The chrome/build/*.orderfile files are used to specify the order in which the linker should lay out symbols in the binary it‘s producing. By ordering functions in the order they’re typically executed during start-up, the start-up time can be reduced slightly.
The order files are used automatically when building with Clang for Windows with the gn flag is_official_build set to true.
To update the order files:
Build with instrumentation enabled:
The instrumentation will capture the couple of million function calls in a binary as it runs and write them to a file in the \src\tmp directory. Make sure this directory exists.
gn gen out\instrument --args="is_debug=false is_official_build=true generate_order_files=true symbol_level=1" ninja -C out\instrument chrome
(If you have access to Goma, add use_goma=true to the gn args and -j500 to the Ninja invocation.)
Run the instrumented binaries:
(Some binaries such as mksnapshot, yasm, and protoc already ran with instrumentation during the build process. The instrumentation output should be available under \src\tmp.)
Open the Task Manager's Details tab or Process Explorer to be able to see the Process IDs of running programs.
Run Chrome with the sandbox disabled (otherwise the render process instrumentation doesn't get written to disk) and with a startup dialog for each renderer:
out\instrument\chrome --no-sandbox --renderer-startup-dialog
Note the Process IDs of the browser and render process (there is sometimes more than one; you want the one that loads the New Tab Page).
Check in \src\tmp\ for instrumentation output from those processes, for example cygprofile_14652.txt and cygprofile_23592.txt. The files are only written once a certain number of function calls have been made, so sometimes the renderer needs to be reloaded in order for the file to be produced.
If the files appear to have sensible contents (a long list of function names that eventually seem related to what the browser and render process should do), copy them into chrome\build\:
copy \src\tmp\cygprofile_25392.txt chrome\build\chrome.x64.orderfile copy \src\tmp\cygprofile_14652.txt chrome\build\chrome_child.x64.orderfile
Re-build the chrome target. This will re-link chrome.dll and chrome_child.dll using the new order files and surface any link errors if the files are broken.
ninja -C out\instrument chrome
Repeat the previous steps with a 32-bit build, i.e. passing target_cpu="x86" to gn and storing the files as .x86.orderfile.
Upload the order files to Google Cloud Storage. They will get downloaded by a gclient hook based on the contents of the .orderfile.sha1 files.
You need to have write access to the chromium-browser-clang GCS bucket for this step.
cd chrome\build\ upload_to_google_storage.py -b chromium-browser-clang/orderfiles chrome.x64.orderfile chrome.x86.orderfile chrome_child.x64.orderfile chrome_child.x86.orderfile gsutil.py setacl public-read gs://chromium-browser-clang/orderfiles/*
Check in the .sha1 files corresponding to the orderfiles created by the previous step.