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Bump WebKit: a transition from a dictionary structure takes its property table instead of copying it - #45774
Bump WebKit: a transition from a dictionary structure takes its property table instead of copying it#45774robobun wants to merge 2 commits into
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…rty table instead of copying it A delete, an attribute change, a prototype change, seal, freeze or a private brand on an object with more than 128 named properties copied the object's property table. The loop of #44583 (delete and re-add one key of a 1000 key object) ran about 100 times slower than node. WEBKIT_VERSION points at the preview build of oven-sh/WebKit#803.
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Updated 4:42 AM PT - Oct 19th, 2026
✅ @robobun, your commit 040e23085358ed1a76e9ff635281a653df3f4778 passed in 🧪 To try this PR locally: bunx bun-pr 45774That installs a local version of the PR into your bun-45774 --bun |
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughThe WebKit version pin now uses the preview tag for PR 803. New Bun tests measure ChangesDictionary property-table allocations
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
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| Title check |  |
The title clearly identifies the main change: updating WebKit to support taking a dictionary structure's property table instead of copying it. |
| Linked Issues check | |
The PR pins WEBKIT_VERSION to autobuild-preview-pr-803-cf96a639, which supplies the JavaScriptCore dictionary-table transition fix for [#44583] and [#803]. The new regression tests cover delete an… |
| Out of Scope Changes check | |
The diff contains only the WebKit version update and regression tests. The version update supplies the linked JavaScriptCore fix. The tests validate the linked large-dictionary transition behavior. No… |
| Description check | |
The description is complete and relevant. It explains the problem, fix, scope, performance impact, downsides, verification steps, and test results. It does not use the exact template headings, but it … |
Full details: Linked Issues check
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Explanation
The PR pins
WEBKIT_VERSIONtoautobuild-preview-pr-803-cf96a639, which supplies the JavaScriptCore dictionary-table transition fix for [#44583] and [#803]. The new regression tests cover delete and re-add, attribute changes, prototype changes, and largeprocess.envobjects. They verify reducedPropertyTableallocation and final object state. The PR reports the 1,000-property workload improving from 2,278 ms to 78 ms per 100,000 pairs.
Full details: Out of Scope Changes check
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Explanation
The diff contains only the WebKit version update and regression tests. The version update supplies the linked JavaScriptCore fix. The tests validate the linked large-dictionary transition behavior. No unrelated changes are present.
Full details: Description check
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Explanation
The description is complete and relevant. It explains the problem, fix, scope, performance impact, downsides, verification steps, and test results. It does not use the exact template headings, but it provides all required information.
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Actionable comments posted: 3
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @test/js/bun/jsc/object-delete-property-dictionary.test.ts:
- Line 43: Group the four subprocess tests in this file under a concurrent
describe block so their subprocess waits can overlap; keep the tests and their
assertions unchanged.
- Around line 50-57: Update the large-object property attribute test and its
result collection to include the final writable state of key0, using its
property descriptor; assert that writable is false after the loop ends at i =
99. Preserve the existing allocation, key-count, and value assertions.
- Around line 59-67: Update the propertyTablesAllocatedBy helper to return a
prototype marker from Object.getPrototypeOf(object), and assert prototypeMarker
is 2 in the prototype-transition test. Keep the existing allocation, key-count,
and value assertions unchanged.
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📒 Files selected for processing: 2
scripts/build/deps/webkit.tstest/js/bun/jsc/object-delete-property-dictionary.test.ts
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| * From https://github.com/oven-sh/WebKit releases. | ||
| */ | ||
| export const WEBKIT_VERSION = "87ee10da087b63e011bd1ada64d1814a850a4395"; | ||
| export const WEBKIT_VERSION = "autobuild-preview-pr-803-cf96a639"; |
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🔴 Every developer and CI build of Bun will fail to download JavaScriptCore the moment oven-sh/WebKit#803 merges or closes, which the base pin never does. scripts/build/deps/webkit.ts:6 pins WEBKIT_VERSION to the preview tag "autobuild-preview-pr-803-cf96a639"; scripts/build/download.ts:319-321 documents that GitHub deletes an autobuild-preview-pr-* release when its PR merges or closes, so every prebuilt fetch 404s at once. Fix: before merging, land oven-sh/WebKit#803 and set WEBKIT_VERSION to the merged main commit sha (or its autobuild- tag), confirming the release carries every platform x flavor tarball prebuiltSuffix() can name. The description says to do this later, but a merged Bun pin to a deleted release breaks main for everyone until someone re-bumps.
Why this was flagged
The trigger is the oven-sh/WebKit PR #803 merging or closing, which happens outside this repo and is the stated plan. After that, cfg.webkitVersion defaults to "autobuild-preview-pr-803-cf96a639" (scripts/build/config.ts:59, :1221) and prebuiltUrl() at scripts/build/deps/webkit.ts:81-82 builds https://github.com/oven-sh/WebKit/releases/download/autobuild-preview-pr-803-cf96a639/bun-webkit--.tar.gz. GitHub removes the preview release with the PR, so the download returns 404 and prebuiltDownloadError at scripts/build/download.ts:328-337 throws "WebKit preview release is gone" for every bun run build and every CI build-bun step on every platform. On the base branch WEBKIT_VERSION is the 40-hex merged sha 87ee10da087b63e011bd1ada64d1814a850a4395 whose autobuild release is permanent. The repo's own upgrade procedure (.claude/commands/upgrade-webkit.md:34) says the bump to the merged sha must happen before the Bun PR merges; nothing in this diff enforces that, and the only safeguard is the error message after the breakage.
