pytorch

057b78b3 - Add option to use ninja to compile ahead-of-time cpp_extensions

Add option to use ninja to compile ahead-of-time cpp_extensions Background ^^^^^^^^^^ Previously, ninja was used to compile+link inline cpp_extensions and ahead-of-time cpp_extensions were compiled with distutils. This PR adds the ability to compile (but not link) ahead-of-time cpp_extensions with ninja. The main motivation for this is to speed up cpp_extension builds: distutils does not make use of parallelism. With this PR, using the new option, on my machine, - torchvision compilation goes from 3m43s to 49s - nestedtensor compilation goes from 2m0s to 28s. User-facing changes ^^^^^^^^^^^^^^^^^^^ I added a `use_ninja` flag to BuildExtension. This defaults to `False`. When `use_ninja` is True: - it will attempt to use ninja. - If we're on windows, then this throws a warning and falls back to distutils. (this PR doesn't add windows support for this functionality, but it can be a future task). - If we can't find ninja, then we hard error. I wasn't sure whether or not to set `use_ninja=True` as the default. If we make that the default, then it instead needs to do a "best effort" and have an automatic fallback path to avoid being bc-breaking. Testing such a change is kind of difficult. I'm happy to figure out the design/behavior of this in this PR or in a follow-up. Implementation Details ^^^^^^^^^^^^^^^^^^^^^^ This PR makes this change in two steps. Please me know if it would be easier to review this if I split this up into a stacked diff. Those changes are: 1) refactor _write_ninja_file to separate the policy (what compiler flags to pass) from the mechanism (how to write the ninja file and do compilation). 2) call _write_ninja_file and _run_ninja_build while building ahead-of-time cpp_extensions. These are only used to compile objects; distutils still handles the linking. Change 1: refactor _write_ninja_file to seperate policy from mechanism - I split _write_ninja_file into: _write_ninja_file and _write_ninja_file_to_build_library - I renamed _build_extension_module to _run_ninja_build Change 2: Call _write_ninja_file while building ahead-of-time cpp_extensions - _write_ninja_file_and_compile_objects calls _write_ninja_file to only build object files. - We monkey-patch distutils.CCompiler.compile to call _write_ninja_files_and_compile_objects - distutils still handles the linking step. The linking step is not a bottleneck so it was not a concern. - This change only works on unix-based systems. Our code for windows goes down a different codepath and I did not want to mess with that. - If a system does not support ninja, we raise a warning and fall back to the original compilation path. Test Plan ^^^^^^^^^ Adhoc testing - I built torchvision using pytorch master and printed out the build commands. Next, I used this branch to build torchvision and looked at the ninja file. I compared the ninja file with the build commands and asserted that they were functionally the same. - I repeated the above for pytorch/nestedtensor. PyTorch test suite - I added cpp extension tests to test BuildExtension with `use_ninja=False` and `use_ninja=True`. - This involves having a duplicate of the cpp extension files. One of the copies is used to build with `use_ninja=False`, the other copy is used to build `use_ninja=True`. I am not sure if it is possible to deduplicate these. - Ran the cpp extension tests. `python run_test.py -i cpp_extension -v` Help wanted and/or future work ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - API: should we have `use_ninja=True` and implement an automatic fallback? - Testing: How do we clean up the tests? [ghstack-poisoned]