[llvm] InstCombine: Teach SimplifyDemandedFPClass to infer nnan and ninf on fmul (PR #175854)

[Matt Arsenault via llvm-commits]

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@@ -2329,7 +2329,18 @@ Value *InstCombinerImpl::SimplifyDemandedUseFPClass(Instruction *I,
     Known = KnownFPClass::fmul(KnownLHS, KnownRHS, Mode);
 
     FPClassTest ValidResults = DemandedMask & Known.KnownFPClasses;
-    return getFPClassConstant(VTy, ValidResults, /*IsCanonicalizing=*/true);
+    if (Constant *SingleVal =
+            getFPClassConstant(VTy, ValidResults, /*IsCanonicalizing=*/true))
+      return SingleVal;
+
+    FastMathFlags InferredFMF =
+        inferFastMathValueFlagsBinOp(FMF, ValidResults, KnownLHS, KnownRHS);
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arsenm wrote:

I'm not sure why the root would matter? This would mean losing noundef as the return attribute on the calling function: https://alive2.llvm.org/ce/z/JK2Hst

https://github.com/llvm/llvm-project/pull/175854