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SubjectRe: [PATCH] x86: make use of inc/dec conditional
>>> Peter Zijlstra <peterz@infradead.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@novell.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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