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

[Uros Bizjak]

On Mon, Jul 27, 2015 at 5:54 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * 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.

No problem, I'll add this information and send a v2 patch.

> 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?

Yes. It was discussed and agreed between GCC and kernel people (HPA)
on GCC mailing list. Please see [3].

[3] https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00725.html

Uros.
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