An orderfile is a list of symbols that defines an ordering of functions which is respected by the linker when generating a binary. It can improve performance by reducing the number of random reads and improving the usefulness of readahead.
To generate an orderfile you can run the orderfile_generator_backend.py script. You will need an Android device connected with adb to generate the orderfile as the generation pipeline will need to run benchmarks on a device.
Example:
tools/cygprofile/orderfile_generator_backend.py --target-arch=arm64 --use-remoteexec
You can specify the architecture (arm or arm64) with --target-arch. For quick local testing you can use --streamline-for-debugging. To build using Reclient, use --use-remoteexec (Googlers only). There are several other options you can use to configure/debug the orderfile generation. Use the -h option to view the various options.
NB: If your checkout is non-internal you must use the --public option.
To build Chrome with a locally generated orderfile, use the chrome_orderfile_path=<path_to_orderfile> GN arg.
Orderfiles can be tested using Pinpoint. To do this, please create and upload a Gerrit change overriding the value of chrome_orderfile_path to, for instance, //path/to/my_orderfile (relative to src), where my_orderfile is the orderfile that needs to be evaluated. The orderfile should be added to the local branch and uploaded to Gerrit along with build/config/compiler/BUILD.gn. This Gerrit change can then be used as an “experiment patch” for a Pinpoint try job.
The orderfile_generator_backend.py script runs several key steps:
Build and install Chrome with orderfile instrumentation. This uses the -finstrument-function-entry-bare gcc flag to insert instrumentation for function entry. The build will be generated in out/arm_instrumented_out/ or out/arm64_instrumented_out, depending on the architecture.
Run the benchmarks and collect profiles. These benchmarks can be found in orderfile.py. These profiles are a list of function offsets into the binary that were called during execution of the benchmarks.
Cluster the symbols from the profiles to generate the unpatched orderfile. The offsets are processed and merged using a clustering algorithm to produce an orderfile.
Build an uninstrumented Chrome and patch the orderfile with it. The orderfile based on an instrumented build cannot be applied directly to an uninstrumented build. The orderfile needs to be patched with an uninstrumented build because the instrumentation has a non-trivial impact on inlining decisions and has identical code folding disabled. The patching step produces the final orderfile which will be in clank/orderfiles/ for internal builds, or in orderfiles/ if running the generator script with --public. The uninstrumented build will be in out/orderfile_arm64_uninstrumented_out.
Run benchmarks on the final orderfile. We run some benchmarks to compare the performance with/without the orderfile. You can supply the --no-benchmark flag to skip this step.
