As far as I can tell all we were trying to do was modulo LowerDiv by (MaxValue+1) and make sure whatever value was effectively subtracted from LowerDiv was also subtracted from UpperDiv.

This patch recognizes that MaxValue+1 is a power of 2 so we can just use a bitwise AND to accomplish a modulo operation or isolate the upper bits.

Differential Revision: https://reviews.llvm.org/D32672

llvm-svn: 304733
This commit is contained in:
Craig Topper 2017-06-05 20:48:05 +00:00
parent 00b6c8abb9
commit 144e4b1e4c
1 changed files with 19 additions and 15 deletions
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34
lib/IR/ConstantRange.cpp
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@ -577,9 +577,6 @@ ConstantRange ConstantRange::truncate(uint32_t DstTySize) const {
if (isFullSet())
return ConstantRange(DstTySize, /*isFullSet=*/true);
APInt MaxValue = APInt::getLowBitsSet(getBitWidth(), DstTySize);
APInt MaxBitValue = APInt::getOneBitSet(getBitWidth(), DstTySize);
APInt LowerDiv(Lower), UpperDiv(Upper);
ConstantRange Union(DstTySize, /*isFullSet=*/false);
@ -587,35 +584,42 @@ ConstantRange ConstantRange::truncate(uint32_t DstTySize) const {
// We use the non-wrapped set code to analyze the [Lower, MaxValue) part, and
// then we do the union with [MaxValue, Upper)
if (isWrappedSet()) {
// If Upper is greater than Max Value, it covers the whole truncated range.
if (Upper.uge(MaxValue))
// If Upper is greater than or equal to MaxValue(DstTy), it covers the whole
// truncated range.
if (Upper.getActiveBits() > DstTySize ||
Upper.countTrailingOnes() == DstTySize)
return ConstantRange(DstTySize, /*isFullSet=*/true);
Union = ConstantRange(APInt::getMaxValue(DstTySize),Upper.trunc(DstTySize));
UpperDiv.setAllBits();
// Union covers the MaxValue case, so return if the remaining range is just
// MaxValue.
// MaxValue(DstTy).
if (LowerDiv == UpperDiv)
return Union;
}
// Chop off the most significant bits that are past the destination bitwidth.
if (LowerDiv.uge(MaxValue)) {
APInt Div(getBitWidth(), 0);
APInt::udivrem(LowerDiv, MaxBitValue, Div, LowerDiv);
UpperDiv -= MaxBitValue * Div;
if (LowerDiv.getActiveBits() > DstTySize) {
// Mask to just the signficant bits and subtract from LowerDiv/UpperDiv.
APInt Adjust = LowerDiv & APInt::getBitsSetFrom(getBitWidth(), DstTySize);
LowerDiv -= Adjust;
UpperDiv -= Adjust;
}
if (UpperDiv.ule(MaxValue))
unsigned UpperDivWidth = UpperDiv.getActiveBits();
if (UpperDivWidth <= DstTySize)
return ConstantRange(LowerDiv.trunc(DstTySize),
UpperDiv.trunc(DstTySize)).unionWith(Union);
// The truncated value wraps around. Check if we can do better than fullset.
UpperDiv -= MaxBitValue;
if (UpperDiv.ult(LowerDiv))
return ConstantRange(LowerDiv.trunc(DstTySize),
UpperDiv.trunc(DstTySize)).unionWith(Union);
if (UpperDivWidth == DstTySize + 1) {
// Clear the MSB so that UpperDiv wraps around.
UpperDiv.clearBit(DstTySize);
if (UpperDiv.ult(LowerDiv))
return ConstantRange(LowerDiv.trunc(DstTySize),
UpperDiv.trunc(DstTySize)).unionWith(Union);
}
return ConstantRange(DstTySize, /*isFullSet=*/true);
}