[llvm] [ValueTracking] Add support for `usub.sat` in `isKnownNonZero (PR #87700)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: None (goldsteinn)

<details>
<summary>Changes</summary>

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
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)

``````````

</details>


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