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

[James Y Knight via llvm-commits]

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@@ -3394,17 +3394,41 @@ 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 default LLVM code in an environment where these assumptions are not met
+can lead to undefined behavior.
+
+The representation bits of a floating-point value do not mutate arbitrarily; if
+there is no floating-point operation being performed, the NaN payload (if any)
+is preserved.
+
+When a floating-point math operation produces a NaN value, the result has a
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jyknight wrote:

I'd like to see some text added to the specification sections for the fneg/fabs/copysign operations. Noting _there_ that exceptions cannot be signaled (in addition to them only touching the sign bit) is meaningful -- it means that you may use them in a "strictfp" function. Unlike other operations, they do not need a "constrained" variant, because their behavior is fully defined regardless.

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