[llvm] [InstCombine] Fold isnan idioms (PR #101510)

[Matt Arsenault via llvm-commits]

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@@ -363,8 +363,28 @@ static Value *foldLogOpOfMaskedICmps_NotAllZeros_BMask_Mixed(
   // deduce anything from it.
   // For example,
   // (icmp ne (A & 12), 0) & (icmp eq (A & 3), 1) -> no folding.
-  if ((*BCst & *DCst) == 0)
+  if ((*BCst & *DCst) == 0) {
+    // Try to fold isNaN idiom:
+    // (icmp ne (A & FractionBits), 0) & (icmp eq (A & ExpBits), ExpBits) ->
+    // isNaN(A)
+    Value *Src;
+    if (match(A, m_ElementWiseBitCast(m_Value(Src))) &&
+        Src->getType()->getScalarType()->isIEEELikeFPTy() &&
+        !Builder.GetInsertBlock()->getParent()->hasFnAttribute(
+            Attribute::NoImplicitFloat)) {
+      APInt ExpBits =
+          APFloat::getInf(Src->getType()->getScalarType()->getFltSemantics())
+              .bitcastToAPInt();
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arsenm wrote:

Early exit on the ExpBits tests here? 

https://github.com/llvm/llvm-project/pull/101510