[Bug middle-end/113921] Output register of an "asm volatile goto" is incorrectly clobbered/discarded

["torvalds@linux-foundation.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921

--- Comment #5 from Linus Torvalds <torvalds@linux-foundation.org> ---
(In reply to Linus Torvalds from comment #2)
>
> So we could make our workaround option be something like
> 
>    config GCC_ASM_GOTO_WORKAROUND
>      def_bool y
>      depends on CC_IS_GCC && GCC_VERSION < 120100

Replying to myself some more, since that kernel side workaround is actually in
two parts: it does the extra empty inline asm as an extra barrier, but it
*also* adds the 'volatile' to the asm with outputs to work around the other gcc
bug.

And that other fix ("Mark asm goto with outputs as volatile") is *not* in
gcc-12.1.0. It has only made it into gcc-13.2.0 (and it's pending in the gcc-12
release branch if I read things right).

I suspect that other bug doesn't affect Sean's kvm case, because the outputs
are always used, but it could affect other kernel code.

So we'd want to have at least gcc-13.2 to be safe.

Hmm.

We could make the "add volatile manually" be the default workaround, though,
since it shouldn't matter.