Linker
- Rust course
- Expert
- Lesson 39/46
- 4 min
Learn linker selection and LTO settings.
The linker
The compiler turns each crate into separate object files; the linker combines them and the libraries into a single binary. Link time becomes significant in large projects; faster linkers (lld, mold) can be used. Settings in the release profile decide the scope of optimization:
[profile.release]
lto = "fat" # cross-crate inlining and dead-code removal
codegen-units = 1 # a single unit: better optimization, slower build
panic = "abort" # no unwind tables: smaller binary
strip = true # strip symbolslto = "thin" gives most of the gain of full LTO in much less time.
Choose settings by measuring
Every setting is a trade-off: speed, size and build time. For size use opt-level = "z", for speed opt-level = 3; different profiles should be tried and measured before shipping. Custom profiles can be defined:
[profile.dagitim]
inherits = "release"
lto = "fat"
codegen-units = 1
$ cargo build --profile dagitimThe profile file is a simple key = value format; tools can read it and produce suggestions.
Practice Read a profile
Read the text "lto = \"fat\"\ncodegen-units = 1\nopt-level = 3" line by line, split each line with split_once('='), trim keys and values, strip the quotes and put them in a BTreeMap. Print the number of settings and the value of lto separated by a space.
Starter code
use std::collections::BTreeMap;
fn main() {
// buraya yaz
}Sample solution
use std::collections::BTreeMap;
fn main() {
let metin = "lto = \"fat\"\ncodegen-units = 1\nopt-level = 3";
let mut ayarlar = BTreeMap::new();
for s in metin.lines() {
if let Some((a, d)) = s.split_once('=') {
ayarlar.insert(a.trim().to_string(), d.trim().trim_matches('"').to_string());
}
}
println!("{} {}", ayarlar.len(), ayarlar["lto"]);
}Expected output
3 fat
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does LTO (link-time optimization) make possible?
- Optimizing functions in different crates together (inlining, dead-code removal) Correct answer
- Faster builds
- Debugging
- Cross compilation
Explanation: Without LTO each crate is optimized separately; small functions in dependencies may not be inlined.
2 Cross-crate optimization
[profile.release]
____ = "fat"[profile.release]
lto = "fat"Explanation: lto turns on link-time optimization.
3 Setting and effect
| Term | Meaning |
|---|---|
lto = "fat" |
Full LTO |
codegen-units = 1 |
Single optimization unit |
panic = "abort" |
Smaller binary |
opt-level = "z" |
Size-focused |