Re: [Tinycc-devel] Anyone else working with the RISC-V port?

[Michael Matz]

Hello,

On Sat, 19 Dec 2020, Charles Lohr wrote:

> I am interested in doing a project using TinyCC, a web-based IDE for the
> ESP32-C3, a wifi-enabled microcontroller. 
> https://github.com/cnlohr/espwebc3/blob/main/README.md
>
> I spent a while trying to decipher riscv-gen, and it's actually not that
> bad.  Very much inline with the cryptic TinyCC (ok, bit of a jab, but it's
> not that bad).  It currently only supports RV64, vs the chip I'll be using
> supports RV32-IMC.  Conveniently, most of the instructions implemented in
> riscv-gen are the 32-bit instructions, only a handful of RV64 instructions,
> which shouldn't be too difficult to avoid.

Right, that should be relatively easy.  One thing to look out for will 
probably be the fact that you then need to use the generic code in tcc for 
open-coding 64bit support (e.g. arithmetic on long long), i.e. 
lexpand/lbuild/gen_opl, which currently is only tested on i386 and arm32. 
Look for uses of PTR_SIZE in tccgen.c.

You also need to adjust the linker/object-file-writer to use ELF32 on 
riscv; that's all supported already of course, but needs the right 
conditionals and some additional defines/codes for the 32bit relocations 
(where they really differ from the ELF64 ones).

> Is anyone else working in this area? Has anyone expressed any possibility at
> adding a few more comments to the riscv-gen code?

Well, what can I say.  Sure, comments might be nice, if they clarify 
things :)

> Are there any landmines I may run into?

Not that I know of, it's all fairly straight forward.  I have tested the 
riscv64 backend only relatively lightly, and am not sure if others have 
played much with it, so it's quite possible you run into genuine bugs 
therein.

> How eager would people be to add a -m32 flag for RISC-V?

I'd suggest going the same way as i386/x86-64, i.e. create a riscv32-tcc 
compiler, which can then be called from tcc when -m32 is used.  Supporting 
both bit-width in the same executable will run into major refactoring work 
as several types are hard-coded (e.g. the whole ELF support isn't 
conditionalized at runtime per word width).


Ciao,
Michael.