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    SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs
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    Not arguing that, but the static aspect is still key... or people will read it as another version of likely/unlikely.  I'd be fine with static_likely/unlikely for example; I wish "static" wasn't such an overloaded word in C but I can't.personally think of a better term.

    Ingo Molnar <mingo@elte.hu> wrote:

    >
    >* H. Peter Anvin <hpa@zytor.com> wrote:
    >
    >> On 02/21/2012 11:25 PM, Ingo Molnar wrote:
    >> >
    >> > There is a fundamental assymetry, and intentionally so. You
    >> > *really* have to think what the common case is, and make
    >> > sure the build defaults to that. It's not the end of the
    >> > world to have it flipped over, but there's costs and those
    >> > costs are higher even in the branch path than a regular
    >> > likely()/unlikely().
    >>
    >> No, not really -- it's still an unconditional branch, which
    >> means you will not tax the branch predictor in any way and
    >> which can be followed by the front end without taking a
    >> speculation hit. [...]
    >
    >You are talking about CPU level costs, I am also talking about
    >costs introduced at build time.
    >
    >Fact is, jump-label unlikely branches are moved *out of line*:
    >they are often in unlikely portions of the function (near other
    >unlikely branches), with instruction cache granularity costs and
    >potentially higher instruction-cache miss costs attached, etc.
    >
    >You are missing three important aspects:
    >
    >Firstly, instead of:
    >
    > ins1
    > ins2
    > ins3
    > ins4
    > ins5
    > ins-compare
    > ins-branch
    > ins6
    > ins7
    > ins8
    > ins9
    > ins10
    >
    >We have:
    >
    > ins1
    > ins2
    > ins3
    > ins4
    > ins5
    > ins-jump
    >
    > [ hole ]
    >
    > ins6
    > ins7
    > ins8
    > ins9
    > ins10
    > ins-jump back
    >
    >Where the 'hole' fragments the instruction cache layout. Given
    >that most of kernel execution is instruction-cache-cold, the
    >'straightness' of kernel code matters quite a bit.
    >
    >Secondly, there's build time instruction scheduling costs as
    >well: GCC will prefer the likely branch over the unlikely one,
    >so we might see extra instructions in the out-of-line code:
    >
    >
    > ins1
    > ins2
    > ins3
    > ins4
    > ins5
    > ins-jump
    >
    > [ hole ]
    >
    > ins-extra-1
    > ins-extra-2
    > ins6
    > ins7
    > ins8
    > ins9
    > ins10
    > ins-jump back
    >
    >In that sense jump labels are unlikely() branches combined with
    >a patching mechanism.
    >
    >Thus *both* aspects are important: if a branch is *truly* 50/50
    >then it's quite possibly *NOT* a correct optimization to use
    >jump-labels as the 'uncommon' code goes through extra hoops and
    >fragments out of the fastpath, which in quite many real life
    >cases can outstrip the advantage of the avoidance of a single
    >branch ...
    >
    >Thirdly,
    >
    >even if it's a correct optimization and both branches happen to
    >outperform the pre-jump-label version, regardless of the
    >direction of the jump label flag, it's *STILL* fundamentally
    >assymetric: due to the hole and due to the possible extra
    >instructions the out of line code will be slower by a few
    >instruction and the NOP fall-through will be faster.
    >
    >This is fundamentally so, and any naming that tries to *hide*
    >that assymetry and the associated micro-costs is confused.
    >
    >Thanks,
    >
    > Ingo

    --
    Sent from my mobile phone. Please excuse my brevity and lack of formatting.


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