Toolchains update policy

We document here the decision making and planning around when we update the various toolchains used and required to build Firefox.

Rust

The Rust update policy is documented in a separate document.

Clang

We ship official stable Firefox with a released Clang.

As a general rule, we update the Clang compiler version used to build Firefox Nightly as soon as possible, considering the constraint that the Clang compiler and the Rust compiler need to use the same version of LLVM.

We don’t upgrade the Clang version in the beta or release branches of Firefox.

The following exception applies:

We may skip the update (or backout the update) if major problems are encountered (typically, we’ve had to do so because of build problems, crash reporting bustage, or performance issues).

Local developer builds use the same version as used for the official builds.

Minimum Supported Clang Version

We will update the Minimum Supported Clang Version to match the smallest LLVM version supported by the Minimum Supported Rust Version.

That smallest LLVM version can be found in the check_llvm_version function in the src/bootstrap/src/core/build_steps/llvm.rs source file in the Rust compiler source code for the Minimum Supported Rust Version.

Rationale

Downstreams that need to procure a Rust compiler will also need an LLVM toolchain, which in practice includes a Clang with the same version. Since Rust enforces a minimum supported LLVM version, setting our Minimum Supported Clang Version to match that same baseline makes the requirements straightforward: a single minimum LLVM version covers both Rust and C/C++ compilation.

This approach strikes a balance between the constraints faced by downstreams in acquiring and maintaining toolchains, and our own need to use reasonably modern compilers for C and C++ code.

libstdc++

We build stable Firefox with a libstdc++ that allows to provide the necessary binary compatibility

The binary compatibility is defined by the Firefox system requirements on Linux. Those can be adjusted but a balance must be made between the breadth of systems supported by Firefox builds, and the desired C++ features on the development side. As of writing there isn’t a policy describing exactly how the system requirements are updated, yet.

Tricks may be put in place to guarantee the required binary compatibility with older versions of libstdc++ than the one used to build. (see build/unix/stdc++compat)

GCC

The Minimum Supported GCC Version matches the version of libstdc++ above

GCC and libstdc++ are, generally speaking, coupled, and it can be difficult to use a version of GCC that doesn’t match the version of libstdc++. As such, we don’t support building with a version of GCC older than the version of libstdc++ Firefox is built against by default (from the development sysroot).