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Generated files

Many files are generated as part of the Chrome build. Fundamentally these are treated the same as static files (files checked into the repo), but have various subtleties. In many cases the build configuration file will be named generated.{gn,gyp,gypi} or similar.

Location

Generated files are output in $CHROMIUM_DIR/out/$BUILD_TYPE/gen, as in src/out/Release/gen.
These are indexed in code search in src/out/Debug/gen.
Because this is a different directory than static files, any behavior must be separately specified for generated files, notably:
  • Include search path: if you compile generated C++ files, you need to list the appropriate directory for generated files in the include search path, e.g., <(SHARED_INTERMEDIATE_DIR)/blink in a .gyp file for Blink generated files.
    • Note that static and generated files are not distinguished by path in includes; this is because qualified paths are for componentization, and static and generated files are in the same component.
  • Possibly different directory tree: life is much simpler if the generated files follow the same directory structure as the static files: this allows them to be treated uniformly, only needing to specify two roots.
    • E.g., for qualified paths in includes, you don't need to distinguish whether a file is generated or not: you just need to list both roots in the include search path.
    • For the same reason, checkdeps is simpler if generated files have the same directory structure.

Outputs (esp. .pyc)

All generated files must be listed in the outputs of the build step. If not, these files won't be cleaned up (notably by ninja -t clean), and then the build won't function properly (it will rebuild excessively). One case of this are byte-compiled Python files (.pyc), which are implicitly generated on import: if you generate Python files, you must also list the .pyc files in the outputs. See Bug 397909 and CL 463063003.

Clobber build

Moving generated files can break incremental builds, and thus requires a "clobber build" (i.e., full, clean build: delete all generated files). This is because the build does not delete old generated files, so a stale file may be used, notably via includes of headers with unqualified paths.
This shows up as generated files appearing to not be regenerated, though in fact the files are being generated in a new location; see for example Issue 381111 (comment).

  • (Required): Warn the tree sheriff or gardener (if other repo).
  • (Required): Add a clobber landmine, which forces a rebuild.
    • This should be done as part of the original CL (if in Chromium), or part of the roll (if in another repo).
    • If you forget this, it can also be done in a followup CL (E.g., CL 316343003).
    • For files in other repos (like Blink), this requires a roll, and thus causes breakage if there are other changes before the roll.
  • (Optional): In case of moving headers, put destination directory after origin directory in search path, so new files are found first. (E.g., CL 325483002 (comment))
    • This does not always work, and can cause cruft if the order is not natural.
    • However, this does not require a roll, and thus is good for changes in other repos.

Dependencies

Currently checkdeps is not run on generated files (Issue 365190), so generated files can break componentization.
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