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

Closes #157238.

Author: dtcxzyw

llvm/lib/Analysis/ValueTracking.cpp

@@ -5070,6 +5070,11 @@ 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))

llvm/test/Transforms/InstCombine/is_fpclass.ll

@@ -3922,6 +3922,38 @@ 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
+}
+
+define i1 @minnum_qnan_commuted(i32 %x, float nofpclass(nnorm nsub nzero ninf nan) %y) {
+; CHECK-LABEL: @minnum_qnan_commuted(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[QNAN_BITS:%.*]] = or i32 [[X:%.*]], -5938
+; CHECK-NEXT:    [[QNAN:%.*]] = bitcast i32 [[QNAN_BITS]] to float
+; CHECK-NEXT:    [[MIN:%.*]] = call float @llvm.minnum.f32(float [[Y:%.*]], float [[QNAN]])
+; CHECK-NEXT:    [[TEST:%.*]] = call i1 @llvm.is.fpclass.f32(float [[MIN]], i32 64)
+; CHECK-NEXT:    ret i1 [[TEST]]
+;
+entry:
+  %qnan_bits = or i32 %x, -5938
+  %qnan = bitcast i32 %qnan_bits to float
+  %min = call float @llvm.minnum.f32(float %y, float %qnan)
+  %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)