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    Subject[tip:perfcounters/urgent] x86: atomic64: Improve atomic64_read()
    Commit-ID:  67d7178f8fc64b7f68d7dd8a1b21dfa0d42c220c
    Gitweb: http://git.kernel.org/tip/67d7178f8fc64b7f68d7dd8a1b21dfa0d42c220c
    Author: Eric Dumazet <eric.dumazet@gmail.com>
    AuthorDate: Fri, 3 Jul 2009 13:23:02 +0200
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Fri, 3 Jul 2009 14:42:59 +0200

    x86: atomic64: Improve atomic64_read()

    Optimize atomic64_read() as a special open-coded
    cmpxchg8b variant. This generates nicer code:

    arch/x86/lib/atomic64_32.o:

    text data bss dec hex filename
    435 0 0 435 1b3 atomic64_32.o.before
    431 0 0 431 1af atomic64_32.o.after

    md5:
    bd8ab95e69c93518578bfaf0ea3be4d9 atomic64_32.o.before.asm
    2bdfd4bd1f6b7b61b7fc127aef90ce3b atomic64_32.o.after.asm

    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Arnd Bergmann <arnd@arndb.de>
    LKML-Reference: <alpine.LFD.2.01.0907021653030.3210@localhost.localdomain>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    arch/x86/lib/atomic64_32.c | 12 ++++++++++--
    1 files changed, 10 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c
    index fd28fd3..cd11803 100644
    --- a/arch/x86/lib/atomic64_32.c
    +++ b/arch/x86/lib/atomic64_32.c
    @@ -62,9 +62,17 @@ void atomic64_set(atomic64_t *ptr, u64 new_val)
    */
    u64 atomic64_read(atomic64_t *ptr)
    {
    - u64 old = 1LL << 32;
    + u64 res;

    - return cmpxchg8b(&ptr->counter, old, old);
    + asm volatile(
    + "mov %%ebx, %%eax\n\t"
    + "mov %%ecx, %%edx\n\t"
    + LOCK_PREFIX "cmpxchg8b %1\n"
    + : "+A" (res)
    + : "m" (*ptr)
    + );
    +
    + return res;
    }

    /**

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