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    SubjectRe: [PATCH 07/12] x86: use cmpxchg_flag() where applicable
    On 08/24/2011 02:56 PM, Linus Torvalds wrote:
    > Ok, I see nothing horrible in this series.
    > The one reaction I have is that the cmpxchg_flag() thing returns an
    > 8-bit value, but then a lot of the users end up having to extend it to
    > a full "int" purely for calling convention reasons (eg I think
    > 'down_write_trylock()' will have 'sete + movzl' - not a new problem,
    > but since the whole point was to remove extraneous instructions and we
    > no longer have the silly 'testl', it now annoys me more).
    > So it seems a bit sad. But I guess it doesn't really matter.

    I think it is a net lose. The most common case is probably going to be
    to use it immediately, in which case we have:

    cmpxchg -> sete -> compare -> conditional


    cmpxchg -> compare -> conditional

    For doubleword cmpxchg it's another matter entirely, because doubleword
    comparisons are significantly more expensive that sete + test.

    So unless there is actual data showing this is better, I would like to
    see this dropped for now.


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      Last update: 2011-08-25 00:05    [W:0.023 / U:11.240 seconds]
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