Weekly report 1, LLVM libc

Hey! I had to start GSoC on sunday last week due to school, and I didn’t
think that I’d write a weekly report for the first week but I decided to
do it anyways.

My plan for week 1 was:
>This week I will set up a LLVM toolchain and sysroot for compiling
>programs targeting LLVM libc. I will also start setting up a
>“llvm-libc/Linux from Scratch” chroot.

Because I played with LLVM libc before last week I had already completed this
goal. Going forward I will only work in the sysroot until setting up
crossdev because it’s simple and gives me everything I need to fix
dependencies like Python.

This far the project has been going pretty smooth, but I’ve also ran
into some issues which I will comment on.

The first issue was regarding SSP when setting up the LLVM
toolchain. LLVM libc currently does not support stack smashing
protection, and somehow compiled binaries automatically wants it if the
toolchain was built with SSP enabled, even when compiling with
-fno-stack-protector. Probably this has something to do with internal
libraries getting built with it. I spent quite a bit of time on this
because I forgot CXXFLAGS was a thing and only set CFLAGS, thinking that
something else was causing it :). Why this works out of the box on some
other distributions, hence not in setup docs, is because Gentoo enables
SSP in the clang config files.

(See:
https://blogs.gentoo.org/mgorny/2022/10/07/clang-in-gentoo-now-sets-default-runtimes-via-config-file/)

I then moved on to work on Python. When I talked with LLVM libc
developers about this project they told me that the biggest obstacle for
Python would be the missing libm functions, so I decided to use Julia’s
openlibm instead of the built in one.

> I’d guess that python wouldn’t quite work yet since we don’t have all
> of the double precision math functions yet, though you might be able
> to fudge it by creating entrypoints that just call the single
> precision versions.

Openlibm just compiled and worked
out of the box with Python by configuring with
–with-libm=*libopenlibm.a*, and then substituting math.h for
openlibm_math.h.

Next week I will mostly work on Python, because libm isn’t the only
issue. Things like fileno, wide strings, pthread_cond are used in Python
and is not in LLVM libc yet. posixsource.c is particularly annoying.
Yes, I have a time machine.

—
catcream