F41 Change Proposal: LLVM 19 (System-Wide)

LLVM 19

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

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Announced

:link: Summary

Update all llvm sub-projects in Fedora Linux to version 19.

:link: Owner

:link: Detailed Description

All llvm sub-projects in Fedora will be updated to version 19, and there will be a soname version change for the llvm libraries. Compatibility packages clang18, llvm18, lld18, compiler-rt18, and libomp18 will be added to ensure that packages that currently depend on clang and llvm version 18 libraries will continue to work. We may add other compatibility packages too if they’re determined to be necessary to maintain functionality in other RPMS that use llvm/clang. Any compatibility packages we add for Fedora 41 will be retired or orphaned before the Fedora 42 branch date. As stated in the LLVM-18 change proposal, we plan to retire or orphan these older compatibility packages prior to the Fedora 41 branch date:

  • llvm17
  • clang17
  • lld17
  • compiler-rt17
  • libomp17

Other notable changes:

  • Build compat packages (e.g. llvm18) as early as possible. When we package a new major release of llvm, we create a compat package so that packages that aren’t compatible with the new version can still use the old version. In the past, we’ve waited to introduce the compat packages until the new version of LLVM was ready (typically during the Beta Freeze). However, this proved to be an issue this release for packages the were ready to switch to the compat packages early in the release cycle, but then had to wait for Beta freeze.

  • Spec file merge. We plan to retire the clang, compiler-rt, lld and libomp packages and merge them in with llvm and have them be sub-packages of the llvm package. All these packages have their sources in the same upstream git repository and use the same versioning. This change will allow us to use the build configuration recommended by upstream and also make it possible to optimize the packages using Profile-Guided Optimizations (PGO). It’s possible that in future releases (f42+), we may decided to merge more packages in with llvm too.

  • Fat LTO. All RPMS built with clang will default to using the -ffat-lto option. Fat LTO is a feature that allows the compiler to produce libraries that contain LTO bitcode along side the traditional ELF binary code so that the libraries can be linked in both LTO mode and non-LTO mode. gcc also supports this feature and has it enabled in Fedora. In Fedora 40 and older, with LTO enabled, clang produces binaries with only LTO bitcode, so we need to run a post-processing script (brp-llvm-compile-to-elf) on the libraries to convert them to ELF code so they can be used by other packages. Enabling Fat LTO will allow us to remove this script and simplify the build process. We originally proposed this feature for Fedora 40, but it was not ready in time.

:link: Planned Schedule

Our plan is to push 19.1.0-rc3 into Fedora 41 as a Beta Freeze exception. Updates after 19.1.0-rc3 will generally be very small and can be done after the Beta Freeze is over. If we are late packaging releases after 19.1.0-rc3, we will not ask for a Final Freeze exception, unless they contain a fix for a critical release blocking bug.

We are not planning to push 19.1.0-rc1 into rawhide because the library ABI is not stabilized at that point. Typically, the ABI stabilizes after -rc3, but there are no guarantees from upstream about this. Given the history of minimal ABI changes after -rc3, we feel like it’s safe to push -rc3 into rawhide and Fedora 41. The worst case scenario would be an ABI change in -rc4 or the final release that would force us to patch LLVM to maintain compatibility with the -rc3 ABI. This scenario would not require rebuilding LLVM library users in Fedora, so it would merely be a self-contained change to LLVM.

:link: Important Dates

Dates may change depending on circumstances.

  • Jun 4: Build llvm18, clang18, lld18, compiler-rt18, and lld18 compat packages in rawhide.
  • July 26: Begin building LLVM 19.1.0-rc1 in COPR.
  • Aug 6: Begin building LLVM 19.1.0-rc2 in COPR.
  • Aug 6: Fedora f41 branches created.
  • Aug 20: Begin building LLVM 19.1.0-rc3 in Rawhide and f41 side-tags.
  • Aug 20: Fedora f41 Beta Freeze
  • Aug 20-> Sep 10: Request Beta Freeze Exception and push 19.1.0-rc3 into f41 stable.
  • Sep 3: Begin building LLVM 19.1.0-rc4 in Rawhide side-tag.
  • Sep 17: Begin building LLVM 19.1.0 in Rawhide and f41 side-tags.
  • Sep 17 → Oct 1: Push 19.1.0 into f41 stable.
  • Oct 1: Fedora f41 Final Freeze.

