tree 033d99c5f0bee5462c9199b5447c178d0ec451ee
parent a155c8c78af4030169a8da4886dc0c6b170535de
author Takuto Ikuta <tikuta@google.com> 1772673223 +0900
committer GN LUCI <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1772673783 -0800

[gn] Optimize Value::LIST with Copy-On-Write using nullptr

To reduce the execution time of `gn gen`, the internal holding of
Value::LIST was changed from `std::vector<Value>` to
`scoped_refptr<ValueList>` to enable Copy-On-Write (COW).

This reduces unnecessary deep copies and allocation overhead for large
lists, significantly improving performance.

Performance benchmark comparing this optimization to the main branch:
Benchmark 1: gn_unoptimized gen out/Default
  Time (mean ± σ):      2.384 s ±  0.085 s    [User: 23.577 s, System: 20.115 s]

Benchmark 2: gn_optimized gen out/Default
  Time (mean ± σ):      2.051 s ±  0.043 s    [User: 21.155 s, System: 19.339 s]

Summary
  gn_optimized ran 1.16 ± 0.05 times faster than gn_unoptimized

Bug: 484863025
Change-Id: I08a3d8f5bc2dd9c794fb06a99de48ec0c36e18d8
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/21340
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Turner <digit@google.com>