From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Monjalon <thomas.monjalon@6wind.com>
Subject: Re: [PATCH v2] ring: check for zero objects mc dequeue /
	mp enqueue
Date: Fri, 18 Mar 2016 11:35:46 +0100
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Cc: Olivier Matz <olivier.matz@6wind.com>, Mauricio =?ISO-8859-1?Q?V=E1squez?=
 <mauricio.vasquezbernal@studenti.polito.it>,
 Lazaros Koromilas <l@nofutznetworks.com>
To: dev@dpdk.org, Bruce Richardson <bruce.richardson@intel.com>
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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 ;)