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

[Joshua Cranmer 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.
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jcranmer-intel wrote:

The `denormal-fp-math` attribute does affect whether subnormals are preserved, although I suspect it is somewhat inconsistently applied at the moment.

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