Summary - Zero-cost abstraction

Combine monomorphization, fusion and inline to write a performance comparison.

Zero-cost summary

Iterators, closures and generics turn into hand-written code in a release build. Monomorphization is fast but bloats code; the inner-function pattern or dyn reduces it. Adapter chains fuse into a single loop; a needless collect in the middle breaks this. #[inline] and #[cold] are hints. Bounds checks disappear with iterators, zip and a hoisted assert. Every claim should be measured: criterion, godbolt, a profile.

Two calling styles

The same closure can be passed in two ways:

fn uygula_generic<F: Fn(i32) -> i32>(f: F, n: i32) -> i32 {
    f(n)     // F is compiled separately for each closure, inlined
}

fn uygula_dyn(f: &dyn Fn(i32) -> i32, n: i32) -> i32 {
    f(n)     // a single copy, called through the vtable
}

Exercise Two calls

Write the uygula_generic and uygula_dyn functions above. Pass the closure |x| x + 1 with 41 to both and print the results separated by a space.

Show example solution

fn uygula_generic<F: Fn(i32) -> i32>(f: F, n: i32) -> i32 {
    f(n)
}

fn uygula_dyn(f: &dyn Fn(i32) -> i32, n: i32) -> i32 {
    f(n)
}

fn main() {
    let artir = |x| x + 1;
    println!("{} {}", uygula_generic(artir, 41), uygula_dyn(&artir, 41));
}

Test yourself

What is the main difference between a generic closure parameter and <code>&dyn Fn</code>?
- No difference
- dyn is faster
- Generic allocates on the heap
- Generic is inlined; dyn is a vtable call but a single copy is compiled
Show answer

Generic is inlined; dyn is a vtable call but a single copy is compiled — Generic in a hot loop, dyn for rarely called callbacks is usually a good choice.

Dynamic closure

fn uygula_dyn(f: &____ Fn(i32) -> i32, n: i32) -> i32 {
    f(n)
}

Show answer

fn uygula_dyn(f: &dyn Fn(i32) -> i32, n: i32) -> i32 {
    f(n)
}

Group summary

Yineleyici

Same code as a loop
Monomorfizasyon

Code bloat
#[cold]

Rare path
zip

Check-free pairing

Next lesson: std::simd

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