On Fri, Mar 18, 2016 at 11:27:18AM +0100, Olivier Matz wrote: > Hi, > > On 03/18/2016 11:18 AM, Bruce Richardson wrote: > >>> diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h > >>> index 943c97c..eb45e41 100644 > >>> --- a/lib/librte_ring/rte_ring.h > >>> +++ b/lib/librte_ring/rte_ring.h > >>> @@ -431,6 +431,11 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * > >>> const *obj_table, > >>> uint32_t mask = r->prod.mask; > >>> int ret; > >>> > >>> + /* 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. > Hi Olivier,
it might be worse if the user makes a lot of calls with n == 0. It almost certainly would depend upon the compiler. Overall, I'd rather see us err on the side of not putting in the calls unless there is a proven case to do so. I don't think the documentation benefit is huge here either, it's just standard parameter checking at the start of the function. /Bruce
