[Webkit-unassigned] [Bug 98913] Change transformation precision from double to float

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=98913





--- Comment #15 from Gabor Rapcsanyi <rgabor at webkit.org>  2012-10-18 06:38:15 PST ---
(In reply to comment #14)
> (In reply to comment #12)
> > Created an attachment (id=168725)
 --> (https://bugs.webkit.org/attachment.cgi?id=168725&action=review) [details] [details]
> > transform.html
> > 
> > Performance test for transformations.
> 
> Profiling this shows that it spends 12% of its time in cssText(), and about 1.5% in TransformationMatrix::multiply(), on desktop at least. So multiply() is far from being the bottleneck. dtoa() takes more time.

Well I did some more investigation in this area.
I measured the speed and memory gain with this transform.html on a Pandaboard.

As I wrote above there was no big speed differences between the float and double types even on this specific test. I changed the matrix multiplication to NEON code to see the improvement but it wasn't too much.

Speed test:
Double: ~6.03 fps
Float:  ~6.32 fps
NEON:   ~6.6 fps

I also did some memory measurements on this test with Smaps.

libWebCore memory usage:
Float: 14 540 kB
Double:14 580 kB
Difference: 40 kB save

Heap usage:
Float: 59 352 kB
Double:59 792 kB
Difference: 440 kb save

Sum memory usage:
Float: 95 864 kB
Double:96 368 kB
Difference: 504 kb save

As we can see the memory difference is not too much.

Summarize all of these results I think changing to float the doubles is not worth. There weren't too much performance gain even with the neonized matrix multiplication on a specific test. It would only worth because of the memory savings but it's not too much either.

So I'll close this bug with wontfix if there are no objections.

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