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 01:47:29PM +0100, Mauricio Vásquez wrote:
> Hi,
> 
> 
> On Fri, Mar 18, 2016 at 11:35 AM, Thomas Monjalon <thomas.monjalon@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.

Yeah, and I wonder how many of those are actually necessary too :-)

It's not a big deal either way, I just think the patch is fine as-is without
the extra macros.

/Bruce