tree 15a70bb1d2dc8ffafddb6dc28ed403c368b41ffc
parent ebe58477f476fe0d9ef0718f209e5b32903b386c
author Philipp Wollermann <philwo@chromium.org> 1782558572 +0900
committer gn-scoped@luci-project-accounts.iam.gserviceaccount.com <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1782710436 -0700

Parallelize per-target JSON rendering in --ide=json

GenerateJSON() built each target's description and serialized it to JSON
on the main thread, one target at a time. The desc building and JSON
serialization are independent per target, so render them on a WorkerPool
into a pre-sized, order-preserving vector; only appending to the shared
output buffer and collecting toolchains stays serial.

The shared ResolvedTargetData is safe for concurrent access -- the
parallel Ninja target writers already share a single instance the same
way (see TargetWriteInfo in command_gen.cc). Verified race-free with a
ThreadSanitizer build over a full Chromium out/debug tree (0 reports).

For that tree (31145 targets) the JSON phase drops from ~2.1s to ~0.4s
(5.2x), bringing total "gn gen --ide=json" to ~2.1s vs ~1.7s for plain
"gn gen". The generated project.json is byte-identical before and after.

Change-Id: I04de9bec3965d5636a431e50d7ddeae2e9cea1c3
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/23461
Commit-Queue: Philipp Wollermann <philwo@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>