Branch: refs/heads/main
Home: https://github.com/WebKit/WebKit
Commit: c74b5b63f801e54fe07c3f95be71f3e57d7d8127
https://github.com/WebKit/WebKit/commit/c74b5b63f801e54fe07c3f95be71f3e57d7d8127
Author: Sosuke Suzuki <[email protected]>
Date: 2026-06-17 (Wed, 17 Jun 2026)
Changed paths:
A JSTests/microbenchmarks/map-set-delete-churn-large.js
A JSTests/microbenchmarks/map-set-delete-churn.js
A JSTests/microbenchmarks/set-add-delete-churn.js
M Source/JavaScriptCore/runtime/JSOrderedHashTableHelper.h
Log Message:
-----------
[JSC] Map/Set `set` + `delete` churn always expands then immediately shrinks
https://bugs.webkit.org/show_bug.cgi?id=317199
Reviewed by Yusuke Suzuki.
expandIfNeeded intends to rehash at the same capacity when tombstones
dominate, but compares deletedEntryCount against capacity / 2 while the
rehash fires at usedCapacity == dataCapacity == capacity / 2 for small
tables, so the branch is unreachable whenever any entry is alive. A Map
kept at constant size by repeated set+delete therefore always expands
x4 only to immediately shrink back, repeating this on every fill while
find() walks an ever-growing tombstone chain.
Compare against dataCapacity / 2 instead (matching V8's policy). After
the same-capacity rehash alive <= capacity / 4 stays above the
capacity / 8 shrink threshold, so this introduces no new grow/shrink
cycle.
baseline patched
map-set-delete-churn-large 2381.5707+-218.4677 ^ 522.1759+-9.4359 ^
definitely 4.5609x faster
map-set-delete-churn 86.2613+-3.8686 ^ 37.2290+-1.5133 ^
definitely 2.3170x faster
set-add-delete-churn 80.6294+-4.6888 ^ 35.3066+-2.2222 ^
definitely 2.2837x faster
set-delete-add 20.3424+-0.2986 ^ 15.7992+-0.2317 ^
definitely 1.2876x faster
map-rehash 18.9758+-0.8362 ? 18.9775+-0.5948 ?
Tests: JSTests/microbenchmarks/map-set-delete-churn-large.js
JSTests/microbenchmarks/map-set-delete-churn.js
JSTests/microbenchmarks/set-add-delete-churn.js
* JSTests/microbenchmarks/map-set-delete-churn-large.js: Added.
* JSTests/microbenchmarks/map-set-delete-churn.js: Added.
* JSTests/microbenchmarks/set-add-delete-churn.js: Added.
* Source/JavaScriptCore/runtime/JSOrderedHashTableHelper.h:
(JSC::JSOrderedHashTableHelper::expandIfNeeded):
Canonical link: https://commits.webkit.org/315431@main
To unsubscribe from these emails, change your notification settings at
https://github.com/WebKit/WebKit/settings/notifications
