fix: add ENABLE_BASELINE support for ARM64 to fix crash on older CPUs

[robobun]

Summary

• Adds ENABLE_BASELINE option for ARM64 architecture (similar to existing x64 baseline support)
• Uses -moutline-atomics flag for ARM64 baseline builds to generate runtime-detected atomic implementations
• Adds -baseline suffix for ARM64 WebKit downloads

Fixes #26556

Problem

Bun 1.3.7 crashes with "Illegal instruction" on older ARM64 CPUs (ARMv8.0-A without LSE support) such as:

• Cortex-A53 (Raspberry Pi 3/4, Amlogic S905X)
• Exynos 9611
• AWS a1 instances

The crash occurs because WebKit's pas_lock.h uses GCC atomic builtins that compile to inline LSE instructions (casa, ldaddl, swpa) when built on ARMv8.1+ CI runners. These instructions don't exist on ARMv8.0-A CPUs.

Solution

The -moutline-atomics flag generates code that performs runtime CPU feature detection and uses the appropriate atomic implementation (LSE on newer CPUs, LL/SC on older CPUs).

Note: This fix requires building and publishing baseline ARM64 WebKit builds with -moutline-atomics for the full fix to work. The Bun build system changes here enable downloading those baseline builds once available.

Test plan

• Code compiles successfully (CMake configuration tested)
• Once baseline ARM64 WebKit is published: Test on Raspberry Pi 4 or AWS a1 instance

🤖 Generated with Claude Code

[robobun]

^{Updated 6:12 AM PT - Jan 29th, 2026}

❌ Your commit 5a48fa9e has 5 failures in Build #36105 (All Failures):

• test/js/node/http/node-http-backpressure.test.ts - SIGKILL on 🐧 25.04 x64
• test/cli/install/bun-upgrade.test.ts - 1 failing on 🐧 3.22 x64
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 14 aarch64
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 13 aarch64
• test/bundler/compile-windows-metadata.test.ts - 1 failing on 🪟 2019 x64
• 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-baseline - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl-baseline - build-zig
• cmake/targets/BuildBun.cmake - cmake error on 🐧 x64-musl-baseline - 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 26557

That installs a local version of the PR into your bun-26557 executable, so you can run:

bun-26557 --bun

[coderabbitai]

Walkthrough

Introduces ENABLE_BASELINE configuration option for ARM64 architecture to enable outline atomics compilation mode. Modifies compiler flags to conditionally apply -moutline-atomics when enabled, and updates WebKit build suffix to include "-baseline" variant.

Changes

Cohort / File(s)	Summary
Configuration Options cmake/Options.cmake	Adds aarch64-specific branch defining ENABLE_BASELINE option for ARMv8.0-A compatibility, defaulting to OFF.
ARM64 Compiler Flags cmake/CompilerFlags.cmake	Introduces conditional logic to switch between -moutline-atomics (when ENABLE_BASELINE enabled) and -mtune=ampere1 for both Windows and non-Windows ARM64 paths.
WebKit Build Configuration cmake/tools/SetupWebKit.cmake	Appends "-baseline" suffix to WEBKIT_SUFFIX variable when ENABLE_BASELINE is true, integrating with existing suffix pipeline.

Suggested reviewers

• Jarred-Sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding ENABLE_BASELINE support for ARM64 to fix crashes on older CPUs.
Description check	✅ Passed	The description covers the problem, solution, and test plan with clear structure and references the linked issue, but deviates from the repository template.
Linked Issues check	✅ Passed	The PR fully addresses the linked issue #26556 by implementing ARM64 baseline support with -moutline-atomics flag and baseline suffix for WebKit downloads.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to implementing ARM64 baseline support as required by issue #26556; no unrelated modifications detected.

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