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    Subject[PATCH] x86: Optimize variable_test_bit()
    While looking at some asm I noticed we produce the most horrid code for
    test_bit():

    1a5e: 49 0f a3 30 bt %rsi,(%r8)
    1a62: 45 19 c0 sbb %r8d,%r8d
    1a65: 45 85 c0 test %r8d,%r8d
    1a68: 75 a6 jne 1a10 <x86_schedule_events+0xc0>

    Since test_bit() doesn't actually have any output variables, we can use
    asm goto without having to add a memory clobber. This reduces the code
    to something sensible:

    1a12: 49 0f a3 30 bt %rsi,(%r8)
    1a16: 72 68 jb 1a80 <x86_schedule_events+0x130>

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    PS. should we kill the memory clobber for __test_and_change_bit()? It
    seems inconsistent and out of place.

    PPS. Jakub, I see gcc5.1 still hasn't got output operands for asm goto;
    is this something we can get 'fixed' ?

    arch/x86/include/asm/bitops.h | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
    index cfe3b954d5e4..bcf4fa77c04f 100644
    --- a/arch/x86/include/asm/bitops.h
    +++ b/arch/x86/include/asm/bitops.h
    @@ -313,6 +313,15 @@ static __always_inline int constant_test_bit(long nr, const volatile unsigned lo

    static inline int variable_test_bit(long nr, volatile const unsigned long *addr)
    {
    +#ifdef CC_HAVE_ASM_GOTO
    + asm_volatile_goto ("bt %1, %0\n\t"
    + "jc %l[cc_label]"
    + : : "m" (*(unsigned long *)addr), "Ir" (nr)
    + : : cc_label);
    + return 0;
    +cc_label:
    + return 1;
    +#else
    int oldbit;

    asm volatile("bt %2,%1\n\t"
    @@ -321,6 +330,7 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr)
    : "m" (*(unsigned long *)addr), "Ir" (nr));

    return oldbit;
    +#endif
    }

    #if 0 /* Fool kernel-doc since it doesn't do macros yet */

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