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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

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 ;)