[libc] support for _BitInt in stdbit.h

[nickdesaulniers]

This was pointed out in #80459 (comment) , that C23 added support for _BitInt types. It would be nice if our type generic macros in stdbit.h would resolve for these types.

I wonder if the support of _BitInt was related to why GCC added a few builtins to help implement c23 stdbit.h?

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

This was pointed out in https://github.com//pull/80459#discussion_r1476478451 , that C23 added support for `_BitInt` types. It would be nice if our type generic macros in stdbit.h would resolve for these types.

I wonder if the support of _BitInt was related to why GCC added a few builtins to help implement c23 stdbit.h?

[pinskia]

I wonder if the support of _BitInt was related to why #79630 (comment) to help implement c23 stdbit.h?

It is.

[SchrodingerZhu]

Those builtins are type generic?

[pinskia]

Those builtins are type generic?

Yes the g builtins are type generic and documented that way.
see starting at https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fffsg .
"Similar to __builtin_ffs, except the argument is type-generic signed integer (standard, extended or bit-precise)."

[nickdesaulniers]

Some issues I'm running into:

• -std=c23 is nice for the tests, but only is supported in clang-18+ and gcc trunk (to be 14). Can stick with -std=c2x for now.
• UWB suffixes on literals a nice for tests, but only supported in clang-15+ and gcc trunk (to be 14). I guess I might be able to stick to casts for now?

[Flandini]

@nickdesaulniers, is there some ongoing work on this? I would be interested in helping with this while my math issues and PRs are on hold.

[nickdesaulniers]

No, please feel free to send a patch!