From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 29 Jan 2021 14:20:09 -0700 From: Nathan Chancellor Subject: Minimum supported version of LLVM Message-ID: <20210129212009.GA2659554@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline List-ID: To: Nick Desaulniers , Sedat Dilek , Miguel Ojeda , Lukas Bulwahn , Kees Cook , Marco Elver , Dmitry Vyukov , Arvind Sankar , Ilie Halip Cc: kernelci@groups.io, clang-built-linux@googlegroups.com 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