Fourth profile-guided pass (#139, docs/benchmarks.md pass 28). scanAttributes
allocated a fresh Set<String>'s native storage on every element with
attributes, just to check a handful of names for duplicates; the set is now a
stored EventReader scratch property cleared with removeAll(keepingCapacity:)
per element, so each element's check reuses the previous element's storage.
Safe because scanAttributes cannot re-enter itself (nothing inside an
attribute scan starts another start-tag scan), so the shared storage never
sees two interleaved scans; duplicate detection, recovering-mode behavior,
and thrown errors are unchanged.
Verified in an isolated worktree at the previous commit: full local suite
(1858 tests) and full W3C XSTS suite (14383 groups) pass unchanged. Measured
with an interleaved same-state A/B, baseline/patched/baseline/patched each
best-of-5, because the machine carried concurrent load this session: both
patched runs (0.1073s, 0.1067s) beat both baseline runs (0.1091s, 0.1117s),
~2-4% on parse.
Recorded alongside, per the benchmarks log's discipline: a sibling candidate
(reserving each tree frame's children capacity on first append) measured
neutral under the same protocol and was reverted rather than landed, and the
TreeNode weak-parent side-table cost (~9% of parse, swift_weakAssign per
child plus a runtime side table per parent) is documented under Known
structure of the gap as needing a design decision, since parent's
nil-after-release semantics are documented public API. Touched lines carry
repo-standard swiftformat rewraps applied by a concurrent formatter run.
Mihaela Mihaljevic
2b6da0d0a0