[MatthiasPortzel]

I understand your point, but LLVM-MOS is a bad example. You gain LLVM’s language optimizations, as you point out. But LLVM’s assumed architecture is so different from the 6502 that lowering the code to assembly introduces many superfluous instructions. (As an example, the 6502 has one general purpose register, but LLVM works best with many registers. So LLVM-MOS creates 16 virtual registers in the first page of memory and then generates instructions to move them into the main register as they are used.) It’s of course possible to further optimize this, but the LLVM-MOS project isn’t that mature yet. So assembly programmers can still very much do better.

[chongli]

So LLVM-MOS creates 16 virtual registers in the first page of memory and then generates instructions to move them into the main register as they are used.

Isn’t this actually good practice on the 6502? The processor treats the first page of memory (called the zero page) differently. Instructions that address the zero page are shorter because they leave out the most significant byte. Addressing any other page requires that extra byte for the MSB.

Furthermore, instructions which accept a zero page address typically complete one cycle faster than absolute addressed instructions, and typically only one cycle slower than immediate addressed instructions.

So if you can keep as much of your memory accesses within the zero page as possible, your code will run a lot faster. It would seem to me that treating the zero page as a table of virtual registers is a great way to do that because you can bring all your register colouring machinery to bear on the problem.

[tredre3]

I understand your point but the beginning of the zero page is almost always used as virtual registers by regular hand-rolled 6502 applications. So it's pretty normal for LLVM to do the same, it's not an example of LLVM doing something weird.