Verification: scripts/build/deps/webkit.ts:6 sets WEBKIT_VERSION = "autobuild-preview-pr-803-cf96a639" (base was a merged sha whose release never expires). scripts/build/download.ts:319-322 documents the trigger: GitHub deletes the preview release when the PR merges or closes, so every default prebuilt build 404s and hard-fails. No in-repo check prevents merging an autobuild-preview-pr-* pin.
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Agreed. The preview tag is for the build and the CI of this PR only. WEBKIT_VERSION moves to the merged main sha of oven-sh/WebKit#803 before this PR merges. Left open until that bump lands.
…es and prototype, own bound for process.env
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Code review found no new issues
No new issues were found in this update; 1 finding from earlier reviews is still open above.
Still open from earlier reviews (1):
- 🔴
scripts/build/deps/webkit.ts:6—Every developer and CI build of Bun will fail to download JavaScriptCore the moment oven-sh/WebKit#803 merges or closes…
If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.
Fixes #44583
Problem
delete object.key0; object.key0 = ion an object with 1000 named properties runs 100000 times in about 3 s in Bun and in 50 to 115 ms in node. The reporter's profile has 41% of the time in thePropertyTablecopy constructor and 42% inPropertyTable::destroy. Mnemonist's LRU cache (Mnemonist LRU eviction is much slower than Node #44580) does this per eviction.Structure::takePropertyTableOrCloneIfPinnedcopied the whole table: O(n) time and 25 KB allocated per delete. An attribute change, a prototype change, seal, freeze, preventExtensions and a private brand copied it too.process.envwith more than 128 variables is such an object.Fix
WEBKIT_VERSIONto the preview build of that PR. Switch it to themainsha once [JSC] A transition from a dictionary structure takes its property table instead of copying it WebKit#874 merges.test/js/bun/jsc/object-delete-property-dictionary.test.ts(new, four cases; on the current pin each allocates one table per iteration, 103 to 109 per 100). In thejscshell the loop above goes from 2278 ms to 78 ms per 100000 pairs, and from 25090 to 160 bytes allocated per pair.Background
Structureis an object's shape. Most structures share one property table along a chain of transitions. A dictionary structure owns a pinned table and has no chain. A cacheable dictionary keeps inline caches valid by changing structure on a delete.Downsides
Structurecell: 160 bytes and about 0.8 us per delete plus re-add pair. Objects with 100 properties never become dictionaries and are unchanged (265 vs 264 ms per 1000000 pairs, noise 20%).delete globalThis.x, and a read site the DFG compiles after a delete: same as before. JavaScriptCore text grows by 348 bytes.Notes
Gate. The fix is in the vendored engine, not under
src/, so a fail-before run that stripssrc/still uses the fixed WebKit and passes. Fail-before was run by hand: with the current pin,bun test test/js/bun/jsc/object-delete-property-dictionary.test.tsfails its four cases with 108, 103, 108 and 35 PropertyTable cells per 100 iterations; with the preview build it passes (8 to 12, the tables the adds grow into on any build).The
process.envcase.process.envin Bun is a JavaScriptCore object with one property per variable, so an environment with more than 128 variables gets the same dictionary structure, anddelete process.env.Xcopied its table (35 tables per 100 delete plus set pairs with 237 variables).Measurements are in oven-sh/WebKit#874 (release
jscshell, Linux x64, interleaved runs against its base): delete plus re-add on 1000 properties 2278 ms to 78 ms per 100000 pairs (JIT off 3471 ms to 84 ms), on 2000 properties 4987 ms to 105 ms, on 100 properties 265 ms vs 264 ms per 1000000 pairs; bytes per delete plus re-add pair 25090 to 160; per add plus attribute change pair 46051 to 570; 10 million inline-cached reads on a 200 property object after one or two deletes 77 and 69 ms vs 78 and 82 ms (both 51 to 87); 10 million bare global reads afterdelete globalThis.x32 vs 38 ms (both 32 to 49); a read site cached before a delete and DFG-compiled after it 81 vs 83 ms, with the same DFG node counts as before (6 CheckStructure, 1 GetById, 10 GetByOffset).Review. A design review weighed a patch at the two symptom sites, the dictionary policy (uncacheable on the first delete) and the helper every transition gets its table through, and picked the helper with a marker for the departed structure. A self-review of the WebKit diff raised 8 concerns, all addressed there (chain cut kept for seal, freeze, preventExtensions, prototype and brand transitions;
PropertyCondition::isWatchableWhenValidno longer asserts when a departed dictionary lost the property between its two reads; the type profiler reads the object's structure; tests for every transition kind and forprocess.env).[policy-decision:webkit] gate passed · iteration 0 · 2 files touched
passes on PR (with fix)
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gate history · 1 passed · 0 rejected · iteration 0
evidence per changed file
root cause · written by the author bot
Deleting a property from an object with more than 128 named properties puts its Structure into uncacheable dictionary mode, and every subsequent delete or reinsertion created a fresh Structure whose PropertyTable was cloned from the previous one and then destroyed, so each operation copied and freed a thousand-entry table. The fix makes a transition from a dictionary structure take ownership of the existing property table instead of copying it, marking the source as a departed dictionary so the DFG status computations and type profiler do not rely on a table it no longer holds. With the tab…