[llvm] [IR] Add new function attribute nocreateundeforpoison (PR #164809)

[Jay Foad via llvm-commits]

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@@ -2741,6 +2741,10 @@ For example:
 ``"nooutline"``
     This attribute indicates that outlining passes should not modify the
     function.
+``nocreateundeforpoison``
+    This attribute indicates that the result of the function will not be undef
+    or poison if all arguments are not undef and not poison. Otherwise, it is
+    undefined behavior.
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jayfoad wrote:

Good point. I'm not sure exactly how to word it. How about:

> Note that a call instruction with poison-generating fast math flags may still generate poison, even if the call site or called function have the `nocreateundeforpoison` attribute. In that sense, poison-generating fast math flags take priority over this attribute.

?

https://github.com/llvm/llvm-project/pull/164809