performance - OpenGL core profile incredible slowdown on OS X

Question

I added a new GL renderer to my engine, which uses the core profile. While it runs fine on Windows and/or nvidia cards, it is like 10 times slower on OS X (3 fps instead of 30). The weird thing is, that my compatibility profile renderer runs fine.

I collected some traces with Instruments and the GL profiler:

https://www.dropbox.com/sh/311fg9wu0zrarzm/31CGvUcf2q

It shows that the application spends its time in glDrawRangeElements. I tried the following things:

• use glDrawElements instead (no effect)
• flip culling (no effect on speed)
• disable some GL_DYNAMIC_DRAW buffers (no effect)
• bind index buffer after VAO when drawing (no effect)
• converted indices to 4 byte (no effect)
• use GL_BGRA textures (no effect)

What I didn't try is to align my vertices to 16 byte boundary and/or convert indices to 4 byte, but seriously, if that would be the issue then why the hell does the standard allow it?

I'm creating the context like this:

NSOpenGLPixelFormatAttribute attributes[] =
{
    NSOpenGLPFAColorSize, 24,
    NSOpenGLPFAAlphaSize, 8,
    NSOpenGLPFADepthSize, 24,
    NSOpenGLPFAStencilSize, 8,
    NSOpenGLPFADoubleBuffer,
    NSOpenGLPFAAccelerated,
    NSOpenGLPFANoRecovery,
    NSOpenGLPFAOpenGLProfile, NSOpenGLProfileVersion3_2Core,
    0
};

NSOpenGLPixelFormat* format = [[NSOpenGLPixelFormat alloc] initWithAttributes:attributes];
NSOpenGLContext* context = [[NSOpenGLContext alloc] initWithFormat:format shareContext:nil];

[self.view setOpenGLContext:context];
[context makeCurrentContext];

Tried on the following specs:

• radeon 6630M, OS X 10.7.5
• radeon 6750M, OS X 10.7.5
• geforce GT 330M, OS X 10.8.3

Do you have any ideas what I might do wrong? Again, it works fine with the compatibility profile (not using VAOs though).

UPDATE: reported to Apple.

UPDATE: Apple doesn't give a damn to the problem...anyway I created a small test program which is actually good. Now I compared the call stack with Instruments, and found out that when using the engine, glDrawRangeElements does two calls:

• gleDrawArraysOrElements_ExecCore
• gleDrawArraysOrElements_Entries_Body

while in the test program it calls only the second. Now the first call does something like an immediate mode render (gleFlushPrimitivesTCLFunc, gleRunVertexSubmitterImmediate), so obviously casues the slowdown.

Answer 1

Finally, I was able to reproduce the slowdown. This is just crazy... It is clearly caused by glBindAttribLocation being called on the "my_Position" attribute. Now I did some testing:

• 1 is default (as returned by glGetAttribLocation)
• if I set it to zero, theres no problem
• if I set it to 1, the rendering becomes slow
• if I set it to any larger number, it is slow again

Obviously I relink the program (check code). It is not a problem in the implementation, I tested it with "normal" values too.

Test program:

https://www.dropbox.com/s/dgg48g1fwgyc5h0/SLOWDOWN_REPRO.zip

How to repro:

• open with XCode
• open common/glext.h (don't be disturbed by the name)
• modify the GLDECLUSAGE_POSITION constant from 0 to 1
• compile and run => slow
• changing back to zero => good