Enable Link-Time Optimization (LTO)

[zamazan4ik]

Hi!

I noticed that in the build scripts Link-Time Optimization (LTO) for the project is not enabled. I suggest switching it on since it will reduce the binary size (always a good thing to have) and will likely improve the application's performance. If you want to read a bit more about LTO, I can recommend starting from this Rustc documentation (similar pages exist for C compilers like GCC and Clang - -flto compiler switch).

I suggest enabling LTO only for the Release builds so as not to sacrifice the developers' experience while working on the project since LTO consumes an additional amount of time to finish the compilation routine. If you think that a regular Release build should not be affected by such a change as well, then I suggest adding an additional dist or release-lto profile where in addition to regular release optimizations LTO will also be added. Such a change simplifies life for maintainers and others interested in the project persons who want to build the most performant version of the application. E.g., check cargo-outdated Release profile.

Basically, it can be enabled with the following lines for the Rust part:

[profile.release]
lto = true

I have made quick tests (Fedora 41, Rustc 1.83) by adding lto = true to the Release profile. The binary size reduction is the following:

• scp_bpflang: from 5 Mib to 3.9 Mib
• scx_flash: from 4.9 Mib to 3.9 Mib
• scx_lavd: from 5.3 Mib to 4.2 Mib
• scx_layered: from 6 Mib to 4.7 Mib
• scx_loader: from 6 Mib to 4.5 Mib
• scx_mitosis: from 2.1 Mib to 1.8 Mib
• scx_rlfifo: from 3.5 Mib to 2.8 Mib
• scx_rustland: from 5 to 3.9 Mib
• scx_rusty: from 5.5 Mib to 4.4 Mib

I also suggest enabling LTO for the C part of the project - didn't perform such tests though but it should work too.

We discussed a bit this topic with @arighi in Discord - I will provide a PR for the Rust part of the project with enabling LTO. Regarding the C part - I am not sure since I am not an expert in Meson.

Thank you.

[hodgesds]

I suggest enabling LTO only for the Release builds so as not to sacrifice the developers' experience while working on the project since LTO consumes an additional amount of time to finish the compilation routine.

I pretty much only run release builds for development. I think this idea makes sense in general, but it should be done with a separate profile/config on Cargo/Meson to work.

I suggest adding an additional dist or release-lto profile where in addition to regular release optimizations LTO will also be added

This is a good idea.

[likewhatevs]

I pretty much only run release builds for development. I think this idea makes sense in general, but it should be done with a separate profile/config on Cargo/Meson to work.

yeah I just figured I'd stop doing this if this change were made, but a separate profile is a better idea tbh. this change as is would make ci a bit slower also.

[berkus]

I think this idea makes sense in general, but it should be done with a separate profile/

It makes sense for release profile to be used for.. uhm.. releases. And a dev profile can be set up for development in the necessary way.

[likewhatevs]

It makes sense for release profile to be used for.. uhm.. releases. And a dev profile can be set up for development in the necessary way.

Not super opposed to this (nor super in support of it, the rust code isn't doing anything too heavy tbh), but we're running into bad queue'ing on GH free tier already and are a bit unsure how to deal with this (🤔), and we would want to run a build with this profile per commit on ci.

Misc btw, if we're gonna add LTO, we may as well set CUs to 1 (so LTO works best), opt level to 3.

[JakeHillion]

I think there is an argument for LTO by default given we care about performance, e.g. if a random person clones scx and does some performance testing having the best we can by default (release) makes sense. Whether it makes any real difference is the other question. I don't think binary size is a good argument for changing the defaults though, that can be done in release-lto. Thanks for the work so far!

[JohnRTitor]

Regarding the C part - I am not sure since I am not an expert in Meson.

I opened #1155, but got stuck at a linker error. (Not experienced enough with meson to debug this, meson is probably stripping the archive index, which ld linker needs).

Any help would be nice!