[ValueTracking] Don't take sign bit from NaN operands

Author: dtcxzyw

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))

llvm/test/Transforms/InstCombine/is_fpclass.ll

@@ -3927,7 +3927,7 @@ define i1 @test_class_is_not_psub_pnorm_pinf__dynamic(float %arg) #3 {
 define i1 @minnum_qnan(i32 %x) {
 ; CHECK-LABEL: @minnum_qnan(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %qnan_bits = or i32 %x, -5938