Upgrade WebKit to 5b194ac887f2

[robobun]

Summary

• Upgrade WebKit to latest upstream (5b194ac887f29e2a222e2b3fd97e1378ba4f0298)
• Fix build issues for local WebKit builds

WebKit Changes Summary

Notable Changes

• bmalloc -> mimalloc migration: Major allocator change in upstream (disabled for Bun since we use our own mimalloc)
• B3 compiler: Removed linear scan register allocator (greedy allocator is now the only option)
• 2.6x faster [...set] spread optimization by Sosuke Suzuki (Bun)
• 2.5x faster Array.prototype.slice on ScopedArguments by Sosuke Suzuki (Bun)
• String constant folding: String.fromCharCode and String.prototype.codePointAt now constant-folded in DFG/FTL

Bug Fixes Included

• RegExp.prototype[Symbol.split] incorrectly used fast path when limit is not a number
• TypedArray.prototype.subarray calculated beginByteOffset incorrectly
• WASM debugger: Fixed deadlock during thread suspension
• DFG: Fixed ValueRep reduction preserving speculation checks

Build Changes (this PR)

• Removed DerivedSources/inspector from include paths (headers already in PrivateHeaders/JavaScriptCore)
• Don't define USE_MIMALLOC when building with WEBKIT_LOCAL (avoids macro redefinition)
• WebKit's build.ts updated to disable USE_MIMALLOC (Bun uses its own mimalloc)

Test plan

• Built WebKit with bun build.ts debug
• Built Bun with bun run build:local
• Verified basic functionality (bun-debug --print '42')

🤖 Generated with Claude Code

[robobun]

^{Updated 9:11 PM PT - Jan 21st, 2026}

❌ Your commit a011c82c has 1 failures in Build #35571 (All Failures):

• cmake/targets/BuildBun.cmake - cmake error on 🍎 aarch64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🍎 x64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🍎 x64 - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 aarch64-musl - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64-baseline - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-baseline - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64 - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl-baseline - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🐧 aarch64-musl - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🐧 aarch64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🐧 aarch64 - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl-baseline - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-asan - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-baseline - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-asan - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64 - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🍎 aarch64 - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64 - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64-baseline - build-cpp
• cmake/targets/BuildBun.cmake - cmake error on 🪟 x64-baseline - build-cpp

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🧪   To try this PR locally:

bunx bun-pr 26352

That installs a local version of the PR into your bun-26352 executable, so you can run:

bun-26352 --bun

[coderabbitai]

Walkthrough

Build system tweaks: BuildBun.cmake now conditionally defines USE_MIMALLOC=1 only when WEBKIT_LOCAL is not set; SetupWebKit.cmake updates the default WEBKIT_VERSION hash and removes the inspector DerivedSources include from WEBKIT_LOCAL include paths.

Changes

Cohort / File(s)	Summary
BuildBun.cmake conditional allocator cmake/targets/BuildBun.cmake	USE_MIMALLOC=1 compilation definition is now wrapped with a NOT WEBKIT_LOCAL check to avoid macro redefinition when WEBKIT_LOCAL is set. Comment added explaining rationale.
WebKit setup and include paths cmake/tools/SetupWebKit.cmake	Updated default WEBKIT_VERSION commit hash; removed ${WEBKIT_PATH}/JavaScriptCore/DerivedSources/inspector from WEBKIT_LOCAL include_directories and added comment noting inspector headers are provided via PrivateHeaders/JavaScriptCore.

Possibly related PRs

• Bump WebKit #22957: Updates the same cmake/tools/SetupWebKit.cmake WEBKIT_VERSION default value.
• fix(cmake): fix include paths for local WebKit #25815: Adjusts WEBKIT_LOCAL include_directories handling—specifically related to inspector/JavaScriptCore include paths.
• Upgrade WebKit to d5bd162d9ab2 #25958: Changes both WEBKIT_VERSION default and WEBKIT_LOCAL include path handling in SetupWebKit.cmake.

Suggested reviewers

• Jarred-Sumner

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Upgrade WebKit to 5b194ac887f2' accurately describes the main objective of the pull request, which is to upgrade WebKit to a specific upstream commit version.
Description check	✅ Passed	The pull request description is comprehensive and well-structured, covering what the PR does, test verification, and detailed changes. However, it does not follow the repository's required template structure with the exact sections 'What does this PR do?' and 'How did you verify your code works?'

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