Weekly report 6, LLVM libc

Hi! This week I have been working on LLVM/Clang support for
Crossdev. This is currently done by swapping out the different Crossdev
stages for ones that make sense for LLVM.

Currently it replaces stage0 with checking whether LLVM can target the
target triple’s architecture by checking the LLVM_TARGETS USE-flag.

Stage1, which normally installs libc headers and compiles a -stage1 C
compiler is replaced by installing libc headers and compiling

Stage2 (kernel headers), is the same.

Stage3 (libc install), is the same.

Stage4, which compiles a full compiler is skipped completely.

Another needed change was to make the compiler-rt ebuild cross-aware,
with changes like making the assembler and C compiler target the target triple, and
including headers from the crossdev /usr/${CTARGET}/usr/include
directory instead of using the host’s libc headers. I got some help from
wikky here, thanks!

Currently doing ‘crossdev –llvm -t riscv64-gentoo-linux-musl &&
riscv64-gentoo-linux-musl-emerge dash’ produces a working binary that
can be run using qemu-user like this ‘qemu-riscv64 -L
/usr/riscv64-gentoo-linux-musl /usr/riscv64-gentoo-linux-musl/bin/dash’
which I think is very cool! However, there is still some issues with
dynamically linking libraries built with the cross compiler. For example
xz-utils installs liblzma.so which fails with “exec format
error”. (http://sprunge.us/HkSmms). I am currently looking into that.

Another thing I’m still a little uncertain about is where to put all
environment variables and compilation options, whether it’s crossdev’s
job or the ebuilds’. This is something I will come back to, and I have
some changes locally on my computer
Crossdev patches:
Compiler-rt patches:

Later during the week I made ebuilds for LLVM libc and
libc-hdrgen (generates LLVM libc headers from TableGen specification
files). Normally you build LLVM libc together with libc-hdrgen, but when
cross compiling it’s a better idea to split these and keep libc-hdrgen a
tool installed on the build host. I have played with only building
libc headers for bootstrapping with crossdev but I haven’t figured it all out yet.
To only install headers you can use the install-libc-headers target, but
it seems like CMake still wants to build things. There’s also the scudo
allocator that needs to be statically linked to LLVM libc. My idea is to
make a USE=static-scudo flag for compiler-rt that gets set in crossdev
when compiling compiler-rt for a LLVM libc target.
These are also kept locally until I’ve figured out how to cross compile
in stages.

Many “small random issues” and technicalities have also poped up during
this week that’ve taken quite a long time, but are not really worth
digging into here.

Next week I will continue with this until I can use it to work on LLVM
libc, worst case scenario I could temporarily make a “franken LLVM libc ebuild” that
does everything (headers, compiler-rt, scudo, llvm libc) in one shot,
but it should definitely be possible to do it separately.

I also forgot to update my llvm-common changes with the new
elisp-site-file-install function that was inspired by my PR 🙁
… will fix that tomorrow.

Thanks for reading!

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