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The Firefox build system uses the technique of "unified builds" (or elsewhere
called "`unity builds <https://en.wikipedia.org/wiki/Unity_build>`_") 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``:
* 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 :ref:`above<unified_build_compilation_failures>`.
* We are happy to accept patches that fix problematic unified build chunks (such as by adding
includes or namespace annotations).