Issue 4071 - Speed up ElemHide.getSelectorsForDomain for domains with no filters

Issue 4071 - Speed up ElemHide.getSelectorsForDomain for domains with no filters

[kzar, 2016-05-25 17:56:53]

Patch Set 1

Very rough so far, also not 100% convinced this idea makes sense. But if it does
it could make the common case of domains with no exceptions quite a bit faster.

[kzar, 2016-05-25 18:10:54]

As discussed in IRC this change sped up my test from 5601ms to 2114ms with
Chrome 50.

(function() {
  var start = performance.now();
  for (var i = 0; i < 1000; i += 1)
    ElemHide.getSelectorsForDomain("blersdfdsfd.com");
  return performance.now() - start;
}());

[Sebastian Noack, 2016-05-26 11:46:44]

https://codereview.adblockplus.org/29344551/diff/29344552/lib/elemHide.js
File lib/elemHide.js (right):

https://codereview.adblockplus.org/29344551/diff/29344552/lib/elemHide.js#new...
lib/elemHide.js:289: else if (count)
It seems the condition to check for non-zero count is redundant. Then you also
don't need the count variable.

However, I wonder whether we should delete properties from
exceptionCountByDomain anyway. It seems we don't do that for filtersByDomain
either.

https://codereview.adblockplus.org/29344551/diff/29344552/lib/elemHide.js#new...
lib/elemHide.js:556: if (filter)
Just for my understanding, what is the scenario when "filter" evaluates to false
here?

https://codereview.adblockplus.org/29344551/diff/29344552/lib/elemHide.js#new...
lib/elemHide.js:557: selectors.push(filters[filterKey].selector);
Use the "filter" variable here?

https://codereview.adblockplus.org/29344551/diff/29344552/lib/elemHide.js#new...
lib/elemHide.js:562: anyExceptions = anyExceptions ||
exceptionCountByDomain[currentDomain];
I think it's preferable to write it like this:

 if (exceptionCountByDomain[currentDomain])
    anyExceptions = true;

If for nothing else, to not change the type of the "anyExceptions" variable.