Unified Builds

The Firefox build system uses the technique of “unified builds” (or elsewhere called “unity builds”) to improve compilation performance. Rather than compiling source files individually, groups of files in the same directory are concatenated together, then compiled once in a single batch.

Unified builds can be configured using the UNIFIED_SOURCES variable in moz.build files.

Why are there unrelated compilation failures when I change files?

Since multiple files are concatenated together in a unified build, it’s possible for a change in one file to cause the compilation of a seemingly unrelated file to fail. This is usually because source files become implicitly dependent on each other for:

  • #include statements

  • using namespace ...; statements

  • Other symbol imports or definitions

One of the more common cases of unexpected failures are when source code files are added or removed, and the “chunking” is changed. There’s a limit on the number of files that are combined together for a single compilation, so sometimes the addition of a new file will cause another one to be bumped into a different chunk. If that other chunk doesn’t meet the implicit requirements of the bumped file, there will be a tough-to-debug compilation failure.

Building outside of the unified environment

As described above, unified builds can cause source files to implicitly depend on each other, which not only causes unexpected build failures but also can cause issues when using source-analysis tools. To combat this, we’ll use a “non-unified” build that attempts to perform a build with as many files compiled individually as possible.

To build in the non unified mode, set the following flag in your mozconfig:

ac_add_options --disable-unified-build

Other notes:

  • Some IDEs (such as VSCode with clangd) build files in standalone mode, so they may show more failures than a mach build.

  • The amount of files per chunk can be adjusted in moz.build files with the FILES_PER_UNIFIED_FILE variable. Note that changing the chunk size can introduce compilation failures as described above.

  • We are happy to accept patches that fix problematic unified build chunks (such as by adding includes or namespace annotations).