Re: [dpdk-dev] [PATCH v2] ring: check for zero objects mc dequeue / mp enqueue

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Mar 18, 2016 at 11:27:18AM +0100, Olivier Matz wrote:
> Hi,
> 
> On 03/18/2016 11:18 AM, Bruce Richardson wrote:
> >>> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
> >>> index 943c97c..eb45e41 100644
> >>> --- a/lib/librte_ring/rte_ring.h
> >>> +++ b/lib/librte_ring/rte_ring.h
> >>> @@ -431,6 +431,11 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void *
> >>> const *obj_table,
> >>>         uint32_t mask = r->prod.mask;
> >>>         int ret;
> >>>
> >>> +       /* Avoid the unnecessary cmpset operation below, which is also
> >>> +        * potentially harmful when n equals 0. */
> >>> +       if (n == 0)
> >>>
> >>
> >> What about using unlikely here?
> >>
> > 
> > Unless there is a measurable performance increase by adding in likely/unlikely
> > I'd suggest avoiding it's use. In general, likely/unlikely should only be used
> > for things like catestrophic errors because the penalty for taking the unlikely
> > leg of the code can be quite severe. For normal stuff, where the code nearly
> > always goes one way in the branch but occasionally goes the other, the hardware
> > branch predictors generally do a good enough job.
> 
> Do you mean using likely/unlikely could be worst than not using it
> in this case?
> 
> To me, using unlikely here is not a bad idea: it shows to the compiler
> and to the reader of the code that is case is not the usual case.
>
Hi Olivier,

it might be worse if the user makes a lot of calls with n == 0. It almost
certainly would depend upon the compiler. Overall, I'd rather see us err on the
side of not putting in the calls unless there is a proven case to do so.

I don't think the documentation benefit is huge here either, it's just standard
parameter checking at the start of the function.

/Bruce