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    SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs
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    On Wed, 2012-02-22 at 15:56 +0100, Ingo Molnar wrote:

    > > Because it really just looks like a stronger "unlikely()" and
    > > fundamentally it really isn't. [...]
    >
    > Well, the fact is that right now it *is* a stronger unlikely()
    > on architectures that have jump-labels and it's mapped to
    > unlikely() on others.
    >

    Has gcc been fix to make it truly an unlikely case and remove the "jmp;
    jmp" problem of before? I'm still using gcc 4.6.0 which has the
    following code for a tracepoint (example is the
    trace_sched_migrate_task().

    5b4a: e9 00 00 00 00 jmpq 5b4f <set_task_cpu+0x5e>

    The above is the jump label that turns into a nop at boot up.

    5b4f: eb 19 jmp 5b6a <set_task_cpu+0x79>

    Here we jump over some of the trace code (this is the fast path)

    5b51: 49 8b 7d 08 mov 0x8(%r13),%rdi
    5b55: 44 89 e2 mov %r12d,%edx
    5b58: 48 89 de mov %rbx,%rsi
    5b5b: 41 ff 55 00 callq *0x0(%r13)
    5b5f: 49 83 c5 10 add $0x10,%r13
    5b63: 49 83 7d 00 00 cmpq $0x0,0x0(%r13)
    5b68: eb 41 jmp 5bab <set_task_cpu+0xba>

    Below is the continuation of the fast path.

    5b6a: 48 8b 43 08 mov 0x8(%rbx),%rax
    5b6e: 44 39 60 18 cmp %r12d,0x18(%rax)
    5b72: 74 0c je 5b80 <set_task_cpu+0x8f>


    Again, I'm using gcc 4.6.0 and maybe it has been fixed.

    -- Steve




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