iPhone shader profiling

Question

I'm using a series of shaders to perform realtime image processing on the iPhone (3GS/4/iPad). The fps isn't what I'd like it to be.

Are there any tools that I can use to help me work out what the bottlenecks are?

Answer 1

I assume you already know that performance tests on the Simulator are worthless and that you're testing on real metal, so Instruments is always a good place to start - specifically in your case you'd be interested in the OpenGL ES and OpenGL ES Analyzer instruments.

Generally speaking for GLSL, there's a list of common GLSL mistakes at the OpenGL.org site. The O'Reilly labs "iPhone 3D Programming" book has some further hints, such as avoiding expensive operations in conditionals, and watching for texture lookups.

Also, it's going to depend on what kind of image processing you're doing; if you're trying to apply heavy Photoshop-esqe filters that would give a quad-core pause to render, it's going to be costly on a lowly phone.

Answer 2

The only currently available tool is the PVRUniSCo editor, which will give you a cycle count for each line of code in your shader (though only on Windows, it seems).