So I'm in agreement with you as a whole. The profile pollution issue is a bigger concern than just requireNonNull; it means the generally useful (and seemingly benign) practice of refactoring common code into a single place can lead to performance regressions. I do hope that's rectified somehow.
sent from my phone On Feb 20, 2015 7:27 AM, "Aleksey Shipilev" <aleksey.shipi...@oracle.com> wrote: > On 02/19/2015 09:36 PM, Vitaly Davidovich wrote: > > The problem is microbenchmarking this may not tell the true story (like > > all microbenchmarks). In this case the explicit test and branch is > > probably perfectly predicted because there's not much code. In a bigger > > application, where the history fell out of the buffer, it'll depend on > > what static prediction is used. Or, this branch will evict another > > useful one from the history buffer. Not saying you're wrong in your > > suggestion to go ahead with this change, but something to keep in mind. > > Noted, however the argument can be made that: > a) that the same thing applies to getClass()-style checks for public > APIs. Once anybody calls the API method with nulls, the getClass() check > would inflate into explicit branch, and then you face the same branch > predictor behavior; > b) the branch misprediction for otherwise stable branch is by > construction transient, and CPUs should re-learn quickly if we keep on > calling the method. If we don't, then the cost of branch mispredict is > not important to begin with. This should be taken with a grain of salt, > because only a selected few (definitely not me, quite probably no one > except HW engineers) know how exactly branch predictors work. > > That said, the reasonable way you can address these (micro)things in > compiler is to intrinsify Object.requireNonNull to resist the profile > pollution and emit the implicit null check unconditionally. But I submit > that class library should not care about low-probability mispredict, and > do the thing that clearly declares intent. The performance cost of > profile pollution, is negligible for our cases. > > -Aleksey. > > >
