Changes/GNUToolchainF36

GNU Toolchain Update (gcc 12, glibc 2.35)

Summary

Update the Fedora 36 GNU Toolchain to gcc 12 and glibc 2.35.

The gcc 12 is currently under development and will be included in Fedora 36 upon release. The glibc 2.35 change will be tracked in this top-level GNU Toolchain system-wide update.

Owner

• Name: Arjun Shankar
• Email: arjun@redhat.com

Current status

• Targeted release: Fedora Linux 36
• Last updated: 2022-01-05
• 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

The GNU Compiler Collection, GNU C Library, GNU Debugger, and GNU Binary Utilities make up the core part of the GNU Toolchain and it is useful for our users to transition these components as a complete implementation when making a new release of Fedora.

The GNU Compiler Collection is expected to release version 12 in Q2, before the Fedora 36 release. It will contain many new features, documented here: https://gcc.gnu.org/gcc-12/changes.html. The latest point release for gcc 12 will be included in Fedora 36, this will most probably be 12.1.

The GNU C Library version 2.35 is expected to be released in the beginning of February 2022; we have started closely tracking the glibc 2.35 development code in Fedora Rawhide and are addressing any issues as they arise. Given the present schedule Fedora 36 will branch after the release of glibc 2.35. However, the mass rebuild schedule means Fedora 36 will mass rebuild (if required) before the final release of glibc 2.35, but after the ABI is frozen.

The GNU Binutils version 2.37 and GNU Debugger version 11.1 currently included in Fedora 35 will continue to be included in Fedora 36. There will be a GNU Binutils version 2.38 released at the end of January, but the inclusion will be scheduled for Fedora 37.

Benefit to Fedora

Stays up to date with latest features, improvements, security and bug fixes from gcc, glibc, binutils, and gdb upstream.

The goal is to track and transition to the latest components of the GNU Toolchain.

Scope

• Proposal owners: Fedora Toolchain Team (gcc, glibc, binutils, gdb, ...) developers need to ensure that gcc, glibc, binutils, and gdb in rawhide are stable and ready for the Fedora 36 branch.

• Other developers: Given that glibc is backwards compatible and we have been testing the new glibc in rawhide it should make very little impact when updated, except for the occasional deprecation warnings and removal of legacy interfaces from public header files. An update to GCC 12.1 would mean a new major release and could have broad scope for change.

• Release engineering: A mass rebuild is strongly encouraged; #Releng issue number

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

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

• Alignment with Objectives: N/A

Upgrade/compatibility impact

The compiler, the static linker and the the library are backwards compatible with the previous version of Fedora.

The upgrade to glibc-2.35 coincides with the removal of nscd.

Some source changes may be required for gcc 12 rebase: https://gcc.gnu.org/gcc-12/changes.html

How To Test

The GNU Compiler Collection has its own testsuite which is run during the package build and examined by the gcc developers before being uploaded.

The GNU C Library has its own testsuite, which is run during the package build and examined by the glibc developers before being uploaded. This test suite has over 6200 tests that run to verify the correct operation of the library. In the future we may also run the microbenchmark to look for performance regressions.

User Experience

Users will see improved performance, many bugfixes and improvements to POSIX compliance, Unicode 14 support, C.UTF-8 locale support, improved experimental support for C++20 and C++23, new compiler warnings and improvements to existing ones, and more.

Dependencies

All packages do not need to be rebuilt due to backwards compatibility. However, it is advantageous if a mass rebuild is performed during the Fedora 36 cycle. The mass rebuild would ensure all packages can be built with the newer compiler and core runtime.

Contingency Plan

• Contingency mechanism glibc: If glibc 2.35 proves too disruptive to compiling the distribution we could revert to 2.34, but given that Rawhide has started tracking glibc 2.35, no show-stopper problems are expected. At this point, we can still revert to upstream version 2.34 if insurmountable problems appear, but to do so may require a mass rebuild to remove new symbols from the ABI/API.
• Contingency mechanism for gcc: If gcc 12 proves too disruptive to compiling the distribution we could revert to gcc 11.

• Contingency deadline: Fedora mass rebuild on 2022-01-19.
• Blocks release? Yes, upgrading to the gcc 12 release blocks the release. Yes, upgrading to glibc 2.35 does block the release.

Documentation

The gcc manual contains the documentation for the release and doesn't need any more additional work.

The glibc manual contains the documentation for the release and doesn't need any more additional work.

Release Notes

The GNU Compiler Collection version 12 is soon to be released. See https://gcc.gnu.org/gcc-12/changes.html.

The GNU C Library version 2.35 will be released at the beginning of August 2021. The current NEWS notes can be seen here as they are added: https://sourceware.org/git/?p=glibc.git;a=blob;f=NEWS;hb=HEAD