Fast Rebuilds

A build through zig build compiles and runs the build script on every invocation. That is most of a no-op rebuild's cost. Measured on an Apple M4 Max with Zig 0.15.2, the no-op is close to a measurement of zig build startup: about 39 ms to run a trivial build script, about 69 ms before any target declared, about 84 ms for the 16 translation unit project used in the benchmark. Ninja parses a static manifest and answers in about 4 ms.

zaza-drive closes that gap. It is a native build driver that reads a manifest, stats the sources and the header dependencies each object recorded via -MMD -MF, compiles only what changed, and links only when an object changed. A no-op is a handful of stat calls and an exit. Same 16 unit workload, same machine, same compiler in every lane:

On the no-op zaza-drive beats Ninja by a wide margin. On the incremental it is level with zig build, because once a real translation unit recompiles the compiler does the same work in every lane and that work dominates.

The native compiler fast path

The remaining cost is process startup, paid twice on an incremental rebuild: once to compile, once to link. The zig c++ wrapper is about twice the per-invocation cost of the system compiler. Measured, one trivial translation unit, median of ten: zig c++ -c at ~31 ms against Apple clang++ -c at ~15 ms.

zig build drive-native writes a manifest that uses the system default compiler. On the same 16 unit workload the incremental rebuild drops from 127 ms to 47 ms, about 2.7x.

The system compiler is a different compiler from Zig's bundled one. It does not cross-compile and can differ in behaviour, so this is an iteration path. Release and cross builds go through zig build, or through zig build drive, which writes a manifest that uses the faithful zig c++. The two caches never mix: native objects live in .zaza-drive-native/, faithful ones in .zaza-drive/.

zig build drive-native
./zig-out/bin/zaza-drive zig-out/build-native.manifest

tools/zaza-drive/README.md documents the driver, its correctness check, and watch mode. The benchmark that produced these numbers is in benchmarks/; run ./tools/zaza-drive/bench.sh to reproduce them on your own machine.