Re: Porting to a custom ISA

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Aug 15, 2023 at 1:38 PM MegaIng via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hello everyone,
>
> I am currently in the process of porting gcc to an ISA I designed with a
> few others (spec [1]). Using the old ggx/moxie tutorial as a guideline
> and looking at the source of other backends, as well as the quite decent
> gccint document, I managed to get basic programs running (binutils,
> especially gas, was also not too hard to get running). I am now
> partially writing here because the gccint documents tells me to do this
> (I am unsure if our project will ever be mature enough to be added to
> gcc, but I don't think it hurts to strive for that), and partially
> because I do need a bit of help with some stuff that I can't find
> documentation for.
>
> One of the big challenges I am facing is that for our backend we
> sometimes support 16bit as the max size a CPU supports, including for
> pointers, and sometimes 32bit or 64bit. Am I seeing it correctly that
> POINTER_SIZE has to be a compile time constant and can therefore not
> easily be changed by the backend during compilation based on command
> line arguments?

POINTER_SIZE needs to be only a C expression, not a constant

> Also, on another backend I saw comments relating to libgcc (or newlib?)
> not working that well on systems where int is 16bit. Is that still true,
> and what is the best workaround?

You could look into the AVR port for example.  I think libgcc should
work reasonably well here.

> And a bit more concrete with something I am having a hard time
> debugging. I am getting errors `invalid_void`, seemingly triggered by an
> absent of `gen_return` when compiling with anything higher than -O0. How
> do I correctly provide an implementation for that? Or disable it's
> usage? Our epilogue is non-trivial, and it doesn't look like the other
> backends use something like `(define_insn "return" ...)`.

Somebody else has to answer this.

Richard.

> Many thanks in advance,
>
> MegaIng
>
>
> [1]: https://github.com/ETC-A/etca-spec
>