F42 Change Proposal: LLVM 20 (system-wide)

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Wiki - https://fedoraproject.org/wiki/Changes/LLVM-20
Discussion thread -
https://discussion.fedoraproject.org/t/f42-change-proposal-llvm-20-system-wide/139540

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 20.

== Owner ==
* Name: [[User:Tstellar| Tom Stellard]]
* Email: <tstellar@xxxxxxxxxx>


== Detailed Description ==

All llvm sub-projects in Fedora will be updated to version 20.  There
will be a soname version change for the llvm libraries, and an llvm19
compat package added to ensure that packages that currently depend on
clang and llvm version 19 libraries will continue to work.

Other notable changes:

* '''Install Prefix Changes''' The install prefix for all new  llvm
packages will be /usr/lib64/llvm$VERSION/ instead of /usr/.  There
will be symlinks for each binary and library added to /usr/ which
point to the corresponding binary in /usr/lib64/llvm$VERSION/  The
goal of this change is to reduce the differences between the compat
and non-compat versions so that it it easier for packages that depend
on llvm to switch between the two.

* '''Merging more packages into llvm'''  In Fedora 41, we merged the
clang, libomp, compiler-rt, lld, python-lit, and lldb packages into
the llvm srpm.  For Fedora 42, we will also be merging llvm-bolt,
polly, libcxx, and mlir into the llvm20 srpm.

=== Planned Schedule ===
Our plan is to push 20.1.0-rc3 into Fedora 42 as a Beta Freeze exception.
Updates after 20.1.0-rc3 will generally be very small and can be done
after the Beta Freeze is over. If we are late packaging releases after
20.1.0-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 20.1.0-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 42. 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 ====

* Jan 31: Begin building LLVM 20.1.0-rc1 in COPR.
* '''''Feb 4: Fedora f42 branches created'''''
* Feb 11: Begin building LLVM 20.1.0-rc2 in COPR.
* '''''Feb 18: Fedora f42 beta freeze
* Feb 25: Begin building LLVM 20.1.0-rc3 in Rawhide and f42 side-tags.
* Feb 25-> Mar 11: Request Beta Freeze Exception and push 20.1.0-rc3
into f42 stable.
* Mar 11: Begin building LLVM 20.1.0-rc4 in Rawhide side-tag.
* Mar 25: Begin building LLVM 20.1.0 in Rawhide and 42 side-tags.
* '''''Apr 1: Fedora f42 final freeze'''''

== Feedback ==


== Benefit to Fedora ==
New features and bug fixes provided by the latest version of LLVM.

== Scope ==
* Proposal owners:

** Build and test early release candidates of LLVM 20 in COPR.
** Build and test later release candidates of LLVM 20 in koji.

* Other developers:
** Fix build issues found with LLVM-20 or switch their package to use
the llvm19 compat libs. The LLVM team will not block Bodhi updates on
dependent packages that fail to build or run with LLVM-20. 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-20 update.

* Release engineering: [https://pagure.io/releng/issues #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 ==


== Early Testing (Optional) ==


== 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
20.


== User Experience ==


== Dependencies ==

Packages that depend on one of the llvm packages will need to be
updated to work with LLVM20 or will need to switch to using one of the
llvm19 compat packages.

== Contingency Plan ==
* Contingency mechanism: (What to do?  Who will do it?)If there are
major problems with LLVM 20, the compatibility package provide a way
for other packages to continue using LLVM 19.
* Contingency deadline: Final Freeze
* Blocks release? No

== Documentation ==

LLVM sub-projects in Fedora have been updated to version 20:

* llvm (now includes polly, llvm-bolt, libcxx, mlir)
* flang
* libclc
* llvm-test-suite


-- 
Aoife Moloney

Fedora Operations Architect

Fedora Project

Matrix: @amoloney:fedora.im

IRC: amoloney

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