Reduce loop size in SkTileGridNextDatum

We don't have a benchmark in Skia, but this seems to give nearly 2x speedup in Chrome for the larger queries that dominate the distribution. That should translate into a 5% CPU savings for the compositor.

From https://code.google.com/p/skia/issues/detail?id=1212, selected costs without this patch:

queryTileCount
   #calls
       totalTime
                averageTime
 1 448  0.934ms   2us
 2 187  0.584ms   3us
 4  18  0.071ms   3us
12   4  0.099ms  24us
24  20  1.039ms  52us
30  44  3.093ms  70us
42  27  3.251ms 120us
84  82 29.138ms 355us

Selected costs with this patch:
 1 408  0.505ms   1us
 2 172  0.549ms   3us
12  27  0.371ms  13us
20  30  0.966ms  32us
30  41  1.745ms  42us
84  96 19.808ms 206us

[TomH, 2013-04-05 13:53:57]

PTAL.

[junov1, 2013-04-05 14:08:08]

Great simple optimization!
lgtm with minor correction.

https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h
File src/core/SkTileGrid.h (right):

https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h#newcode108
src/core/SkTileGrid.h:108: } else if (!((*minVal) < (*candidate))) {
I think what you want here is "else if (*minVal == *candidate)"

[TomH, 2013-04-05 14:10:17]

On 2013/04/05 14:08:08, junov1 wrote:
> Great simple optimization!
> lgtm with minor correction.
> 
> https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h
> File src/core/SkTileGrid.h (right):
> 
> https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h#newcode108
> src/core/SkTileGrid.h:108: } else if (!((*minVal) < (*candidate))) {
> I think what you want here is "else if (*minVal == *candidate)"

SkPictureStateTree::Draw defines operator< but not operator==.

if !(A<B) && !(B<A), B==A; we don't need to require the additional operator.

[junov1, 2013-04-05 14:13:11]

On 2013/04/05 14:10:17, TomH wrote:
> On 2013/04/05 14:08:08, junov1 wrote:
> > Great simple optimization!
> > lgtm with minor correction.
> > 
> > https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h
> > File src/core/SkTileGrid.h (right):
> > 
> >
https://codereview.appspot.com/8315044/diff/1/src/core/SkTileGrid.h#newcode108
> > src/core/SkTileGrid.h:108: } else if (!((*minVal) < (*candidate))) {
> > I think what you want here is "else if (*minVal == *candidate)"
> 
> SkPictureStateTree::Draw defines operator< but not operator==.
> 
> if !(A<B) && !(B<A), B==A; we don't need to require the additional operator.

Oh yeah. So to compensate for the poor readability, a comment would be nice.

[TomH, 2013-04-05 14:22:01]

On 2013/04/05 14:13:11, junov1 wrote:
> Oh yeah. So to compensate for the poor readability, a comment would be nice.

Touche! Commented and landed as r8543.