tree ee0ed4d7bc9025fbf82f612160bb3ea2ed3a9e5b
parent 47b40ef49e678319c0dac767a95a363cc3f1f6cb
author Takuto Ikuta <tikuta@google.com> 1772001303 +0900
committer GN LUCI <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1772069763 -0800

Optimize HeaderChecker by parallelizing ReachabilityCache pre-calculation and removing lock contention.

This change introduces a pre-calculation phase in HeaderChecker::Run
that concurrently populates the ReachabilityCache for all relevant
targets using a WorkerPool. It also eliminates lock contention during
the file-checking phase by using atomic completion flags and a lock-free
fast path in ReachabilityCache::SearchForDependencyTo.

Key optimizations:
- Concurrent BFS pre-calculation during HeaderChecker::Run.
- Reusing the WorkerPool across pre-calculation and file-checking phases.
- Lock-free dependency lookup using std::atomic<bool> completion flags
  with acquire/release semantics in the common case where the cache is warm.
- Significantly reduces lock contention (shared_mutex) on macOS.

Verified with existing HeaderChecker unittests and profiling on
Chromium.

Benchmark 1: ./gn_main gen --root=/Users/tikuta/chromium/src --check
/Users/tikuta/chromium/src/out/no_clang_modules
  Time (mean ± σ):      6.306 s ±  0.025 s    [User: 27.219 s, System: 21.382 s]
  Range (min … max):    6.283 s …  6.333 s    5 runs

Benchmark 2: ./gn_optimized gen --root=/Users/tikuta/chromium/src
--check /Users/tikuta/chromium/src/out/no_clang_modules
  Time (mean ± σ):      6.083 s ±  0.024 s    [User: 27.499 s, System: 26.226 s]
  Range (min … max):    6.049 s …  6.112 s    5 runs

Summary
  ./gn_optimized gen --root=/Users/tikuta/chromium/src --check /Users/tikuta/chromium/src/out/no_clang_modules ran
    1.04 ± 0.01 times faster than ./gn_main gen --root=/Users/tikuta/chromium/src --check /Users/tikuta/chromium/src/out/no_clang_modules

Bug: 484862257
Change-Id: I4c9609f0b57693a1a209f4c36b810bb1597a39a9
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/21260
Reviewed-by: David Turner <digit@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>