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

[Mauricio Vásquez]

Hi,


On Fri, Mar 18, 2016 at 11:35 AM, Thomas Monjalon <thomas.monjalon at 6wind.com
> wrote:

> 2016-03-18 11:27, Olivier Matz:
> > On 03/18/2016 11:18 AM, Bruce Richardson wrote:
> > >>> +       /* 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.
>
> It would be nice to have a guideline section about likely/unlikely in
> doc/guides/contributing/design.rst
>
> Bruce gave a talk at Dublin about this kind of things.
> I'm sure he could contribute more design guidelines ;)
>

There is a small explanation in the section "Branch Prediction" of
doc/guides/contributing/coding_style.rst, but I do not know if that is
enough to understand when to use them.

I've made a fast check and there are many PMDs that use them to check if
number of packets is zero in the transmission function.