[clang] Fix math-errno issue (PR #66381)

[Zahira Ammarguellat via cfe-commits]

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@@ -2339,6 +2345,28 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
   // LLVM counterparts if the call is marked 'const' (known to never set errno).
   // In case FP exceptions are enabled, the experimental versions of the
   // intrinsics model those.
+  bool ConstAlways =
+      getContext().BuiltinInfo.isConst(BuiltinID);
+
+  // There's a special case with the fma builtins where they are always const
+  // if the target environment is GNU or the target is OS is Windows and we're
+  // targeting the MSVCRT.dll environment.
+  switch (BuiltinID) {
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zahiraam wrote:

> @zahiraam I think the best way to do that would be to add a new letter vcode to the attributes here:
> 
> https://github.com/llvm/llvm-project/blob/cbdccb30c23f71f20d05b19256232419e7c5e517/clang/include/clang/Basic/Builtins.def#L74
> 
> -- something like "// m -> const when we GNU or MSVCRT libraries are targeted" and then fma, for example, would become "BUILTIN(__builtin_fma, "dddd", "Fmne")"
> I'd suggest doing that as a separate patch, because it isn't obvious exactly how it should be managed.
> 


Yes, I can work on that in a subsequent patch. Will keep the FIXME comment then.

https://github.com/llvm/llvm-project/pull/66381