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Comment by nmilo | Hacker News Reader

No, GenericWriter takes a function at compile time and it gives you a GenericWriter struct that calls that function (at compile time), no function pointers needed.

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There's definitely overhead with the GenericWriter, seeing as it uses the AnyWriter for every call except `write` (1)

    genericWriter        - 31987.66ns per iterations
    appendSlice          - 20112.35ns per iterations
    appendSliceOptimized - 12996.49ns per iterations

`appendSliceOptimized` is implemented using knowledge of the underlying writer, the way that say an interface implementation in Go would be able to. It's a big part of the reason that reading a file in Zig line-by-line can be so much slower than in other languages (2)

(1) https://gist.github.com/karlseguin/1d189f683797b0ee00cdb8186...

(2) https://github.com/ziglang/zig/issues/17985

Nice! It'd be fun to see a comparison in rust if anyone is keen. - Both code and resulting performance.

I was curious, so I ran your zig version myself and ported it to rust[1].

I think you forgot to run your benchmark in release mode. In debug mode, I get similar results to you. But in release mode, it runs ~5x faster than you reported:

    genericWriter        - 4035.47ns per iterations
    appendSlice          - 4026.41ns per iterations
    appendSliceOptimized - 2884.84ns per iterations

I bet the first two implementations are emitting identical code. But appendSliceOptimized is clearly much more efficient.

For some reason, rust is about twice as fast as zig in this test:

    generic_writer          time:   [1.8812 µs 1.8837 µs 1.8870 µs]
    append_slice            time:   [2.0019 µs 2.0065 µs 2.0126 µs]
    append_slice_raw        time:   [1.6932 µs 1.7041 µs 1.7182 µs]

(1µs = 1000ns)

[1] https://gist.github.com/josephg/e2dd6e7baf0a764a21bd724f8a2e...

Tests running on linux 6.8.0, CPU: Ryzen 7950x. zig v0.13 / rust v1.84.1.

Zig command: zig build --release=fast -Dtarget=native run

Rust command: RUSTFLAGS='-C target-cpu=native' cargo run --release -- --bench

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