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Rust sort_unstable_by with more complex closure unexpectedly shrunk binary (lemmy.ml)
submitted 3 weeks ago by tracyspcy@lemmy.ml to c/rust@lemmy.ml
4 comments fedilink hide all child comments

Crossposted from https://lemmy.ml/post/49521856

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 ca 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

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[-] TehPers@beehaw.org 2 points 3 weeks ago

This was also posted here: https://programming.dev/post/52906691

Just FYI if you want to follow that discussion.

[-] tracyspcy@lemmy.ml 2 points 3 weeks ago

thank you! Oh. it seems user is blocked on lemmy.ml or something like that, so i cannot participate in this discussion or even see the post and answers within my instance.

Since I cannot reply where, you can post my reply if you wish: 2 points:

  1. more complex closure and not only control flow, it can be simple arithmetic expression as well, results in actually smaller assembly on arm-unknown-linux-gnueabihf target.
  2. I don't think the loop unrolling plays a big role here. sort_unstable_by() is using quicksort and I suspect that complex closure was outlined to a separate fn, and not copied to quicksort anymore.
this post was submitted on 02 Jul 2026
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