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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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