[llvm] [LangRef] Clarify the semantics of fast-math flags (PR #89442)

[Nikita Popov via llvm-commits]

================
@@ -3656,6 +3656,45 @@ floating-point transformations.
 ``fast``
    This flag implies all of the others.
 
+When performing a transformation that involves more that one instruction, the
+flags required to enable the transformation must be set on all instructions
+involved in the transformation, and any new instructions created by the
+transformation should have only those flags set which were set on all the
+original instructions that are being transformed.
----------------
nikic wrote:

> I don't have great wording to suggest at the moment (not unless you want a page of text), but maybe it would be better to delineate the flags for which the all-instructions-must-have-the-flags-to-be-rewritten property is true?

I think for the purposes of *this* change, explicitly listing the flags for which this guidance applies would indeed be best.

https://github.com/llvm/llvm-project/pull/89442