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    Subject[PATCH 2/6] x86-64: Don't generate cmov in vread_tsc
    Date
    vread_tsc checks whether rdtsc returns something less than
    cycle_last, which is an extremely predictable branch. GCC likes
    to generate a cmov anyway, which is several cycles slower than
    a predicted branch. This saves a couple of nanoseconds.

    Signed-off-by: Andy Lutomirski <luto@mit.edu>
    ---
    arch/x86/kernel/tsc.c | 15 +++++++++++----
    1 files changed, 11 insertions(+), 4 deletions(-)

    diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
    index 80e6017..a159fba 100644
    --- a/arch/x86/kernel/tsc.c
    +++ b/arch/x86/kernel/tsc.c
    @@ -791,14 +791,21 @@ static cycle_t __vsyscall_fn vread_tsc(void)
    * ensures that rdtsc is ordered wrt all later loads.
    */

    - /* This doesn't multiply 'zero' by anything, which *should*
    - * generate nicer code, except that gcc cleverly embeds the
    - * dereference into the cmp and the cmovae. Oh, well.
    + /* This doesn't multiply 'zero' by anything, which generates
    + * very slightly nicer code than multiplying it by 8.
    */
    last = *( (cycle_t *)
    ((char *)&__vsyscall_gtod_data.clock.cycle_last + zero) );

    - return ret >= last ? ret : last;
    + if (likely(ret >= last))
    + return ret;
    +
    + /* GCC likes to generate cmov here, but this branch is extremely
    + predictable (it's just a funciton of time and the likely is
    + very likely) and there's a data dependence, so force GCC
    + to generate a branch instead. */
    + asm volatile ("");
    + return last;
    }
    #endif

    --
    1.7.4


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