Messages in this thread | | | Date | Wed, 22 Feb 2012 08:48:39 +0100 | From | Ingo Molnar <> | Subject | Re: [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
| |