7 The Firefox build system uses the technique of "unified builds" (or elsewhere
8 called "`unity builds <https://en.wikipedia.org/wiki/Unity_build>`_") to
9 improve compilation performance. Rather than compiling source files individually,
10 groups of files in the same directory are concatenated together, then compiled once
13 Unified builds can be configured using the ``UNIFIED_SOURCES`` variable in ``moz.build`` files.
15 .. _unified_build_compilation_failures:
17 Why are there unrelated compilation failures when I change files?
18 =================================================================
20 Since multiple files are concatenated together in a unified build, it's possible for a change
21 in one file to cause the compilation of a seemingly unrelated file to fail.
22 This is usually because source files become implicitly dependent on each other for:
24 * ``#include`` statements
25 * ``using namespace ...;`` statements
26 * Other symbol imports or definitions
28 One of the more common cases of unexpected failures are when source code files are added or
29 removed, and the "chunking" is changed. There's a limit on the number of files that are combined
30 together for a single compilation, so sometimes the addition of a new file will cause another one
31 to be bumped into a different chunk. If that other chunk doesn't meet the implicit requirements
32 of the bumped file, there will be a tough-to-debug compilation failure.
37 * Some IDEs (such as VSCode with ``clangd``) build files in standalone mode, so they may show
38 more failures than a ``mach build``.
39 * The amount of files per chunk can be adjusted in ``moz.build`` files with the
40 ``FILES_PER_UNIFIED_FILE`` variable. Note that changing the chunk size can introduce
41 compilation failures as described :ref:`above<unified_build_compilation_failures>`.
42 * We are happy to accept patches that fix problematic unified build chunks (such as by adding
43 includes or namespace annotations).