Slightly faster linking for 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,
##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
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 [build] rustflags = ["-C", "link-arg=-fuse-ld=lld"]
Cargo can be configured at multiple
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! :)