I’m not sure when/if this changed, but as of Zig 0.15.2 you can run both zig build -Doptimize=ReleaseFast and zig build --release=fast, and they seem to be equivalent. Is there a difference? Is one of them more canonical? Is either planned to be deprecated?

build-system

squeek502 October 29, 2025, 9:48pm 2

Tricks to improve optimization level? Help

As far as I understand, --release is also a rather confusing flag that was originally (and still is?) only intended for use by “system package maintainers” For an example of something that might be unexpected, if the preferred_optimize_mode field is set in StandardOptimizeOptionOptions, then the <mode> in --release=<mode> is silently ignored (and this behavior is intentional). Relevant issues/PRs: `--release=x` ignored when `preferred_optimize_mode` is set · Issue #19732 · ziglang/zig · GitH…

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-Doptimize vs --release