linux-kernel - Re: [PATCH] x86: Introduce ASM flags to bitops

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Date:	Mon, 27 Jul 2015 17:50:12 +0200
From:	Uros Bizjak <ubizjak@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Uros Bizjak <uros_bizjak1@....net>, linux-kernel@...r.kernel.org,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] x86: Introduce ASM flags to bitops

On Mon, Jul 27, 2015 at 5:40 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Uros Bizjak <uros_bizjak1@....net> wrote:
>
>> From: Uros Bizjak <ubizjak@...il.com>
>>
>> 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@...il.com>
>> ---
>>  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

Uros.
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