[llvm] [ValueTracking] Don't take sign bit from NaN operands (PR #157250)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Closes https://github.com/llvm/llvm-project/issues/157238.


---
Full diff: https://github.com/llvm/llvm-project/pull/157250.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+5) 
- (modified) llvm/test/Transforms/InstCombine/is_fpclass.ll (+15) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 07950f5341d6c..ff9b9f4f8d121 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5058,6 +5058,11 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
                      KnownRHS.isKnownNeverPosZero()) &&
                     (KnownLHS.isKnownNeverPosZero() ||
                      KnownRHS.isKnownNeverNegZero()))) {
+          // Don't take sign bit from NaN operands.
+          if (!KnownLHS.isKnownNeverNaN())
+            KnownLHS.SignBit = std::nullopt;
+          if (!KnownRHS.isKnownNeverNaN())
+            KnownRHS.SignBit = std::nullopt;
           if ((IID == Intrinsic::maximum || IID == Intrinsic::maximumnum ||
                IID == Intrinsic::maxnum) &&
               (KnownLHS.SignBit == false || KnownRHS.SignBit == false))
diff --git a/llvm/test/Transforms/InstCombine/is_fpclass.ll b/llvm/test/Transforms/InstCombine/is_fpclass.ll
index c1809b8bec61c..e5f72b415d441 100644
--- a/llvm/test/Transforms/InstCombine/is_fpclass.ll
+++ b/llvm/test/Transforms/InstCombine/is_fpclass.ll
@@ -3922,6 +3922,21 @@ define i1 @test_class_is_not_psub_pnorm_pinf__dynamic(float %arg) #3 {
   ret i1 %class
 }
 
+; Make sure we don't take sign bit from NaN operands.
+
+define i1 @minnum_qnan(i32 %x) {
+; CHECK-LABEL: @minnum_qnan(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  %qnan_bits = or i32 %x, -5938
+  %qnan = bitcast i32 %qnan_bits to float
+  %min = call float @llvm.minnum.f32(float %qnan, float 0.000000e+00)
+  %test = call i1 @llvm.is.fpclass.f32(float %min, i32 64)
+  ret i1 %test
+}
+
 declare i1 @llvm.is.fpclass.f32(float, i32 immarg)
 declare i1 @llvm.is.fpclass.f64(double, i32 immarg)
 declare <2 x i1> @llvm.is.fpclass.v2f32(<2 x float>, i32 immarg)

``````````

</details>


https://github.com/llvm/llvm-project/pull/157250