[PATCH] D69272: Restricted variant of '#pragma STDC FENV_ACCESS'

[Kevin P. Neal via Phabricator via cfe-commits]

kpn added a comment.

In D69272#1794630 <https://reviews.llvm.org/D69272#1794630>, @sepavloff wrote:

> Putting restriction on use of the pragma is of course, a temporary solution, it is not usable in all cases. But for some cases it is usable in production code. Where small pieces of code may be extracted into separate functions, this solution can provide tolerable performance loss, if most part of the program doesn't use constrained intrinsics. Warning prevents users from false expectations. Use in production code ensures further development of the feature.
>
> Full-fledged solution requires full support of constrained intrinsics in optimizations. It is not clear how large this work is, but odd are that it would require substantial efforts. It this case usable implementation of 'pragma STDC FE_ACCESS' would postponed. The restricted solution can be implemented much faster and it does not impede development of the full-fledged one.


It depends on the definition of "usable" or "tolerable", and the definition that matters is the one users have.

If #pragma is restricted to no smaller than whole functions then a function that uses it inside the body won't compile. We made the decision instead of the user.

If #pragma is not restricted, but triggers worse performance in the rest of the function, then users can decide for themselves if that is acceptable.

We should leave the decision up to the end-user. The idea of always using constrained intrinsics if any constrained intrinsic is used in a function is our best bet at getting something in the hands of users sooner rather than later and without them having to do any rewriting of existing code.


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https://reviews.llvm.org/D69272