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    SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs

    * 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


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