Single pass autoprof

[TTsangSC]

This PR mainly exists for documentation/discussion purposes. See also pyutils#440, which this builds upon.

The gist is that with the previous setup (as of pyutils#440) we walk the AST in line_profiler.autoprofile 1–3 separate times:

• ~.run_module.ImportFromTransformer walks it once to resolve relative imports, if running kernprof -m.
• ~.profmod_extractor._ImportFinder does another walk to glean all ~._import_targets.ImportTarget covered by --prof-mod.
• If doing full-script rewriting, ~.ast_profile_transformer.AstProfileTransformer walks the AST a final time to insert @profile decorators to functions and methods, and to profile imports missed by ~.profmod_extractor.ProfmodExtractor.extract_all() when using --prof-imports.

This resulted in code duplication, and also in bookkeeping overhead to guard against duplicating the on-import AST nodes, because some were inserted in the body of ~.ast_tree_profiler.AstTreeProfiler._profile_ast_tree(), and some in AstProfileTransformer._transform() which the former conditionally calls.

Changes

To streamline things in line_profiler.autoprofile:

• Many of the shared components are moved to the new ~._single_pass_transformer submodule.
• Said submodule also defines a new SinglePassTransformer class, which handles all the aforementioned AST transformations in one pass.
• The superseded line_profiler.autoprofile components are refactored to wrap around ~._single_pass_transformer components, but are kept around for backward compatibility.
• A new boolean switch line_profiler._diagnostics.USE_LEGACY_AUTOPROF (backed by the environment variable ${LINE_PROFILER_AUTOPROFILE_CORE}) is used so that when using ~.autoprofile.run(), one can switch between:
  • The "new"/"single-pass" core, which uses SinglePassTransformer, and
  • The "legacy"/"old"/"multi-pass" core, which uses the previous workflow.

Tests in tests/test_autoprofile.py have been updated to test both cores and their resp. components where appropriate.

Discussions/TODOs

• Should we deprecate the old interfaces (line_profiler.autoprofile.ast_profile_transformer, .ast_tree_profiler, .profmod_extractor, and .run_module)?

• Or maybe even just wait for version 6 to remove them outright? (This knocks about 1.1k lines out, putting our LoC at around the same level as ENH: handle nested and star imports pyutils/line_profiler#440. Further savings may be possible, since some of the ~._single_pass_transformer and ~.autoprofile components are mostly there to allow for keeping the behaviors consistent with ENH: handle nested and star imports pyutils/line_profiler#440.)

• The single-pass approach streamlines the workflow and reduces inter-component bookkeeping which is a pain, but so far has failed to either actually

  • Reduce the codebase size (see above), or
  • Improve performance.

  For reference regarding the latter, I ran pytest -k "(test_autoprofile and not star_import and not nested_import and not ast_tree_profiler and not ast_profile_transformer and not profmod_extractor and not single_pass_transformer and not multitarget) or not test_autoprofile" to deselect all the tests directly affected by the refactor, and the timings are:

  • This PR: mean 26 s, best 25.3 s
  • ENH: handle nested and star imports pyutils/line_profiler#440: mean 26.4 s, best 25.3 s

  This may be a function of how our test suite tends to use small and shallow snippets, which makes multiple AST passes inexpensive. SinglePassTransformer is probably also pretty unoptimized as-is, e.g. not having much in the way of checks against descending into a composite-statement node.