Changes/LLVM-19

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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.

Summary

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

Owner

• Name: Tom Stellard
• Email: <tstellar@redhat.com>

Current status

• Targeted release: <VERSION>/ Fedora Linux <VERSION>
• Last updated: 2024-05-04
• [Announced]
• [<will be assigned by the Wrangler> Discussion thread]
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

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 merge the clang, compiler-rt, and libomp packages in with llvm and have them be sub-packages of the llvm package. This 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).

• Switch to python-style compat/main packages. In order to make the packaging more consistent between the main package (e.g. llvm) and the compat package (e.g. llvm18), we would retire the un-versioned dist-git for llvm, and create a new versioned dist-git for each new release (e.g. llvm19, llvm20, llvm21 etc.). We would then designate one of these as the 'main version', and that version would produce binary rpms that look like the current main package (i.e. llvm-libs instead of llvm19-libs).

• 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.

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.

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.

Benefit to Fedora

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

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 18 in COPR.

• Other developers:
  • Fix build issues found with LLVM-18 or switch their package to use the llvm17 compat libs. The LLVM team will not block Bodhi updates on dependent packages that fail to build or run with LLVM-18. 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-18 update.

• Release engineering: #Releng issue number

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

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

• Alignment with the Fedora Strategy:

Upgrade/compatibility impact

This change should not impact upgrades.

Early Testing (Optional)

Do you require 'QA Blueprint' support? Y/N

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.

Contingency Plan

• Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
• Contingency deadline: N/A (not a System Wide Change)
• Blocks release? N/A (not a System Wide Change), Yes/No

Documentation

N/A (not a System Wide Change)