Packaging LLVM

Serge « sans paille » Guelton

Compiler Engineer / Wood Craft Lover / RedHat employee

PackaginCon — 9 November 2021

Fedora in one slide

.~= Fedora =~.           Close to upstream philosophy


Several Releases             /:-------------:\\
. rawhide ~ Debian sid    :-------------------::
. f33, f34, f35         :-----------/shhOHbmp---:\\
                      /-----------omMMMNNNMMD  ---:
Architectures        :-----------sMMMMNMNMP.    ---:
. X86_64            :-----------:MMMdP-------    ---\\
. arm              ,------------:MMMd--------    ---:
. aarch64          :------------:MMMd-------    .---:
. ppc64le          :----    oNMMMMMMMMMNho     .----:
                   :--     .+shhhMMMmhhy++   .------/
. riscV            :-    -------:MMMd--------------:
. mips             :-   --------/MMMd-------------;
                   :-    ------/hMMMy------------:
Package            :-- :dMNdhhdNMMNo------------;
. RPM              :---:sdNMMMMNds:------------:
                   :------:://:-------------::
                   :---------------------://

LLVM in one slide

A compiler Infrastructure

+---------------------+ +---------------+
| - Inlining          | | - ADT         |
| - Unrolling         | | - System      |
| - ...               | | - STLExtra    |
|       ______________| | - ...         |
|      / Optimizations| |           ____|
+---------------------+ |          /Core|
                        +---------------+
+---------------------+
| X86, ARM, RiscV...  +
|             ________|
|            /Backends|
+---------------------+

LLVM Project dependency tree

            +------------> mlir > ---------+
           /                                \
          +-----> lld                        \
         /                                    +--> flang
LLVM >--+--> compiler-rt >--+                /
         \                   \              /
          +--> polly          +--> clang >-+
           \                                \
            +-------------- +----------------+--> lldb
                           /
libomp >------------------+

libcxx >

libunwind >

LLVM Upstream

• Monorepo: https://github.com/llvm/llvm-project

• Release cycle:

  • One major release every 6 months
  • Minor release in between
  • Release candidate for major and minor releases

• CMake-based build system
• Mostly C++14

LLVM Packaging In Fedora

1. Build the world (git based)
   • (+) Supported upstream: easy packaging
   • (-) Small patch rebuilds the world
2. Build package independently (tarball based)
   • (+) Faster recompilation
   • (-) More downstream maintenance

Pick your poison! We chose 2.

ABI Compatibility

LLVM Ships two kinds of libraries

1. C++ libraries, no ABI stability, major version bump each release
2. C interface, stable ABI across releases

Minor releases should not break the ABI

Compiling the compiler

Building with...

1. GCC:
   • (+) Fedora default compiler
   • (-) LTO build fails
2. Clang
   • (-) Policy only recently evolved to allow it
   • (+) -flto=thin
   • (-) LTO build fails

Library or Archive

1. Shared Libs:
   • (+) follow Fedora Policy
   • (+) much better disk usage
   • (-) less configuration tested upstream
2. Archive:
   • (+) upstream default
   • (-) huge disk usage

We chose 1.

Slow linking

Linking libLLVM.so is super-slow

• [x] Decrease debug quality: -g1

• [x] Break parallelism:

  • [x] Split build: cmake --build --target LLVM
  • [x] Sequential link: -DLLVM_PARALLEL_LINK_JOBS=1

• [ ] Use lld linker

Multi-versionning

We don't support multiple LLVM version at the same time, but...

• Use update-alternatives to ease transition between major releases
• Provide versioned compatibility packages for packages that don't live on the bleeding edge

Testing

Yeah! Tests! Breakage! Pain!

• [x] unit tests in each package, from upstream

• [x] Fedora CI

  • [x] integration test LLVM Toolchain Integration Test Suite

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The LLVM logo is a stylized wyvern (a kind of dragon). Dragons have connotations of power, speed and intelligence, and can also be sleek, elegant, and modular (err, maybe not).