Slightly faster linking for Rust 08 Jan 2020
otto rust

Build performance has always been important to me, but my pain tolerance has always varied widely depending on the project. The projects I have worked on which require the JVM, such as Jenkins or JRuby/Gradle, anything under 30 seconds seems amazing. For small Node and Ruby projects, anything over a few seconds feels atrocious. Since I’ve been hacking with Rust lately, I haven’t been able to figure out what constitutes “acceptable.” For my relatively small project, incremental compilation was very quick, but for some reason linking the project would talk almost 10 seconds. That seemed pretty unacceptable.

My first course of action for things I don’t understand hasn’t really changed much in the last 15 years: I go complain and look pitiful on IRC. Fed up with this link time adversely affecting my test-driven development workflow in Rust, I visited ##rust on the Freenode network and asked for some tips on how to improve link time.

Unfortunately nobody had any good suggestions for measuring or understanding link time, but since Otto is open source, I shared the repository and a couple lurkers in the channel were able to reproduce the link time, and we all decided: yes, this is way too slow. One suggestion was to try the CLANG linker, rather than use the default GNU ld linker.

We were all independently able to verify that using lld brought the link time from 10s down to just around 3s, which is a great improvement! This change can be codified for Cargo for the snippet below:


# Dramatically increases the link performance for the eventbus
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

Cargo can be configured at multiple levels, but for my project I opted for $GITROOT/.cargo/config rather than adding a user-global Cargo configuration. I don’t believe there is any downside to making it a user-global, but in my case I wanted to check the configuration into the project to make sure that other contributors would benefit from the change as well.

Three seconds for linking my relatively small sample project still feels a bit long, but then I remind myself that I’m pulling in approximately 255 crates (!) as dependencies, and that’s a lot of linking that needs to be done by the compiler, even for incremental builds!

If you’re looking for faster link-time with Rust, try lld, and if that doesn’t work, remove some of the fatter dependencies! :)