Bear with me, because I will raise several questions at once. I still feel, though, that overall this can be treated as one question that may be answered succinctly.

I recently dove into solidifying my understanding of the assembly language, low-level memory operations, CPU structure, and program optimizations. This also sparked my interest in how higher-level shading languages, GLSL and HLSL in particular, are compiled and optimized, as well as what formats they are reduced to before machine code is generated (assuming they are not converted directly into machine code).

After a bit of research into this, the best resource I've found is this presentation from ATI about the compilation of and optimizations for HLSL. I also found sample ARB assembly code.

This sort of addressed my original curiosity, but it raised several other questions. The assembler code in the ATI presentation seems like it contains instructions specifically targeted for the GPU, but is this merely a hypothetical example created for the purpose of conceptual understanding, or is this code really generated during shader compilation? If so, is it possible to inspect it, or even write it in place of the higher-level syntax? My initial searches for an answer to the last question tell me that this may be disallowed, but I have not dug too deep yet.

Also, along the same lines, are GLSL shader programs compiled into ARB assembly code before machine code is generated, and is it possible to write direct ARB assembly?

Lastly, and perhaps what I am most interested in finding out: are there comprehensive resources on shader compilation and low-level GPU code? I have been unable to find any thus far.

I ask simply because I am curious :)