Share shaders between browser compositors.

Share shaders between browser compositors.

Each compositor has it's own context. We should share shaders between them because compiling shaders is expensive. To do this, I pin the very first context created with a ref count. Then I use context sharing to share the shaders.

BUG=none
TEST=by hand with intel, nv, nouveau, and tegra drivers on a TOUCH_UI build

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=91853

[jonathan.backer, 2011-07-07 20:03:09]

wjmaclean is on vacation for the next 3 weeks, so I've added apatrick to review
the GL.  I'll still need sky for owners.

I'm about to try using just one context and many surfaces. I'll post a separate
CL if that works.

[apatrick_chromium, 2011-07-07 20:19:41]

Do all GLs support sharing of shaders between contexts in the same share group?
The GLES2 spec says this:

"A.2 Sharing objects across multiple OpenGL ES contexts
This section defines some of the behavior of OpenGL ES objects which can be
shared by multiple contexts.
Objects which can be shared in this manner include:
 textures
 buffer objects
 renderbuffers
 framebuffers
Traditionally, the specification of shared object and multi-context behavior was
left to the windowsystem. However, this section describes the portions of
behavior of shared objects that apply to all OpenGL
ES implementations, regardless of window-system."


It doesn't explicitly mention shaders and they are a core GLES2 feature. It's a
little ambiguous. ANGLE apparently _does_ support sharing programs and shaders.

If it doesn't work though, one way to avoid compiling shaders multiple times
would be to compile it from source on one context, read it back as a compiled
binary and then upload it on the others with glShaderBinary. Looks like that
approach would work here as well. Just mentioning it.

LGTM

[sky, 2011-07-07 20:30:23]

I'm rubber stamping LGTM

[jonathan.backer, 2011-07-08 13:56:49]

On 2011/07/07 20:19:41, apatrick_chromium wrote:
> Do all GLs support sharing of shaders between contexts in the same share
group?

Seems to work on all the driver combos I've tested so far (both GLES and GL).

> If it doesn't work though, one way to avoid compiling shaders multiple times
> would be to compile it from source on one context, read it back as a compiled
> binary and then upload it on the others with glShaderBinary. Looks like that
> approach would work here as well. Just mentioning it.

I investigated. GLES doesn't support binary readback in the core spec. You are
guaranteed to be able to (a) compile form source or (b) load binaries, but not
necessarily both (presumably so that driver vendors can provide an offline
compiler). There is an extension to provide binary readback ---
OES_get_program_binary --- but I don't know how well supported it is.