[clang] [Clang][doc] -ffp-contract options and standard compliance (PR #127621)

[Andy Kaylor via cfe-commits]

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@@ -1681,19 +1681,25 @@ for more details.
    permitted to produce more precise results than performing the same
    operations separately.
 
-   The C standard permits intermediate floating-point results within an
+   The C/C++ standard permits intermediate floating-point results within an
    expression to be computed with more precision than their type would
    normally allow. This permits operation fusing, and Clang takes advantage
-   of this by default. This behavior can be controlled with the ``FP_CONTRACT``
-   and ``clang fp contract`` pragmas. Please refer to the pragma documentation
-   for a description of how the pragmas interact with this option.
+   of this by default (``on``). Fusion across statements is non-C/C++ standard
+   compliant behavior and can be enabled (``fast``).
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andykaylor wrote:

This last sentence is a bit unclear. I would suggest " Fusion across statements is not compliant with the C/C++ standard but can be enabled using ``-ffp-contract=fast``."

BTW, the C standard allows contraction of "expressions" saying that "a floating expression may be contracted" but the language gets imprecise if we start talking about "contraction across expressions" or "contractions across statements." I think saying "contractions across statements" is not allowed is more accurate, because we always have sub-expressions within what we're contracting. On the other hand, there are cases where we will not contract even within a statement. For example, an explicit cast in the middle of a statement prevents contraction (e.g. `x = c + (float)(a * b)` when a, b, and c are double). @AaronBallman can you suggest more precise wording?

https://github.com/llvm/llvm-project/pull/127621