On Fri, Mar 18, 2016 at 01:47:29PM +0100, Mauricio V?squez wrote: > 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.
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
