Minimum supported version of LLVM

[Nathan Chancellor <nathan@kernel.org>]

Hi all,

I am starting this thread to spur some discussion about our support
model for the minimum supported version of LLVM and how that might
impact bumping the minimum supported version of LLVM/Clang in the
future. I am adding what I hope are relevant parties (individual testers
and maintainers/lists of CI systems). Feel free to add anyone that I
might have missed.

Some history:

For the longest time, ClangBuiltLinux as a project rejected the idea of
a minimum supported version as we did not have as many resources for
testing or a good idea of "what works" and "what doesn't work". After
Linux Plumbers 2020, we decided that we would support from clang 10.0.1
on, which resulted in commit 1f7a44f63e6c ("compiler-clang: add build
check for clang 10.0.1").

The problem:

I would say there are three different levels of support:

1. CC=clang: Compiling with clang, GNU binutils for everything else
2. LLVM=1: Compiling with clang, LLVM binutils, GNU as
3. LLVM=1 LLVM_IAS=1: Compiling with clang, LLVM binutils, and LLVM's
   integrated assembler.

We have started to address these different levels of support in the LLVM
documentation in the kernel:

https://lore.kernel.org/linux-doc/20210114003447.7363-1-natechancellor@gmail.com/

The issue that has been coming up more recently is whether or not the
minimum supported version of clang (the first tier of support) equals
the minimum supported version of LLVM (the second and third tier of
support) when it comes to inserting workarounds or disabling configs
that are known broken.

Some considerations:

1. LLVM 10.0.1 has a few issues that are not present in LLVM 11.x that
   will or have required workarounds in the kernel:

   * https://github.com/ClangBuiltLinux/linux/issues/732
   * https://github.com/ClangBuiltLinux/linux/issues/1187

2. There are still outstanding issues with LLVM utilities that prevent
   recommending LLVM=1 for some architectures so committing to a minimum
   supported version might mean we would need to insert more workarounds
   as time goes on. For example:

   * ld.lld for MIPS big-endian:
     https://github.com/ClangBuiltLinux/linux/issues/1025
   * ld.lld and llvm-objdump for PowerPC64 big-endian:
     https://github.com/ClangBuiltLinux/linux/issues/602
     https://github.com/ClangBuiltLinux/linux/issues/666
   * ld.lld for PowerPC64 little-endian:
     https://github.com/ClangBuiltLinux/linux/issues/811
   * ld.lld for RISC-V:
     https://github.com/ClangBuiltLinux/linux/issues/1020

Questions to address:

I believe the first place to start is answering the question is there a
minimum supported version of LLVM? Is it LLVM as a whole or one
particular utility (such as ld.lld)?

If there is a minimum supported version of LLVM or ld.lld, is it the
same as clang?

If there is not a minimum supported version of LLVM, do we insert
workarounds for issues that we know are fixed with newer versions of
LLVM or do we just tell people to upgrade the version of LLVM they are
using?

If we decide that it is worth defining a minimum supported version of
LLVM, I would vote that it be LLVM 11.0.0. If we decide that that
version should stay in sync with the supported clang version, then I
would vote that we update that for Linux 5.12. Taking a look across
various distributions:

archlinux/base:latest: clang version 11.0.1
debian:stable-slim: clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)
debian:testing-slim: Debian clang version 11.0.1-2
debian:unstable-slim: Debian clang version 11.0.1-2
fedora:latest: clang version 11.0.0 (Fedora 11.0.0-2.fc33)
fedora:rawhide: clang version 11.1.0 (Fedora 11.1.0-0.3.rc2.fc34)
opensuse/leap:latest: clang version 9.0.1
opensuse/tumbleweed:latest: clang version 11.0.1
ubuntu:18.04: clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
ubuntu:20.04: clang version 10.0.0-4ubuntu1
ubuntu:latest: clang version 10.0.0-4ubuntu1
ubuntu:rolling: Ubuntu clang version 11.0.0-2
ubuntu:devel: Ubuntu clang version 11.0.1-2

I am not so concerned for Ubuntu and Debian, as there are updated
versions of LLVM available from apt.llvm.org and OpenSUSE Leap is
already unable to build the upstream kernel with its version of
clang. Everyone else should be unaffected by that bump.

Hopefully all of that makes sense and spurs some good discussion.

Cheers,
Nathan