iOS audio acceleration

Question

Is anybody using OpenGLES2.0 shaders (GLSL) successfully for audio synthesis?

I already use vDSP to accelerate audio in my iOS app, which provides a simple vector instruction set from C code. The main problem with vDSP is that you have to write what amounts to vector oriented assembly language, because the main per-sample loop gets pushed down into each primitive operation (vector add, vector multiply). Compiling expressions into these sequences is the essence of what shader languages automate for you. OpenCL is not public in iOS. It is also interesting that GLSL is compiled at runtime, which means that if most of the sound engine could be in GLSL, then users could make non-trivial patch contributions.

Answer 1

Although the iOS GPU shaders can be relatively "fast", the paths to load and recover data (textures, processed pixels, etc.) from the GPU are slow enough to more than offset any current shader computational efficiencies from using GLSL.

For real-time synthesis, the latencies of the GPU pixel unload path are much larger than the best possible audio response latency using just CPU synthesis to feed RemoteIO. e.g. display frame rates (to which the GPU pipeline is locked) are slower than optimal RemoteIO callback rates. There's just not enough parallelism to exploit within these short audio buffers.