[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:

Those 3 shouldn't be considered floating point math operations (as the term is used here and before). We definitely need to make it clear in their definitions -- it has implications beyond just the NaN behavior.

In particular, they all ought to be specified as: "This operation never raises a floating-point exception, and the result is an exact copy of the input, other than the sign bit. It is not considered a floating-point math operation, but rather, bit-manipulation which operates on floating-point values."

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