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

From: Michael Matz
Subject: Re: [Tinycc-devel] Anyone else working with the RISC-V port?
Date: Sun, 20 Dec 2020 23:40:54 +0100 (CET)
User-agent: Alpine 2.21 (LSU 202 2017-01-01)


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. 

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).


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