Emscripten WebGL inefficiency with glEnable/DisableVertexAttribArray

[juj]

Looking at a bgfx demo in Spector.js shows the two following profiles:

The renderer first disables all attrib arrays, and then re-enables them. These API calls are quite inefficient in web browsers. It would be better to use a bitfield to track which are currently set, and which want to be set, and only do the minimal delta of enable/disable calls to switch from old state to new state. This can improve CPU performance on the order of +5%.

[bkaradzic]

This can improve CPU performance on the order of +5%.

Cool. Send PR.

[juj]

A quick test with removing the redundant calls to glEnableVertexAttribArray and glDisableVertexAttribArray from the generated JS build output code shows a -9.9% reduction in used CPU time (or +11% performance increase).

Before:

After:

(glDisableVertexAttribArray removed to be never called, glEnableVertexAttribArray only called to enable array indices if they haven't been enabled yet)

[bkaradzic]

You still have to call glDisableVertexAttribArray for attributes not used by vertex shader. This type of optimization affects other platforms. Could you check on some other platform other than desktop?

[juj]

You still have to call glDisableVertexAttribArray for attributes not used by vertex shader.

Actually, why? A honest question, I was thinking the same, but cannot find a reason in the GL spec to see why any attrib arrays need to be disabled. The GL spec does not seem to give a reason, technical or behavioral, that I could find. Disabling an attrib array means "I want to use a constant vector value for this attribute instead of sourcing the attribute from a VBO" (i.e. source that attribute via glVertexAttrib*f instead of glVertexAttribPointerfv), it does not mean that that attribute index would be disabled (there is no function to disable attribute indices). Also disabling an attrib array does not unbind the VBO from that attribute index so calling glDisableVertexAttribArray does not contribute to releasing memory either. Extra attributes not consumed by a VS are just ignored.

This type of optimization affects other platforms. Could you check on some other platform other than desktop?

I did not actually modify the engine code, but just the build output of a page from Emscripten that I was given to consult. I don't have the source project to test other platforms.

[bkaradzic]

See: #1515

[juj]

Thanks, very interesting. In such case, using the bitfield approach to keep the enabled set of attribute array indices to a minimum can be used. In the profile numbers above where I removed the glDisableVertexAttribArray calls altogether there was no effect to rendering, so the profiles show an upper limit as to what the performance effect of these api calls is.