From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bruce Richardson <bruce.richardson@intel.com>
Subject: Re: [PATCH v2] ring: check for zero objects mc dequeue /
 mp enqueue
Date: Fri, 18 Mar 2016 14:16:32 +0000
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Cc: Thomas Monjalon <thomas.monjalon@6wind.com>, dev@dpdk.org,
 Olivier Matz <olivier.matz@6wind.com>,
 Lazaros Koromilas <l@nofutznetworks.com>
To: Mauricio =?iso-8859-1?Q?V=E1squez?=
 <mauricio.vasquezbernal@studenti.polito.it>
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On Fri, Mar 18, 2016 at 01:47:29PM +0100, Mauricio V=E1squez wrote:
> Hi,
>=20
>=20
> On Fri, Mar 18, 2016 at 11:35 AM, Thomas Monjalon <thomas.monjalon@6win=
d.com
> > wrote:
>=20
> > 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 =3D=3D 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 takin=
g 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 compi=
ler
> > > 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 ;)
> >
>=20
> 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.
>=20
> I've made a fast check and there are many PMDs that use them to check i=
f
> 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 with=
out
the extra macros.

/Bruce