Upgrade WebKit: keep frozen, sealed and non-extensible array elements in the ArrayStorage vector

[robobun]

Fixes #44305

Problem

• Object.freeze on 300k arrays of 16 ints takes about 700 ms to 1.2 s in bun and 25 to 50 ms in node. Reads from the frozen arrays are 8 to 13x slower than from plain arrays (node: about 4x). 300k frozen arrays take 172 MB of heap instead of 53 MB.
• JSObject::enterDictionaryIndexingMode in JavaScriptCore moved every element of a frozen, sealed or non-extensible array into the SparseArrayValueMap (a hash map). Every later a[i] read missed the JIT vector paths and did a hash lookup in C++.

Fix

• Upgrade WebKit to Keep the elements of a frozen, sealed or non-extensible array in the ArrayStorage vector WebKit#752 (pinned to its preview build autobuild-preview-pr-752-98c438b4 until it merges). The engine keeps the elements in the ArrayStorage vector, switches the array to the SlowPutArrayStorage shape so every write goes through C++, and records the element attributes and the length writability in three Structure bits.
• Every tier already reads a SlowPutArrayStorage vector inline, so reads from a frozen array run at the speed of a plain array. The DFG and FTL in-bounds store into such a vector now tests the structure first.
• Verified: test/js/bun/jsc/frozen-array-representation.test.ts (12 cases, 8 fail on bun 1.4.3: bun:jsc's describeArray shows vector length 0 there). In the engine: two new stress tests, the full JSC stress suite and the related test262 directories on the WebKit PR's CI.

Background

• An ArrayStorage array has a vector plus an optional sparse map for attributed or far-away indices. "Sparse mode" means the vector is empty and the map holds every element. That was the only representation JSC had for a non-configurable element.
• SlowPutArrayStorage is the shape JSC already uses when an indexed write cannot be done inline (indexed accessors on the prototype chain). The JITs inline its reads and call C++ for its writes.
• Designs weighed in the engine PR: Object.seal / Object.freeze: take the JSObject fast path for JSArray and JSFinalObject with indexed properties WebKit#337 alone (cheaper freeze loop, reads stay a hash lookup), a frozen copy-on-write butterfly (covers Object.freeze only), and a new indexing shape (spends the last free shape and two ArrayModes bits, touches 13 JIT files). The vector-resident design fixes every producer at their one chokepoint with no change to the plain array paths.

Downsides

• A DFG / FTL inlined store into a having-a-bad-time array (writable SlowPutArrayStorage) pays one structure load and one branch more per store. Plain Int32 / Double / Contiguous / ArrayStorage stores are unchanged.
• Object.defineProperty on one element of a sealed or frozen array still moves the whole array into the sparse map, as before.
• jsc text size grows by 14,216 bytes (size, x86_64 release: 45,907,907 to 45,922,123).

Notes

Measurements (x86_64 release jsc shell, the issue's repro with 300k x 16-int arrays):

• freeze300kMs: 970 to 1072 ms -> 34 to 47 ms (node 26: 24 to 53 ms).
• frozenRead / plainRead: 9.5x to 9.8x -> 0.9x to 1.2x (node: 3.7x to 5.5x).
• heap for 300k frozen arrays: 171.7 MB -> 52.9 MB (plain 52.7 MB). 300k SparseArrayValueMap cells -> 0.

Debug bun with this preview (assertions on, so the freeze itself is not representative): plainReadMs 118 to 161, frozenReadMs 112 to 162 (0.95x to 1.01x); bun 1.4.3 on the same machine: 8.1x to 8.9x.

Also in the WebKit change: Object.isFrozen / isSealed answer from the structure for an array that Object.freeze / seal froze, Object.freeze(Array.prototype) keeps the prototype blank (no ArrayStorage allocation, so the array prototype chain watchpoint survives), and a frozen array notes its read-only elements for objects that inherit from it only when it becomes a prototype, so a frozen array that is no prototype keeps the builtin fast paths.

The WebKit PR overlaps oven-sh/WebKit#622 (Jarred's, open) on one Structure bit; the PR comment there explains how the two merge in either order. The pin moves to the merge commit once oven-sh/WebKit#752 lands.

