Speed up freelist by using intrinsics to find last set bit.

Speed up freelist by using intrinsics to find last set bit.

Also add unit test for BitSet.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40057

[Ivan Posva, 2014-09-08 06:58:08]

https://codereview.chromium.org/538213003/diff/20001/runtime/vm/bit_set.h
File runtime/vm/bit_set.h (right):

https://codereview.chromium.org/538213003/diff/20001/runtime/vm/bit_set.h#new...
runtime/vm/bit_set.h:78: } while (--w >= 0);
I find this loop hard to read. In addition it visually bothers me that the
data_[w] value is read twice, once outside and once inside the loop.

[koda, 2014-09-09 20:51:41]

https://codereview.chromium.org/538213003/diff/20001/runtime/vm/bit_set.h
File runtime/vm/bit_set.h (right):

https://codereview.chromium.org/538213003/diff/20001/runtime/vm/bit_set.h#new...
runtime/vm/bit_set.h:78: } while (--w >= 0);
On 2014/09/08 06:58:08, Ivan Posva wrote:
> I find this loop hard to read. In addition it visually bothers me that the
> data_[w] value is read twice, once outside and once inside the loop.

I aimed for consistency with existing code.

Uploaded a new version that separates the read from the write and moves the
return out of the loop; do you find it easier to read?

[Ivan Posva, 2014-09-09 21:07:31]

LGTM. I think the additional comments help too.

-Ivan

[koda, 2014-09-09 21:17:15]

Committed patchset #3 (id:40001) manually as r40057 (presubmit successful).