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[ValueTracking] Add support for usub.sat in `isKnownNonZero #87700

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[ValueTracking] Add support for usub.sat in `isKnownNonZero #87700

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14 changes: 14 additions & 0 deletions llvm/lib/Analysis/ValueTracking.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2816,6 +2816,20 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
case Intrinsic::bswap:
case Intrinsic::ctpop:
return isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q);
case Intrinsic::usub_sat: {
KnownBits XKnown =
computeKnownBits(II->getArgOperand(0), DemandedElts, Depth, Q);
if (XKnown.isUnknown())
break;
KnownBits YKnown =
computeKnownBits(II->getArgOperand(1), DemandedElts, Depth, Q);
if (YKnown.isUnknown())
break;
std::optional<bool> NonZero = KnownBits::ugt(XKnown, YKnown);
if (NonZero.has_value())
return *NonZero;
break;
}
case Intrinsic::ssub_sat:
return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,
II->getArgOperand(0), II->getArgOperand(1));
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6 changes: 1 addition & 5 deletions llvm/test/Analysis/ValueTracking/known-non-zero.ll
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Expand Up @@ -882,11 +882,7 @@ define i1 @usub_sat_nonzero(i8 %xx, i8 %yy, i8 %ind) {
; CHECK-LABEL: @usub_sat_nonzero(
; CHECK-NEXT: [[Y_ULT_31:%.*]] = icmp ult i8 [[YY:%.*]], 31
; CHECK-NEXT: call void @llvm.assume(i1 [[Y_ULT_31]])
; CHECK-NEXT: [[XO:%.*]] = or i8 [[XX:%.*]], 34
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It has been handled in InstCombine by converting the usub.sat into sub nuw: https://godbolt.org/z/he76W3K3v

I think this patch just duplicates the logic in InstCombine:

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

Lines 2150 to 2160 in 8461d90

// Make use of known overflow information.
OverflowResult OR = computeOverflow(SI->getBinaryOp(), SI->isSigned(),
Arg0, Arg1, SI);
switch (OR) {
case OverflowResult::MayOverflow:
break;
case OverflowResult::NeverOverflows:
if (SI->isSigned())
return BinaryOperator::CreateNSW(SI->getBinaryOp(), Arg0, Arg1);
else
return BinaryOperator::CreateNUW(SI->getBinaryOp(), Arg0, Arg1);

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Ah yes, good point. Actually, now that you mention it, I'm pretty sure that @goldsteinn already submitted a patch along these lines on phabricator and we reached the same conclusion there... Maybe we should leave a comment so we don't get a third implementation :)

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You are right, I forgot :)

I'll close this and push a comment

; CHECK-NEXT: [[X:%.*]] = call i8 @llvm.usub.sat.i8(i8 [[XO]], i8 [[YY]])
; CHECK-NEXT: [[Z:%.*]] = or i8 [[X]], [[IND:%.*]]
; CHECK-NEXT: [[R:%.*]] = icmp eq i8 [[Z]], 0
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: ret i1 false
;
%y_ult_31 = icmp ult i8 %yy, 31
call void @llvm.assume(i1 %y_ult_31)
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