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.
