On 3/22/2016 6:13 PM, Richardson, Bruce wrote: > On Mon, Mar 21, 2016 at 05:47:44PM +0000, Xie, Huawei wrote: >> On 3/18/2016 10:17 PM, Bruce Richardson wrote: >>> 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. >> IMO we use likely/unlikely in two cases, catastrophic errors and the >> code nearly always goes one way, i.e, preferred/favored fast path. >> Likely/unlikely helps not only for branch predication but also for cache > For branch prediction, anything after the first time through the code path > the prediction will be based on what happened before rather than any static > hints in the code.
Yes, maybe i didn't make myself clear? My main concern isn't about branch predication... >> usage. The code generated for the likely path will directly follow the >> branch instruction. To me, it is reasonable enough to add unlikely for n >> == 0, which we don't expect to happen. >> I remember with/without likely, compiler could generate three kind of >> instructions. Didn't deep dive into it. >> >>> /Bruce >>>
