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

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Message-Id: <1250668860.7583.327.camel@twins>
Date:	Wed, 19 Aug 2009 10:01:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: make use of inc/dec conditional

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.
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