[llvm] [CodeGenPrepare] Reverse the canonicalization of isInf/isNanOrInf (PR #81572)

[Yingwei Zheng via llvm-commits]

================
@@ -1943,6 +1943,36 @@ static bool swapICmpOperandsToExposeCSEOpportunities(CmpInst *Cmp) {
   return false;
 }
 
+static bool foldFCmpToFPClassTest(CmpInst *Cmp, const TargetLowering &TLI,
+                                  const DataLayout &DL) {
+  FCmpInst *FCmp = dyn_cast<FCmpInst>(Cmp);
+  if (!FCmp)
+    return false;
+
+  // Reverse the canonicalization if it is a FP class test
+  auto ShouldReverseTransform = [](FPClassTest ClassTest) {
+    return ClassTest == fcInf || ClassTest == (fcInf | fcNan);
+  };
+  auto [ClassVal, ClassTest] =
+      fcmpToClassTest(FCmp->getPredicate(), *FCmp->getParent()->getParent(),
+                      FCmp->getOperand(0), FCmp->getOperand(1));
+  if (!ClassVal)
+    return false;
+
+  if (!ShouldReverseTransform(ClassTest) && !ShouldReverseTransform(~ClassTest))
+    return false;
+
+  // Don't fold if the target offers free fabs.
+  if (TLI.isFAbsFree(TLI.getValueType(DL, ClassVal->getType())))
----------------
dtcxzyw wrote:

Emm, TTI/TLI doesn't provide a hook for querying the legality of fcmp predicates :(

Could you please provide a test for demonstration?


https://github.com/llvm/llvm-project/pull/81572