svn commit: r358439 - head/sys/amd64/include

[Kyle Evans]

On Wed, Mar 4, 2020 at 11:27 AM Ryan Libby <rlibby at freebsd.org> wrote:
>
> On Wed, Mar 4, 2020 at 3:27 AM Guido Falsi <madpilot at freebsd.org> wrote:
> >
> > On 02/03/20 18:13, Ryan Libby wrote:
> > > On Mon, Mar 2, 2020 at 12:45 AM Alexander V. Chernikov <melifaro at ipfw.ru> wrote:
> > >>
> > >> 28.02.2020, 18:32, "Ryan Libby" <rlibby at freebsd.org>:
> > >>> Author: rlibby
> > >>> Date: Fri Feb 28 18:32:36 2020
> > >>> New Revision: 358439
> > >>> URL: https://svnweb.freebsd.org/changeset/base/358439
> > >>>
> > >>> Log:
> > >>>   amd64 atomic.h: minor codegen optimization in flag access
> > >>>
> > >>>   Previously the pattern to extract status flags from inline assembly
> > >>>   blocks was to use setcc in the block to write the flag to a register.
> > >>>   This was suboptimal in a few ways:
> > >>>    - It would lead to code like: sete %cl; test %cl; jne, i.e. a flag
> > >>>      would just be loaded into a register and then reloaded to a flag.
> > >>>    - The setcc would force the block to use an additional register.
> > >>>    - If the client code didn't care for the flag value then the setcc
> > >>>      would be entirely pointless but could not be eliminated by the
> > >>>      optimizer.
> > >>>
> > >>>   A more modern inline asm construct (since gcc 6 and clang 9) allows for
> > >> This effectively restricts kernel builds by all older compilers.
> > >> Is there any chance of making it conditional depending on the compiler version/features?
> > >
> > > Yes, it is possible to test for __GCC_ASM_FLAG_OUTPUTS__.  It is more
> > > maintenance effort going forward.  If building current with an old cross
> > > compiler is an important scenario, we can either revert this and the
> > > following revision or work up a patch to make it conditional.  I'll see
> > > what that might look like.
> > >
> >
> > Actually this causes emulators/virtualbox-ose port to fail to build:
> >
> > In file included from /usr/src/sys/sys/systm.h:44:
> > /usr/include/machine/atomic.h:230:1: error: invalid output constraint
> > '=@cce' in asm
> > ATOMIC_CMPSET(char);
> > ^
> > /usr/include/machine/atomic.h:205:4: note: expanded from macro
> > 'ATOMIC_CMPSET'
> >         : "=@cce" (res),                /* 0 */         \
> >           ^
> > /usr/include/machine/atomic.h:230:1: error: invalid output constraint
> > '=@cce' in asm
> >
> > (and so on)
> >
> >
> > the virtualbox-ose port is forced to use an older clang version due to
> > crashes when compiled with newer ones.
> >
> > Not sure whose responsibility is to fix this.
> >
> > Should I file a bug report on bugzilla?
> >
> > --
> > Guido Falsi <madpilot at freebsd.org>
>
> We've discussed whether to provide compatibility code for older compilers here:
> https://reviews.freebsd.org/D23937
>
> There's a bug tracking virtualbox-ose being pinned to an old compiler here:
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236616
>
> I see you have commented on that bug.

I've opened https://reviews.freebsd.org/D23967 and PR 244603
specifically for coercing the port to use GCC9 instead, which does
indeed fix the breakage. Once that lands, I'll investigate 236616 some
more to see if newer llvm (including 10) is still broken here.

Thanks,

Kyle Evans