[PATCH] D78827: Add support for #pragma clang fp reassociate(on|off) -- floating point control of associative math transformations

[Steve Canon via Phabricator via cfe-commits]

scanon added a comment.

TS18661-5 is quite vague on what the intended semantics for the pragma are.

These pragmas are intended to be bindings of clause 10.4 of IEEE 754, which is also pretty wishy-washy on the whole, but it's worth noting that clause 10 is titled *expression evaluation* specifically. The relevant text here is:

> A language standard should also define, and require implementations to provide, attributes that allow and disallow value-changing optimizations, separately or collectively, for a block. These optimizations might include, but are not limited to:
>  ― Applying the associative or distributive laws.
>  ― Synthesis of a fusedMultiplyAdd operation from a multiplication and an addition.
>  ― Synthesis of a formatOf operation from an operation and a conversion of the result of the 40 operation.
>  ― Use of wider intermediate results in expression evaluation.

So IEEE-754 appears to view this as being "just like" contraction. (Note that this is all under a "should", so #yolo).


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https://reviews.llvm.org/D78827