Zig's Incremental Compilation Internals

Original Article ↗ Hacker News Discussion ↗

Zig’s new incremental compilation system aims to make rebuilds after small code changes nearly instantaneous by caching fine-grained IR and patching binaries in place, though it currently targets debug builds and self-hosted backends without heavy optimizations. Commenters compare this model to Rust, C, Java, and Fil-C, debating how language design, compilation pipelines, and features like generics, macros, and borrow checking affect both compile times and memory safety guarantees. The thread also touches on alternative strategies such as dynamic linking many small shared libraries, and on how much complexity or runtime cost is acceptable in pursuit of faster iteration and safer low-level code.

Incremental compilation in Zig

  • Current incremental mode works only with Zig’s self-hosted backends (not LLVM), primarily x86_64, and is effectively “debug-only” for now.
  • Future plan: add optimisation passes to self-hosted backends and eventually support some kind of release builds, but cross-function optimizations (e.g., inlining) are fundamentally at odds with this model and will remain limited.
  • Zig’s language design has been adjusted over time (sometimes controversially) specifically to make fine‑grained incremental compilation and semantic analysis tractable.

C compilation and LLVM reliance

  • Incremental recompilation applies only to Zig sources in mixed Zig/C projects; C is compiled via LLVM and not cached at the same granularity.
  • There is a Zig-based C compiler (Aro/arocc) used for translate-c (header translation), but not yet as a general C backend. Longer‑term C-compilation plans are still being designed.

Why a single big binary and incremental linking

  • Some questioned why Zig patches a large debug binary instead of composing many small shared libraries.
  • Responses:
    • Zig uses a single compilation unit model; separate files are organizational, not compilation boundaries, so most incremental work (parsing, semantic analysis) is still needed either way.
    • Splitting into many shared libraries would mostly shift work from the static linker to the dynamic loader, hurting runtime startup; measured experiments with hundreds–thousands of shared libs show noticeable overhead.
    • Incremental linking is acknowledged as complex but considered the better tradeoff; corruption concerns are to be handled via cache separation, corruption detection, and safe cancellation.

Rust, other compilers, and compile times

  • Several comparisons to Rust:
    • Rust already has incremental compilation but suffers from language complexity (macros, proc macros, name resolution, monomorphized generics) and the traditional “compile libraries then link” model.
    • There is ongoing work in Rust to improve incremental behavior (e.g., more granular queries, “relink don’t rebuild”), but re‑architecting is hard.
    • Some argue many languages still waste work by compiling entire libraries even when only a small portion is used; Zig’s demand‑driven model is seen as cleaner.

Memory safety vs language tradeoffs (Rust, Zig, Java, Fil‑C)

  • One camp treats “memory safety by default, with explicit escape hatches” (Rust/Java style) as baseline; another argues that what matters is the useful safe subset and total tradeoff (complexity, performance, iteration speed).
  • Zig is viewed as improving on C (notably spatial safety tools) but still “unsafe by default”; some consider this acceptable given its goals and tooling advantages, others see lack of full memory safety as a deal‑breaker.
  • Extended debate compares Rust, Zig, Java, C, ATS, and Fil‑C:
    • Disagreements over what “memory safe language” should mean, whether Rust’s position is “table stakes,” and how much value explicit safe/unsafe boundaries provide.
    • Fil‑C is discussed as an experimental C variant using capabilities and a runtime to prevent many exploit classes. Supporters highlight stronger guarantees; critics point to runtime overhead, reliance on trapping rather than compile‑time proofs, platform limitations, and immature implementation.
    • Several participants stress that different projects and teams will rationally choose different points on the safety–complexity–performance spectrum.

Hello World and ergonomics

  • The official Zig “hello world” is seen by some as verbose compared to C‑style examples.
  • Defenses:
    • It is “more correct”: explicit IO handles, explicit error handling, and integration with Zig’s IO model.
    • Simpler variants exist (e.g., std.debug.print to stderr) for quick demos; the verbose example intentionally surfaces real‑world concerns instead of hiding them.