Analysis: bun15 branch (oven-sh/zig fork) vs official Zig 0.15.2

[cataggar]

bun15 branch vs official Zig 0.15.2 — change analysis

bun15 tracks Bun's downstream fork (oven-sh/zig, the upgrade-0.15.2 line), pinned at commit c6df1f40 ("Merge oven-sh/zig#21 — llvm: improve IR attributes and cast flags for better optimization").

It descends cleanly from tag 0.15.2 with 35 commits on top (3 merged PRs: oven-sh/zig#16, oven-sh/zig#17, oven-sh/zig#21), touching ~60 files (+1,641 / −812). The reported compiler version is still 0.15.2 — this is a Bun-tuned 0.15.2, not an upgrade to a newer Zig.

1. LLVM codegen improvements (the headline — oven-sh/zig#21)

• src/codegen/llvm.zig (+339), src/zig_llvm.cpp (+209), lib/std/zig/llvm/Builder.zig, ir.zig, src/codegen/llvm/bindings.zig, src/zig_llvm.h — richer IR attributes and integer/cast flags (nuw/nsw/trunc combinations, etc.) for better optimization.
• LTO bitcode module summary + EnableSplitLTOUnit on -flto.
• Coverage in test/llvm_ir.zig (+90) and new behavior tests (cast/floatop/vector/widening).

2. Target & linker additions

• New aarch64-windows-gnu build target.
• MachO: match arm64e TBD targets when linking arm64 (src/link/MachO.zig, src/link/MachO/Dylib.zig).
• f16 ABI fix on Darwin (CBE + compiler_rt); gate strlen extern off the C backend.

3. Language / tooling tweaks Bun relies on

• "Show private fields in @typeInfo" (src/Sema.zig).
• zig fmt "unprivate" flag + always-call-format-fn (lib/std/zig/Ast/Render.zig, src/fmt.zig).
• Forward build-runner progress when ZIG_PROGRESS is set (backport of Inherit build runner progress node from compiler if stderr is not a TTY ziglang/zig#24733).

4. Downstream std patches ("our patches")

Broad edits to lib/std/posix.zig (548 lines churned), fs/Dir.zig, mem.zig, mem/Allocator.zig (+15), os/windows.zig, os/linux.zig, Io/Writer.zig (+43), crypto (argon2, bcrypt), and Build/Module.zig + Build/Step/Compile.zig (a no_link_obj path).

5. Build system / CI / caching

• Replaces upstream ci.yaml with .github/workflows/bun_build.yaml — Bun's matrix (macos / linux-musl / windows-gnu × {safe, debug}) built via zig-bootstrap, ReleaseSafe vs ReleaseFast variants, releases named by branch (so stable and -fast are distinguishable).
• LLVM build caching (cache keys, include out/host), several "fix build crash" commits, "skip lf flush when no link is set", CMakeLists.txt, build.zig, and a heavily trimmed README.md.

Net assessment

A Bun-tuned 0.15.2 focused on better LLVM optimization/LTO, extra macOS/Windows target support, compiler introspection / zig fmt conveniences, and a custom multi-target build/CI pipeline — layered on the unchanged 0.15.2 base.

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Branch pushed to cataggar/zig as bun15. Comparison base: tag 0.15.2 (e4cbd752c8). Generated from git log/diff 0.15.2..bun15.

[cataggar]

This was a good discussion about this in this thread.
https://ziggit.dev/t/bun-s-zig-fork-got-4x-faster-compilation-times/15183/19