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    SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs
    On Wed, Feb 22, 2012 at 09:18:55AM +0100, Ingo Molnar wrote:

    > The problem with static_branch_def_false/def_true was that the
    > very intuitively visible bias that we see with
    > likely()/unlikely() is confused in jump label constructs through
    > two layers of modifiers. And the fix is so easy, a simple rename
    > in most cases ;-)
    >
    > So instead of that, in this series we have:
    >
    > + if (very_unlikely(&perf_sched_events.key))
    >
    > which is a heck of an improvement IMO. I'd still up its
    > readability a notch, by also signalling the overhead of the
    > update path by making it:
    >
    > + if (very_unlikely(&perf_sched_events.slow_flag))
    >
    > ... but I don't want to be that much of a readability nazi ;-)

    I have to say I don't like the "very_unlikely" name. It's confusing
    because the condition being evaluated appears to be the address of
    something, i.e. &perf_sched_events.key in your example, and that looks
    to me to be very very likely to be true, i.e. non-zero. But the code
    is telling me that's very *un*likely, which is confusing.

    In other words I can't think about "very_unlikely" as being similar to
    "unlikely". Clearly very_unlikely isn't just passing through the
    true/falseness of its argument to the surrounding if, the way that
    unlikely does; it (very_unlikely) is a real function that is doing
    something more complicated with its argument -- dereferencing it,
    conceptually at least, for a start.

    Just from a readability perspective, I like the static_branch name,
    and if we want to add information about the execution bias, I'd
    suggest we do:

    if (likely(static_branch(&x))) ...

    if (unlikely(static_branch(&x))) ...

    if that can be made to work, or else have variants like this:

    if (likely_static_branch(&x)) ...

    if (unlikely_static_branch(&x)) ...

    Paul.


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