PulkoMandy

I'm making some progress getting the vbcc compiler to generate code for the #vtech #vsmile. However, the CPU has only 4 general purpose registers (R1-R4) with the other ones (SP, PC, SR/flags and BP/framepointer) already in useby the cpu itself or the code generator.

The register allocation in vbcc endsup allocating all 4 registers at the start of a function and freeing them at the end, despite not actually using them that much. So all code that needs a temporary register has to push/pop one

Jun 2, 2024 at 12:03pmWeb
Show threadPulkoMandyJun 2, 2024

This works ok, until th.point where a PUSH operation (to push a function call parameter on the stack) tries to use a temporary register, and emits an extra push/pop that results in the wrong parameter value being passed.

Of course I could reserve one extra register permanently for use by the compiler, but that would make the code generation worse everywhere else (since there are already few available registers).

Show threadPulkoMandyJun 2, 2024
Maybe I can write an optimization pass that tries to propagate the register allocations/frees closer to where they are acually used? But I'm not sure how easy that is, since the code has various branches.
Maybe I can find some clever use of the instruction set, or allocate myself some scratch area in ram at a fixed address to backup my temporary register in this case?
Show threadLudovic :Firefox: :FreeBSD:Jun 2, 2024
@pulkomandy one more than a 6502