CI: the one red lane on build 122412, test/js/workerd/html-rewriter-leak.test.ts on debian 13 x64-asan, is the 15 s timeout that #44359 fixes (red on main, 1.79 million handler registrations under ASAN). The other failures passed on retry.

Follow-ups noted: tagged-template strings / raw arrays are still built in sparse mode (JSTemplateObjectDescriptor puts each index with attributes before freezing); class X extends Array instances and Proxies still take the generic SetIntegrityLevel loop.

---

[policy-decision:webkit] gate passed · iteration 5 · 2 files touched

passes on PR (with fix)

Test-only change.

Debug/ASAN (expected pass):
$ bun bd test 'test/js/bun/jsc/frozen-array-representation.test.ts'
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test test/js/bun/jsc/frozen-array-representation.test.ts
bun test v1.4.3 (367d939d9)

test/js/bun/jsc/frozen-array-representation.test.ts:
(pass) frozen arrays keep their elements in the vector > Object.freeze > int32 [131.64ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > double [6.55ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > contiguous [3.99ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > holey [3.83ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > length beyond the vector [6.45ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > an empty array [9.19ms]
(pass) frozen arrays keep their elements in the vector > Object.freeze > an array that already owns a sparse map entry [7.86ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > int32 [6.02ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > double [6.05ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > contiguous [2.18ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > holey [2.55ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > length beyond the vector [4.71ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > an empty array [2.60ms]
(pass) frozen arrays keep their elements in the vector > Object.seal > an array that already owns a sparse map entry [2.49ms]
(pass) frozen arrays keep their elements in the vector > Object.preventExtensions > int32 [5.09ms]
(pass) frozen arrays keep their elements in the vector > Object.preventExtensions > double [2.54ms]
(pass) frozen arrays keep their elements in the vector > Object.preventExtensions > contiguous [2.89ms]
(pass) frozen arrays keep their elements in the vector > Object.preventExtensions > holey [2.36ms]
(pass
... (truncated)
Exit: 0

diff hotspot

scripts/build/deps/webkit.ts                       |   2 +-
 .../js/bun/jsc/frozen-array-representation.test.ts | 332 +++++++++++++++++++++
 2 files changed, 333 insertions(+), 1 deletion(-)

gate history · 3 passed · 2 rejected · iteration 5

evidence per changed file

file                                                 reads  edits  tests
scripts/build/deps/webkit.ts                             0      0     32
test/js/bun/jsc/frozen-array-representation.test.ts      0      3     32

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Walkthrough

The build selects a WebKit autobuild preview. New JavaScriptCore tests check array storage and behavior after freezing, sealing, preventing extensions, and making length non-writable.

Changes

WebKit preview and frozen-array coverage

Layer / File(s)	Summary
Preview selection and locked-array representation scripts/build/deps/webkit.ts, test/js/bun/jsc/frozen-array-representation.test.ts	The configured WebKit build identifier changes. Tests check array values, keys, vector capacity, descriptors, and extensibility across array types and lock operations.
Locked-array operations test/js/bun/jsc/frozen-array-representation.test.ts	Tests cover strict-mode mutations, inherited writes, warmed store sites, and descriptor changes on frozen, sealed, and non-extensible arrays.
Literal isolation and prototype cases test/js/bun/jsc/frozen-array-representation.test.ts	Subprocess tests check later array literals after freezing and operations on a frozen Array.prototype.

Priority: ➖ Normal

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 19757

No actionable merge-blocking issue is established. The preview selection and frozen-array tests are consistent; merge after normal build and test checks pass.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR addresses the coding objective in [#44305]. scripts/build/deps/webkit.ts pins WebKit to autobuild-preview-pr-752-98c438b4, which provides the array-storage change for locked arrays. `test/j…
Out of Scope Changes check	✅ Passed	The changes stay within [#44305]. The WebKit pin supplies the engine fix. The added tests cover the same array-storage behavior, JavaScript mutation semantics, and sparse fallback for frozen, sealed, …
Title check	✅ Passed	The title clearly identifies the main change: upgrading WebKit to keep frozen, sealed, and non-extensible array elements in the ArrayStorage vector.
Description check	✅ Passed	The description explains the problem, implementation, performance impact, limitations, and verification results. It does not use the template headings exactly, but it provides the required information…

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[claude]

Beyond the inline findings, I also checked Bun's native array readers for the shape this bump gives frozen/sealed/non-extensible arrays (ArrayWithSlowPutArrayStorage with a populated vector): the fast paths in src/jsc/bindings/bindings.cpp (getContiguousVector, nextPresentIndex), SerializedScriptValue.cpp and JSDOMConvertSequences.h either gate on Int32/Contiguous/Double and fall back to the generic path, or already switch on ALL_ARRAY_STORAGE_INDEXING_TYPES, which covers SlowPut — so no new native mismatch from the shape change.