:link: Feedback

:link: Benefit to Fedora

New features and bug fixes provided by the latest version of LLVM.

:link: Scope

  • Proposal owners:

    • Review existing llvm and clang compatibility packages and orphan any packages that are no longer used.
    • Build and test early release candidates of LLVM 19 in COPR.
  • Other developers:

    • Fix build issues found with LLVM-19 or switch their package to use the llvm18 compat libs. The LLVM team will not block Bodhi updates on dependent packages that fail to build or run with LLVM-19. There should be around 6-8 weeks between when -rc1 lands in the koji side-tag and the Final Freeze for package maintainers to fix issues uncovered with the LLVM-19 update.
  • Release engineering: [1]

  • Policies and guidelines: N/A (not needed for this Change)

  • Trademark approval: N/A (not needed for this Change)

  • Alignment with the Fedora Strategy:

:link: Upgrade/compatibility impact

This change should not impact upgrades.

:link: Early Testing (Optional)

Do you require ‘QA Blueprint’ support? Y/N

:link: How To Test

The CI tests for the llvm sub-packages in Fedora will be used to catch regressions that might be potentially introduced by the update to LLVM 19.

:link: User Experience

:link: Dependencies

Packages that depend on one of the llvm packages will need to be updated to work with LLVM19 or will need to switch to using one of the llvm18 compat packages.

:link: Contingency Plan

  • Contingency mechanism: (What to do? Who will do it?)

If there are major problems with LLVM 19, the compatibility package provide a way for other packages to continue using LLVM 18.

  • Contingency deadline:Final Freeze
  • Blocks release? No (not a System Wide Change), Yes/No

:link: Documentation

LLVM sub-projects in Fedora have been updated to version 19:

  • llvm (now includes clang, lld, compiler-rt, libomp)
  • lldb
  • llvm-test-suite
  • libcxx
  • python-lit
  • flang
  • mlir
  • polly
  • libclc
  • llvm-bolt

:link: Release Notes

Last edited by @amoloney 2024-05-31T08:50:05Z

How do you feel about the proposal as written?

  • Strongly in favor
  • In favor, with reservations
  • Neutral
  • Opposed, but could be convinced
  • Strongly opposed
0 voters

If you are in favor but have reservations, or are opposed but something could change your mind, please explain in a reply.

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In favor with reservations:
The timing is really tight, as tight as with LLVM 18. In addition, the “hot phase” (rc3 landing in branched during freeze) coincides with summer break in a few regions.
What I “learned” from the LLVM 18 change is to just switch my package to “-compat” and be done, because the time for figuring out the real problem is too short. But that’s really the wrong learnings: We do the integration to find and really fix any problems, not to defer them until later, or until someone else has taken care of them.

Would it work for you to test your package in COPR? We usually have the early -rc builds there and it would give a little more time for you to find issues with the new version.

I know it when it breaks :wink:

Seriously, I think that early testing (earlier than rc3/freeze) makes a lot of sense even though ABI might not be stable yet. This works out very well with python: early mass rebuilds in COPR to alert about possible problems. I don’t know whether it’s possible to do this automatically for all dependencies.
I could imagine early testing, though, e.g. by setting up my own COPR project to depend on your LLVM 19 copr and/or by testing locally in mock with a config referring to your copr.

Let it be also known that during the LLVM 18 change, our Fedora LLVM people were right there supporting everyone with their LLVM related problems during freeze! I just want to make it less hectic for everyone.

When exactly will the llvm17 packages be retired? We’re still waiting for llvm18 support to land in upstream ROCm and need to know if we can wait for the upstream release or if we’ll need to get pre-release bits into rawhide before the llvm17 packages are retired

Ideally, we would retire them before the f41 branch is created, which is Aug 6. If ROCm still needs them at that point, we can postpone the retirement until f42 and the ROCm maintainers can take over ownership of the llvm17 packages in f41.

This change proposal has now been submitted to FESCo with ticket #3220 for voting.

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