ValueTracking: Fix bug with fcmp false to nan constant

If we had a comparison to a literal nan with a false predicate,
we were incorrectly treating it as an unordered compare. This was
correct for fcmp true, but not fcmp false. I noticed this in the
review for e44d3b3 but misdiagnosed
the reason. Also change the test for the fcmp true case to be more
useful, but it wasn't wrong previously.

Author: arsenm

llvm/lib/Analysis/ValueTracking.cpp

@@ -3972,10 +3972,16 @@ std::tuple<Value *, FPClassTest, FPClassTest>
 llvm::fcmpImpliesClass(CmpInst::Predicate Pred, const Function &F, Value *LHS,
                        FPClassTest RHSClass, bool LookThroughSrc) {
   assert(RHSClass != fcNone);
+  Value *Src = LHS;
+
+  if (Pred == FCmpInst::FCMP_TRUE)
+    return exactClass(Src, fcAllFlags);
+
+  if (Pred == FCmpInst::FCMP_FALSE)
+    return exactClass(Src, fcNone);
 
   const FPClassTest OrigClass = RHSClass;
 
-  Value *Src = LHS;
   const bool IsNegativeRHS = (RHSClass & fcNegative) == RHSClass;
   const bool IsPositiveRHS = (RHSClass & fcPositive) == RHSClass;
   const bool IsNaN = (RHSClass & ~fcNan) == fcNone;
@@ -3994,12 +4000,6 @@ llvm::fcmpImpliesClass(CmpInst::Predicate Pred, const Function &F, Value *LHS,
   if (Pred == FCmpInst::FCMP_UNO)
     return exactClass(Src, fcNan);
 
-  if (Pred == FCmpInst::FCMP_TRUE)
-    return exactClass(Src, fcAllFlags);
-
-  if (Pred == FCmpInst::FCMP_FALSE)
-    return exactClass(Src, fcNone);
-
   const bool IsFabs = LookThroughSrc && match(LHS, m_FAbs(m_Value(Src)));
   if (IsFabs)
     RHSClass = llvm::inverse_fabs(RHSClass);

llvm/test/Transforms/Attributor/nofpclass-implied-by-fcmp.ll

@@ -2641,8 +2641,8 @@ define float @assume_false_smallest_normal(float %arg) {
 }
 
 define float @clamp_false_nan(float %arg) {
-; CHECK-LABEL: define nofpclass(nan inf nzero sub norm) float @clamp_false_nan(
-; CHECK-SAME: float returned nofpclass(nan inf nzero sub norm) [[ARG:%.*]]) #[[ATTR2]] {
+; CHECK-LABEL: define float @clamp_false_nan(
+; CHECK-SAME: float returned [[ARG:%.*]]) #[[ATTR2]] {
 ; CHECK-NEXT:    ret float [[ARG]]
 ;
   %fcmp = fcmp false float %arg, 0x7FF8000000000000
@@ -2784,12 +2784,12 @@ define float @clamp_true_smallest_normal_0.0(float %arg) {
 }
 
 define float @clamp_true_nan(float %arg) {
-; CHECK-LABEL: define noundef nofpclass(nan inf nzero sub norm) float @clamp_true_nan(
-; CHECK-SAME: float nofpclass(nan inf nzero sub norm) [[ARG:%.*]]) #[[ATTR2]] {
-; CHECK-NEXT:    ret float 0.000000e+00
+; CHECK-LABEL: define float @clamp_true_nan(
+; CHECK-SAME: float returned [[ARG:%.*]]) #[[ATTR2]] {
+; CHECK-NEXT:    ret float [[ARG]]
 ;
   %fcmp = fcmp true float %arg, 0x7FF8000000000000
-  %select = select i1 %fcmp, float 0.0, float %arg
+  %select = select i1 %fcmp, float %arg, float 0.0
   ret float %select
 }