https://ekaitz.elenq.tech/bootstrapGcc0.html

Home Series
---------------------------------------------------------------------
Feed About

Ekaitz's tech blog:
I make stuff at ElenQ Technology and I talk about it

Intro to GCC bootstrap in RISC-V

Mon 14 February 2022
By Ekaitz Zarraga

You probably already know about how I spent more than a year having
fun with RISC-V and software bootstrapping from source.

As some may know from my FOSDEM talk, NLNet / NGI-Assure put the
funds to make me spend more time on this for this year and I decided
to work on GCC's bootstrapping process for RISC-V.

Why GCC

GCC is probably the most used compiler collection, period. With GCC
we can compile the world and have a proper distribution directly from
source, but who compiles the compiler?^1

Well, someone has to.

The bootstrap

Bootstrapping a compiler with a long history like GCC for a new
architecture like RISC-V involves some complications, starting on the
fact that the first version of GCC that supports RISC-V needs a C++98
capable compiler in order to build. C++98 is a really complex
standard, so there's no way we can bootstrap a C++98 compiler at the
moment for RISC-V. The easiest way we can think of at this point is
to use an older version of GCC for that, one of those that are able
to build C++98 programs but they only require a C compiler to build.
Older versions of GCC, of course, don't have RISC-V support so... We
need a backport^2.

So that's what I'm doing right now. I'm taking an old version of GCC
that only depends on C89 and is able to compile C++98 code and I'm
porting it to RISC-V so we can build newer GCCs with it.

Only needing C to compile it's a huge improvement because there are
Tiny C Compilers out there that can compile C to RISC-V, and those
are written using simple C that we can bootstrap with simpler tools
of a more civilized world.

In summary:

  * C++98 is too complex, but C89 is fine.
  * GCC is the problem and also the solution.

What about GNU Mes?

When we^3 started with this effort we wanted to prepare GNU Mes, a
small C compiler that is able to compile a Tiny C Compiler, to work
with RISC-V so we could start to work in this bootstrap process from
the bottom.

Some random events, like someone else working on that part, made us
rethink our strategy so we decided to start from the top and try to
combine both efforts at the end. We share the same goal: full source
bootstrap for RISC-V.

Tiny C Compilers?

There are many small C compilers out there that are written in simple
C and are able to compile an old GCC that is written in C. Our
favorite is TinyCC (Tiny C Compiler).

GNU Mes is able to build a patched version of TinyCC, which already
supports RISC-V (RV64 only), and we can use that TinyCC to compile
the GCC version I'm backporting.

We'd probably need to patch some things in both projects to make
everything work smoothly but that's also included in the
project plan.

Binutils

Binutils is also a problem mostly because GCC, as we will talk about
in the future, does not compile to binary directly. GCC generates
assembly code and coordinates calls to as and ld (the GNU Assembler
and Linker) to generate the final binaries. Thankfully, TinyCC can
act as an assembler and a linker, and there's also the chance to
compile a modern binutils version because it is written in C.

In any case, the binary file generation and support must be taken in
account, because GCC is not the only actor in this film and RISC-V
has some weird things on the assembly and the binaries that have to
be supported correctly.

Conclusion

This is a very interesting project, where I need to dig in BIG stuff,
which is cool, but also has a huge level of uncertainty, which scares
the hell out of me. I hope everything goes well...

In any case, I'll share all I learn here in the blog and I keep you
all posted with the news we have.

That's all for this time. If you have any question or comment or want
to share your thoughts and feelings with me^5 you can find my contact
information here.

---------------------------------------------------------------------


    PS: Big up to NlNet / NGI-Assure for the money.

[nlnet] [NGIAssure]
---------------------------------------------------------------------

 1. wHo wATcHes tHE wAtchMEN? -

 2. Insert "Back to the Future" music here. -

 3. "We" means I shared my thoughts and plans with other people who
    have a much better understanding of this than myself. -

 4. But there are some others that are really interesting (see cproc,
    for example) -

 5. Or even hire me for some freelance IT stuff  -


Supported by: