934796 - Gallery: border should be drawn by separate draw operations, not branching in fragment shader

Status: RESOLVED DUPLICATE of bug 932919

(Firefox OS Graveyard :: Gaia::Gallery, defect)

Description

[Benoit Jacob [:bjacob] (mostly away)]

Before someone says "but I tested on my device, and it didn't make a speed difference":

  1) there is considerable variation between different GPUs, of how much this matters. Basically, this is going to matter very little on desktop GPUs, a little bit on Adrenos, and hugely on PowerVR. See e.g. this branching-in-fragment-shader benchmark
http://people.mozilla.org/~bjacob/webglbranchingbenchmark/webglbranchingbenchmark.html
and some results here on various devices:
http://people.mozilla.org/~bjacob/webglbranchingbenchmark/webglbranchingbenchmarkresults.txt
See how some mild branching, even based on just uniforms, made a fragment shaders run dozens of times slower on PowerVR. (And we do care about performance specifically on PowerVR, for 1.3).

  2) There is other low-hanging fruit in this fragment shader, and on a given GPU we might have to fix that too before we reap the real benefits.

So, instead of drawing the border by branching in this fragment shader, you want to have a separate simple fragment shader for the border, and render the border with separate WebGL draw operations.

Comment 1

[Benoit Jacob [:bjacob] (mostly away)]

Bug 932919 already exists to remove the border feature altogether, and I hear that it is being planned for 1.3. Duping.