[llvm] specify NaN behavior more precisely (PR #66579)

[Ralf Jung via llvm-commits]

================
@@ -3394,17 +3394,75 @@ Floating-Point Environment
 The default LLVM floating-point environment assumes that traps are disabled and
 status flags are not observable. Therefore, floating-point math operations do
 not have side effects and may be speculated freely. Results assume the
-round-to-nearest rounding mode.
+round-to-nearest rounding mode, and subnormals are assumed to be preserved.
+
+Running LLVM code in an environment where these assumptions are not met can lead
+to undefined behavior. The ``stricfp`` and ``denormal-fp-math`` attributes as
+well as :ref:`Constrained Floating-Point Intrinsics <constrainedfp>` can be used
+to weaken LLVM's assumptions and ensure defined behavior in non-default
+floating-point environments; see their respective documentation for details.
+
+.. _floatnan:
+
+Behavior of Floating-Point NaN values
+-------------------------------------
----------------
RalfJung wrote:

This wasn't really about the "FP environment" any more so I moved this all into a new section on NaN values.

https://github.com/llvm/llvm-project/pull/66579