March 19, 2020

Removal of GCC 4.2.1 marks significant milestone in FreeBSD’s move to an integrated, modern, permissively licensed tool chain

As announced, on February 29, 2020 we removed GNU Compiler Collection (GCC) version 4.2.1 from the FreeBSD base system. Although GCC has not been used by default for anything in FreeBSD-current / svn head for some time before this commit, this milestone marks the completion of a major journey to modernize the FreeBSD toolchain.

The purpose of this blog is to provide detail about the motivation, scope, and timeline for this initiative. None of what is in this blog is new, per se. Nevertheless, we felt it was important to pull all the relevant bits that are currently spread across mailing lists and man pages into one canonical post for easy reference.

Motivation

License needs clearly played a role in motivating the Toolchain Project. Most readers will know that GCC adopted the strong copy-left GPLv3 license in 2007, presenting obvious incompatibilities with the ethos of this community.

A desire to provide a performant, modern toolchain played an important role as well. This is important to support research and development efforts and to give the project more freedom to drive the tool chain’s roadmap. Many of the tools in the Toolchain project (see full list in the Scope section below) represent technical improvements. In most cases, they are relatively new projects without much cruft in their code bases, e.g. they only support operating systems, executable formats, and architectures people actually use today.

Take for example the new compiler, Clang/LLVM, which brings advantages like: 

  • Vastly improved warnings and error messages due to parser design1
  • Compilation that is fast enough to use for syntax highlighting
  • LLVM is always a cross compiler
  • Ability to JIT code
  • Blocks support
  • Extensible and modular design allowing developers and researchers the ability to integrate, reuse, or replace individual parts of LLVM

One industry observer goes so far as to credit the recent blossoming of new development languages like Rust, Kotlin, and Swift in part on LLVM. We are gratified that our design decision to get on board early with LLVM has positioned FreeBSD to be one of (if not *the*) best supported open source operating systems when using LLVM.

Scope

In addition to Clang/LLVM, other tools in the Toolchain project are:

  • The ELF Tool Chain Project – A project to create BSD-licensed replacements for GNU binutils, and that also adds some other tools
  • libc++ – A BSD-licensed C++ standard library
  • libcxxrt – A BSD implementation of the Itanium C++ ABI used by FreeBSD. This provides the low-level parts of the C++ implementation: exception handling, RTTI, etc.

This project also adds support for external toolchains, which provides significant benefits to embedded developers who often need to run vendor-provided toolchains. An explicit goal of external toolchain support is to support any modern compiler with a gcc compatible driver. Another goal is that using external toolchains should be easy.

Timeline

These efforts have been underway for some time, and so the February 29 announcement really marks the culmination of what has been years of effort by countless contributors.

Thanks to much hard work, FreeBSD has used Clang by default for x86 and LE arm targets since version 10.0, released in January 2014, and lld by default since 12.0.

Here’s the complete timeline:

Closing Thoughts

Clearly license concerns were a big part of the initial motivation to replace GCC. But the FreeBSD Toolchain project has been and is about much more. Brooks Davis said it well:

Comprehensive LLVM support in FreeBSD is enormously advantageous in the research world where LLVM is the research compiler of choice. In the DARPA SSITH program, three of five teams are using FreeBSD specifically due to our LLVM support. Further, our collaboration with Arm to bring CHERI to ARMv8-A in the form of the Morello prototype would have been much harder if not impossible without a permissively licensed operating system and toolchain.

Finally, we would like to give a shout out to a few of the contributors without whose leadership and efforts, this major milestone would not have been possible.

FreeBSD Toolchain Hall of Fame

  • Brooks Davis
  • Roman Divacky
  • Dimitry Andric
  • Ed Maste
  • David Chisnall
  • Joseph Koshy
  • Kai Wang

Honorable Mention

  • Warner Losh

1. Compared to the pre-GPLv3 GCC version 4.2.1 that we were using. Contemporary GCC versions do not have these problems.↩