linux-kernel - Re: [PATCH] x86: make use of inc/dec conditional

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Date:	Wed, 19 Aug 2009 10:00:33 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86: make use of inc/dec conditional

>>> Peter Zijlstra <peterz@...radead.org> 19.08.09 10:01 >>>
>On Wed, 2009-08-19 at 08:48 +0100, Jan Beulich wrote:
>> According to gcc's instruction selection, inc/dec can be used without
>> penalty on most CPU models, but should be avoided on others. Hence we
>> should have a config option controlling the use of inc/dec, and
>> respective abstraction macros to avoid making the resulting code too
>> ugly. There are a few instances of inc/dec that must be retained in
>> assembly code, due to that code's dependency on the instruction not
>> changing the carry flag.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...ell.com>
>> 
>> ---
>>  arch/x86/Kconfig.cpu                |    4 ++++
>>  arch/x86/include/asm/asm.h          |   27 +++++++++++++++++++++++++++
>>  arch/x86/include/asm/atomic_32.h    |    8 ++++----
>>  arch/x86/include/asm/atomic_64.h    |   16 ++++++++--------
>>  arch/x86/include/asm/checksum_32.h  |    2 +-
>>  arch/x86/include/asm/spinlock.h     |    6 +++---
>>  arch/x86/lib/checksum_32.S          |   11 ++++++-----
>>  arch/x86/lib/clear_page_64.S        |    3 ++-
>>  arch/x86/lib/copy_page_64.S         |    5 +++--
>>  arch/x86/lib/copy_user_64.S         |   17 +++++++++--------
>>  arch/x86/lib/copy_user_nocache_64.S |   17 +++++++++--------
>>  arch/x86/lib/memcpy_64.S            |   11 ++++++-----
>>  arch/x86/lib/memset_64.S            |    7 ++++---
>>  arch/x86/lib/rwlock_64.S            |    5 +++--
>>  arch/x86/lib/semaphore_32.S         |    7 ++++---
>>  arch/x86/lib/string_32.c            |   23 ++++++++++++-----------
>>  arch/x86/lib/strstr_32.c            |    5 +++--
>>  17 files changed, 108 insertions(+), 66 deletions(-)
>
>What's the performance gain? This seems like a rather large and ugly
>patch if the result is borderline.

The performance gain isn't very significant, but if the compiler cares to
avoid/use certain instructions on certain CPU models, the kernel shouldn't
artificially introduce uses of those instructions.

And while the patch is maybe large, I don't think the resulting code is
significantly more ugly than it already was (if it was). I'd consider
removing the .S/.c changes, though, but I think the inline assembly
changes to headers should go in at least.

Jan

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