macOS: no x64 baseline build — @oven/bun-darwin-x64-baseline is byte-identical to the AVX2 default #34206

Summary

There is no baseline build for x64 macOS

@oven/bun-darwin-x64-baseline ships the same binary as @oven/bun-darwin-x64 — the AVX2 (-march=haswell) build.

Linux, by contrast, ships a genuine baseline.

Why it matters

On a pre-AVX2 Mac (Sandy/Ivy Bridge — no AVX2/FMA) that binary crashes, and there is no baseline package to fall back to.

This is the upstream cause of downstream reports like anomalyco/opencode#29039 — opencode's -baseline Mac binary SIGILLs on Ivy Bridge.

Who hits it

Pre-Haswell Intel Macs running a macOS that bun supports. Quoting the reports:

anomalyco/opencode#24876:

macOS: 13.7.4
Machine: Macmini6,2 (Intel Ivy Bridge, 2012)
CPU: x86_64 (supports AVX, but NOT AVX2)

macOS 13.7.4 clears bun's 13.0 deployment target; the CPU has no AVX2. That combination is the gap. Same reporter:

This makes the tool unusable on: Intel Macs pre-Haswell (≈ 2013 and earlier) / Any x86_64 CPU without AVX2 support. These machines are still relatively common in development environments.

anomalyco/opencode#29039:

MacBook Pro, Intel Core i7-3520M @ 2.90GHz
macOS 24.6.0 (Darwin)
sysctl -n hw.optional.avx2_0 returns 0 — no AVX2 support

That reporter independently confirmed the packaging bug straight from the Mach-O header:

otool -h shows cpusubtype 3 (CPU_SUBTYPE_X86_64_H — Haswell) for both

For my part, I run OpenCore Legacy Patcher on my mom's 2013 Intel Mac — current macOS, so it clears the deployment target, and the CPU is the only blocker.

Evidence

Every step is reproducible with curl — no bun internals involved.

1. The two darwin packages ship the same binary.

Download both npm packages and hash the bun binary inside each:

ea2f223e94bb2f4bf3050895113c3cf346438f6fa0501c8532284e063f72f7a0  @oven/[email protected]
ea2f223e94bb2f4bf3050895113c3cf346438f6fa0501c8532284e063f72f7a0  @oven/[email protected]

Identical hash, identical 69,173,328 bytes — byte-for-byte the same file. The "baseline" package is not a distinct build.

2. That binary is the AVX2 build.
Disassembling it (Mach-O, x86_64) finds 248,019 AVX2 (ymm) + 7,049 AVX-512 (zmm) instructions — i.e. the modern (-march=haswell) build, which a pre-AVX2 CPU can't run.

3. Linux ships a real baseline; darwin doesn't.

Same comparison on the linux packages — default and baseline are different binaries:

9fd36f87e4b90b07632b987a2e4ec81ca15a62c81bf983190cea6d715be2ad74  @oven/[email protected]
a8f9ebd1770ddc8e55dab7a68d4ec1ec1eebf374bb97cc65cf2c3cb373fc6791  @oven/[email protected]

And the linux baseline carries ~24× fewer AVX2 instructions:

4. Systemic, not a 1.3.14 regression.
bun-darwin-x64 == bun-darwin-x64-baseline at 1.2.20 as well.

5. Confirmed on real pre-AVX2 hardware.
On a rented Sandy Bridge Xeon (AVX + SSE4.2, no AVX2/FMA), the AVX2 build — once it touches the simdutf text-processing path — crashes with a segfault after ~21s / ~3.9 GB RSS. The real SSE4.2 baseline build runs fine on the same machine. So the AVX2 build is what crashes, and a proper darwin baseline would work.

Root cause

The baseline WebKit prebuilt doesn't exist for macOS

From scripts/build/deps/webkit.ts:

"Baseline WebKit artifacts (-march=nehalem, /arch:SSE2 ICU) exist for Linux amd64 (glibc + musl) and Windows amd64. No baseline variant for arm64 or macOS."

bun links a prebuilt WebKit, so with no macOS baseline WebKit there's no macOS baseline bun.

For arm64 that's correct — there's no AVX on arm64, so baseline is x64-only. macOS x64 looks like it got grouped in by accident.

The knock-on effects

• .buildkite/ci.mjs buildPlatforms has no darwin baseline lane (linux has one).
• packages/bun-release/src/platform.ts:41 still declares the bun-darwin-x64-baseline npm package, so it publishes anyway — fed the default binary.
• prebuiltSuffix() already appends -baseline for any x64 target, so bun's URL was always right. Only the artifact was missing.

Fix

I've built the missing artifact and confirmed it works.

Using oven-sh/WebKit's own unmodified macos-cross-release.sh with MACOS_ARCH=x86_64 MARCH_FLAG="-march=nehalem", the baseline macOS WebKit builds cleanly:

So it's two small changes, both mirroring lanes that already exist:

1. oven-sh/WebKit — add a bun-webkit-macos-amd64-baseline lane (+ forward MARCH_FLAG): macOS: build a baseline (non-AVX2) x64 WebKit WebKit#290
2. oven-sh/bun — add { os: "darwin", arch: "x64", baseline: true, … } to buildPlatforms, and correct the now-stale comment: macOS: build a real darwin-x64 baseline #34207 (draft, gated on the WebKit PR)

Prebuilt artifacts from this recipe (a baseline bun + an opencode compiled against it), if anyone with a pre-AVX2 Mac wants to test: https://github.com/tobocop2/opencode-bun-pre-avx2-mac