this post was submitted on 02 Jul 2026
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Rust sort_unstable_by with more complex closure unexpectedly shrunk binary (lemdro.id)
submitted 6 days ago by cm0002@lemdro.id to c/rust@programming.dev
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Thing I stumbled upon:

I added boolean argument & if statement inside sort_unstable_by closure:

//from
|&a, &b| compare(a, b) 

//to
|&a, &b| if is_reverse { compare(b, a) } else { compare(a, b) }

I expected binary to grow slightly, but actually got 1KB smaller.

I dug into & found that the reason is: quicksort fn became 980 bytes smaller.

Why? Not sure. Inlining looks same, quicksort just has fewer instructions.

target: arm-unknown-linux-gnueabihf

commit: https://github.com/tracyspacy/fli/commit/a87a98be77cc1972a53e6399716a227d1135e76b

details: https://pastebin.com/qJdTw49n

OC by @tracyspcy@lemmy.ml

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[–] FizzyOrange@programming.dev 3 points 6 days ago

Almost certainly prevents some inlining or loop unrolling or something like that.