Re: [PATCH] x86: Introduce ASM flags to bitops

[Ingo Molnar]

* Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> On Mon, Jul 27, 2015 at 5:40 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Uros Bizjak <uros_bizjak1@xxxxxxx> wrote:
> >
> >> From: Uros Bizjak <ubizjak@xxxxxxxxx>
> >>
> >> This patch introduces GCC ASM flags to bitops. Instead of e.g.
> >>
> >>    136d7:     48 0f a3 3d 00 00 00    bt     %rdi,0x0(%rip)
> >>    136de:     00
> >>    136df:     19 ff                   sbb    %edi,%edi
> >>    136e1:     85 ff                   test   %edi,%edi
> >>    136e3:     0f 95 c0                setne  %al
> >>
> >> following code is generated:
> >>
> >>    13767:     48 0f a3 3d 00 00 00    bt     %rdi,0x0(%rip)
> >>    1376e:     00
> >>    1376f:     0f 92 c0                setb   %al
> >>
> >> Similar improvement can be seen in following code:
> >>
> >>     7a6c:     48 0f a3 11             bt     %rdx,(%rcx)
> >>     7a70:     19 d2                   sbb    %edx,%edx
> >>     7a72:     85 d2                   test   %edx,%edx
> >>     7a74:     74 eb                   je     7a61
> >>
> >> which becomes:
> >>
> >>     7a8c:     48 0f a3 11             bt     %rdx,(%rcx)
> >>     7a90:     73 ef                   jae    7a81
> >>
> >> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> >> ---
> >>  arch/x86/include/asm/bitops.h      | 26 ++++++++++++++++++++++++--
> >>  arch/x86/include/asm/percpu.h      | 18 +++++++++++++++++-
> >>  arch/x86/include/asm/signal.h      |  6 ++++++
> >>  arch/x86/include/asm/sync_bitops.h | 18 ++++++++++++++++++
> >>  4 files changed, 65 insertions(+), 3 deletions(-)
> >
> > Nothing in your patch seems to be setting __GCC_ASM_FLAG_OUTPUTS__, and the patch
> > does not seem to be mailed as part of a larger series ...
> >
> > So how is this supposed to work?
> 
> GCC version 6+ will automatically define __GCC_ASM_FLAG_OUTPUTS__ when
> this feature is supported. Please see [1] for RFC GCC patch series and
> [2] for final committed patch.
> 
> [1] https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00594.html
> [2] https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02087.html

Ok, great. This information should be part of the changelog and such, as it's not 
obvious.

Does the GCC project treat this as an ABI kind of thing, i.e. can the kernel rely 
on it from now on, without the GCC side semantics of this feature not ever 
changing and breaking the kernel?

Thanks,

	Ingo
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