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

[goldsteinn]

A missed case (that we infact had tests for already).

Proof: https://alive2.llvm.org/ce/z/54zsbS

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: None (goldsteinn)

Changes

A missed case (that we infact had tests for already).

Proof: https://alive2.llvm.org/ce/z/54zsbS

---

Full diff: https://github.com/llvm/llvm-project/pull/87700.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ValueTracking.cpp (+14)
• (modified) llvm/test/Analysis/ValueTracking/known-non-zero.ll (+1-5)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 5ad4da43bca7db..57115a78360c1b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -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));
diff --git a/llvm/test/Analysis/ValueTracking/known-non-zero.ll b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
index 0159050d925c3e..00a0c07ede9763 100644
--- a/llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -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
-; 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)

[nikic]

Probably not useful, but LGTM.

[dtcxzyw]

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);

[nikic]

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 :)

[goldsteinn]

You are right, I forgot :)

I'll close this and push a comment