[llvm] LangRef: rint, nearbyint: mention that default rounding mode is assumed (PR #77191)

[Ralf Jung via llvm-commits]

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@@ -15751,7 +15751,9 @@ Semantics:
 """"""""""
 
 This function returns the same values as the libm ``rint`` functions
-would, and handles error conditions in the same way.
+would, and handles error conditions in the same way. The rounding mode
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RalfJung wrote:

Yeah, but there's no way for surrounding LLVM IR code to make use of that, since LLVM doesn't promise to preserve any of that state -- so instruction reordering can easily clobber the exception state in the very next moment.

https://github.com/llvm/llvm-project/pull/77191