[robobun]

MIMALLOC_COMMIT is the head of oven-sh/mimalloc#62. Move it to the merged commit before this lands.

Behaviour change: none

Problem

• [WTF][JSC] An idle thread releases its free memory again while the allocator holds some of it back WebKit#911 calls mi_on_thread_idle_pending_own, which mimalloc does not have at the commit that main pins (92ef6587). A WebKit archive with that change leaves the symbol undefined.
• If that WebKit change merges first, the next WebKit bump in bun fails to link.

Fix

• Move MIMALLOC_COMMIT to theap: mi_on_thread_idle_pending_own, for a thread that comes back only for its own large pages mimalloc#62, alone, before the WebKit change. It adds the export and one flag in padding of mi_tld_t.
• Correct because nothing calls the export yet. sizeof(mi_tld_t) stays 248, and mi_malloc and mi_free compile to the same bytes.
• Verified: a release build for Linux x64 links and passes test/js/bun/jsc (312 pass), test/js/bun/wasm, test/js/web/atomics.test.ts and test/js/web/workers (444 pass). mimalloc's own suite passes 41 of 41 with the new test cases.

Background

• mi_on_thread_idle() is the sweep a thread makes when it goes idle. It leaves the free blocks of a large page that was just in use, and mi_on_thread_idle_pending() says so. The new answer is true only when such a page belongs to the calling thread.
• Bump WebKit (oven-sh/WebKit#911 preview): an idle JSC thread releases again while mimalloc holds its large blocks back #47013 bumps WebKit to the caller and has the tests. It is based on this branch.
• Considered one PR for both pins. The WebKit pin has to wait for [WTF][JSC] An idle thread releases its free memory again while the allocator holds some of it back WebKit#911. This one does not.

Downsides

• None found. Checked: mimalloc .text grows by 139 bytes, 697 of 703 code sections are identical, and an idle sweep makes two more byte stores.

[robobun]

^{Updated 3:27 AM PT - Oct 27th, 2026}

✅ @robobun, your commit 451f7c4e891063940c82cdbc826592bf0db7382e passed in Build #133529! 🎉

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🧪   To try this PR locally:

bunx bun-pr 47042

That installs a local version of the PR into your bun-47042 executable, so you can run:

bun-47042 --bun