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slides/2025-10-10-how-rust-compiles/index.html
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## and thne you're done
## and then you're done
```
&lt;add&gt;:
@ -277,7 +277,8 @@ fn main() { math::add() }
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# so compile times just depend on the amount of functions?
- no!
- yes...
- but not source functions!
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</pre>
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# generics are slow to compile
- spend N times optimizing the function
- and there's duplicate instances!
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# and the duplicates get worse
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## `#[inline]` (⚙️)
- `#[inline]` enables cross-crate inlining of non-generic functions
- for non-generic functions
- for very small functions, this happens automatically
- for other functions, it doesn't, because it would be slow (try with `-Zcross-crate-inline-threshold=always`)
- for other functions, it doesn't, because it would be slow
- don't over-apply it in a library, but also don't forget about it
- benchmark!
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## what if I want to be able to inline everything across crates?
## being lazy has advantages
- that's why i wrote most of this talk last week
- `#[inline]` means that the function is *never* instantiated if it's never used!
https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/
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## but performance is great, i love performance
- its ok i can wait forever
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## inlining across codegen units in the same crate
## there was some LTO all along
- ThinLTO across different codegen units by default
- in release mode, automatic ThinLTO across codegen units in the same crate
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## all of this sucks
## how do i make my program run quickly?
- wasted duplicate work
- missing optimizations without LTO
- let the compiler inline functions
- libraries: remember `#[inline]`
- binaries: you want LTO
- really, it really needs to inline your functions
- without it, it's so over
- and read this: https://nnethercote.github.io/perf-book
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## `Cargo.toml` config
## how do i make my program compile quickly?
```toml
[profile.release]
lto = "thin"
codegen-units = 1
```
- reduce the amount and size of functions
- importantly: in LLVM IR, not necessarily source!
- duplicate and frequent instantiations are bad
- and read this:
- https://corrode.dev/blog/tips-for-faster-rust-compile-times
- https://doc.rust-lang.org/nightly/cargo/guide/build-performance.html
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## and both? 🥺👉👈
- no
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