this post was submitted on 23 Jan 2025
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11
Auto debloat unused pub items from dependencies. (infosec.pub)
submitted 2 weeks ago* (last edited 2 weeks ago) by [email protected] to c/[email protected]
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I shaved off 10 MiB from my binary in 2 hours!

I made a program using Macroquad, then I built it in release mode, the binary was 63 MiB in size.

So I used cargo vendor to have a better look at Macroquad and one of its dependencies, glam.

I then started to delete code, like, lots and lots of code(about 30_000 lines of code); none of it affected my main project, some of it became 'dead_code' just by removing the pub keyword.

The result is that my project was unaffected and the binary went down to 52 MiB.

Is there a way to automate removal of unneeded elements from dependencies? This is potentially huge.

EDIT: I FIGURED IT OUT!!!

My mistake was measuring the size of "target/release", I discovered that that folder contains many "unnecessary" files, like "deps", which greatly bloat the folder'r size, the actual size of my binary is 864K.

I am so relieved.

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[–] [email protected] 9 points 2 weeks ago (3 children)

Actually, dead code eliminination should do the trick, if you're compiling a binary at least (same for a library I think, but there could be re-exports there). Did you compile in release mode?

[–] [email protected] 3 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

To expand: Just configure whatever profile you're using (dev, release, ...) to have link time optimization (lto) enabled:

[profile.release]
lto = "fat"

Reference

[–] [email protected] 1 points 2 weeks ago* (last edited 2 weeks ago)

This really doesn't seem to do the trick, the binary's still at 63MiB.

Also "fat" and true are identical.

Edit: I'm not sure I replied to the right person, ignore this.

[–] [email protected] 1 points 2 weeks ago

I don't recall what the default behavior is with the linker, but it might also benefit from at least thin LTO.

[–] [email protected] 1 points 2 weeks ago (1 children)

The compiler doesn't consider it to be dead code since it's marked pub.

[–] [email protected] 3 points 2 weeks ago (1 children)

Sure, but isn't this in a dependency? Can't be reached when only importing your crate anyways? And if you're building a binary, I don't think this could really considered exported, is what I mean :)

[–] [email protected] 1 points 2 weeks ago

Yes that's exactly what I want. The compiler should stop considering it accessible.

[–] [email protected] 3 points 2 weeks ago (1 children)

@Doods are you using release ? If you want the best dead-code elimination, you can also enable Link time optimisations (LTO): https://doc.rust-lang.org/cargo/reference/profiles.html#lto

[–] [email protected] 1 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

Yes, I am using both LTO and release mode, I can show you:

[profile.release]
opt-level = 3
codegen-units = 1
panic = "abort"
strip = true
lto = true

cargo build --release

[–] [email protected] 1 points 2 weeks ago (1 children)

@Doods I'm surprised you can gain that much with that already enabled!

[–] [email protected] 2 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

Should I bring it up to the 'min-sized-rust' working group or the forums or something?

[–] [email protected] 4 points 2 weeks ago (1 children)

Yes. This behavior seems strange, so either an explanation or investigation by a compiler dev seems like it would be helpful.

[–] [email protected] 2 points 2 weeks ago

Just so you know I figured it out, re-read the post please.

[–] [email protected] 3 points 1 week ago

I'm glad you figured it out.

I don't know what is it with new rustaceans and binary size, but many of them seem to be incessant on wasting time uselessly optimizing it, usually without actually understanding what they're doing, without gaining any relevant benefit, and possibly even causing relevant harm (e.g. degrading run-time performance for no good reason).