Zig Devlog: Self-Hosted x86 Back End Is Now Default in Debug Mode

Summary

This page contains a curated list of recent changes to main branch Zig. June 08, 2025 Author: Andrew KelleyNow, when you target x86_64, by default, Zig will use its own x86 backend rather than using LLVM to lower a bitcode file to an object file.The default is not changed on Windows yet, because more COFF linker work needs to be done first.The x86 backend is now passing 1987 behavior tests, versus 1980 passed by the LLVM backend. In reality there are 2084 behavior tests, but the extra ones there are generally redundant with LLVM’s own test suite for its own x86 backend, so we only run those when testing with self-hosted x86. Anyway, my point is that Zig’s x86 backend is now more robust than its LLVM backend in terms of implementing the Zig language.Why compete with LLVM on code generation? There are a handful of reasons, but mainly, because we can dramatically outperform LLVM at compilation speed.Benchmark 1 (6 runs): zig build-exe hello.zig -fllvm measurement mean ± σ min … max outliers delta wall_time 918ms ± 32.8ms 892ms … 984ms 0 ( 0%) 0% peak_rss 214MB ± 629KB 213MB … 215MB 0 ( 0%) 0% cpu_cycles 4.53G ± 12.7M 4.52G … 4.55G 0 ( 0%) 0% instructions 8.50G ± 3.27M 8.50G … 8.51G 0 ( 0%) 0% cache_references 356M ± 1.52M 355M … 359M 0 ( 0%) 0% cache_misses 75.6M ± 290K 75.3M … 76.1M 0 ( 0%) 0% branch_misses 42.5M ± 49.2K 42.4M … 42.5M 0 ( 0%) 0% Benchmark 2 (19 runs): zig build-exe hello.zig measurement mean ± σ min … max outliers delta wall_time 275ms ± 4.94ms 268ms … 283ms 0 ( 0%) ⚡- 70.1% ± 1.7% peak_rss 137MB ± 677KB 135MB … 138MB 0 ( 0%) ⚡- 36.2% ± 0.3% cpu_cycles 1.57G ± 9.60M 1.56G … 1.59G 0 ( 0%) ⚡- 65.2% ± 0.2% instructions 3.21G ± 126K 3.21G … 3.21G 1 ( 5%) ⚡- 62.2% ± 0.0% cache_references 112M ± 758K 110M … 113M 0 ( 0%) ⚡- 68.7% ± 0.3% cache_misses 10.5M ± 102K 10.4M … 10.8M 1 ( 5%) ⚡- 86.1% ± 0.2% branch_misses 9.22M ± 52.0K 9.14M … 9.31M 0 ( 0%) ⚡- 78.3% ± 0.1% For a larger project like the Zig compiler itself, it takes the time down from 75 seconds to 20...