Extended reasoning...

The diff is a WebKit pin change to a preview tag plus one new bun:jsc-based test file; no security-sensitive surface is touched. The inline findings (ephemeral preview pin, test-coverage gaps) already signal human attention is needed; this note only records the native-consumer check that was ruled out.

Findings marked 🟡 are optional suggestions and need no follow-up push.

[claude]

🔴 Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview release GitHub deletes. scripts/build/deps/webkit.ts:6 sets WEBKIT_VERSION to autobuild-preview-pr-752-98c438b4, an autobuild-preview-pr-* tag that only exists while oven-sh/WebKit#752 is open. Fix: before merge, pin WEBKIT_VERSION to the merged main commit's autobuild-<sha> (or the 40-hex sha) and confirm prebuilt tarballs exist for every platform x flavor. The description calls the preview pin temporary; that note does not protect main once the engine PR lands.

Why this was flagged

The pin at scripts/build/deps/webkit.ts:6 is autobuild-preview-pr-752-98c438b4. scripts/build/download.ts:320-323 documents that GitHub deletes the autobuild-preview-pr-* release when the WebKit PR merges or closes, so every prebuilt download 404s at once; prebuiltDownloadError at download.ts:328-336 then fails the build with "WebKit preview release is gone". .claude/commands/upgrade-webkit.md:34 requires the bump to the merge commit's autobuild-<sha> before the bun PR merges. On the base branch the pin is the 40-hex sha fb1167ebf2cb9edc1f6771a2c11771b024693ae0 whose release is permanent, so CI and local bun bd keep working. After this merges, the moment oven-sh/WebKit#752 lands or closes, every CI build and every developer build on main breaks until someone edits the pin. The PR description says the pin moves to the merge commit once the WebKit PR lands, but nothing in this checkout enforces that ordering.

Verification: The PR description calls the pin temporary, but the diff as it stands still merges it. scripts/build/deps/webkit.ts:6 sets WEBKIT_VERSION to autobuild-preview-pr-752-98c438b4, which prebuiltUrl() at webkit.ts:68-76 uses verbatim as the release tag. On the base branch the pin is a permanent autobuild-<sha> release, so once oven-sh/WebKit#752 merges or closes every prebuilt bun build fails to download JavaScriptCore.

[robobun]

Intentional for now: the pin follows the preview of oven-sh/WebKit#752 until that PR merges. Before this PR merges the pin moves to the merge commit's autobuild release. Leaving this thread open as the reminder.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

Still open from earlier reviews (5):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…
• Also unresolved: 4 minor or pre-existing.

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.

[robobun]

^{Updated 5:07 PM PT - Oct 1st, 2026}

❌ @autofix-ci[bot], your commit e492787 has 1 failures in Build #122590 (All Failures):

• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

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Actionable comments posted: 1

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Inline comments:
Review comments at @test/js/bun/jsc/frozen-array-representation.test.ts:
- Around line 43-44: Mark the two subprocess-spawning tests, “freezing one
literal leaves the next literal from the same site writable” and “freezing
Array.prototype keeps it blank,” as concurrent; leave the synchronous
parameterized tests in the describe.each/ test.each block unchanged.

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[claude]

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Still open from earlier reviews (1):

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[claude]

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Still open from earlier reviews (1):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.

[claude]

Code review completed

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Still open from earlier reviews (1):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.

[claude]

Still open from earlier reviews (1):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.

[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

Still open from earlier reviews (1):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.

[claude]

Still open from earlier reviews (1):

• 🔴 scripts/build/deps/webkit.ts:6 — Every build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.

[claude]

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 build after the WebKit PR merges or closes will fail to download JavaScriptCore, because the pin is to a preview…

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.