perf: improvements to $q

[jeffbcross]

This is a meta issue to track performance improvements to be made in $q for 1.3.0.

The main problems to be solved within the scope of this issue:

• Reduction of deferred instance memory to reduce GC pressure. One quick win is to move instance methods to prototype instead of created inside constructor.
• Some learnings should be taken from bluebird, which has a very fast promise implementation.

Related issues:

• $q is probably due for a rewrite #5223 (suggests rewriting $q, mentions memory pressure)
• Decouple $q in AngularJS 2.0 (and/or switch to bluebird) #6697 (focused on decoupling promise lib in Angular)

There is a PR from @caitp which updates A+ compliance, adds ES6-like constructor API, and begins to experiment with porting bluebird to $q (but with many tests failing): #8067. I've suggested simplifying that PR to not include perf improvements.

Benchmarks:

• JSPerf comparing $q, Native Promise, and Bluebird Promise
• JSPerf comparing many promise libraries, not including $q

[caitp]

Many of those failing tests have already been addressed, shouldn't take too long to have the rest of them cleaned up

[caitp]

Well, using your first jsperf, the new angular implementation is about half the speed of prior angular, although it should have a much smaller memory footprint.

Potential reasons include having to walk higher up the prototype chain to access common methods, Object.defineProperty being used to hide private methods and properties, and calling a promises base class. Might also be calling nextTick() more times than is really necessary.

Interestingly, in FF Nightly (33.0a1 (2014-07-04)), angular's implementation beats bluebird, by quite a bit.

I'm not sure if the benchmarks necessarily measure the right thing, though, because most likely what this is really measuring is our way of deferring to the next turn, which in angular means $rootScope.$evalAsync, which is not necessarily fast.

[caitp]

Without using defineProperties() on the instances of the class, it performs about 5 times faster than angular 1.2.20's $q. This means that each property is enumerable, which is not that big of a deal I guess.

http://jsperf.com/angular-q-vs-bluebird2/2

[jeffbcross]

I think the performance gain is worth not using defineProperties.

[caitp]

yeah I agree

[jeffbcross]

Plus we can save some bytes!