Static Dispatch vs Dynamic Dispatch

lib.rs

#![feature(test)]

extern crate test;

// 지극히 평범한 함수. 아래 두 함수와의 형평성을 위해 일부러 레퍼런스를 받도록 정의했습니다.
pub fn plain(t: &i64) -> i64 {
    t.double()
}

// 트레이트 객체를 인자로 받는 함수.
// https://doc.rust-lang.org/book/ch17-02-trait-objects.html
pub fn trait_object(t: &dyn Double) -> i64 {
    t.double()
}

// 트레이트로 타입 인자에 조건을 건 제네릭 함수.
pub fn generic<T>(t: &T) -> i64 where T: Double {
    t.double()
}

// Rust 1.26 이후부터는 위의 함수를 아래처럼 줄여 쓸 수도 있습니다.
// https://blog.rust-lang.org/2018/05/10/Rust-1.26.html
pub fn universal_impl_trait(t: &impl Double) -> i64 {
    t.double()
}

pub trait Double {
    fn double(&self) -> i64;
}

impl Double for i64 {
    fn double(&self) -> i64 {
        *self
    }
}


#[cfg(test)]
mod bench {
    use test::{Bencher, black_box};
    use super::*;

    const N: i64 = 10000;

    #[bench]
    fn bench_plain(b: &mut Bencher) {
        let a: Vec<i64> = (1..=N).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &a {
                sum += plain(i);
            }
            black_box(sum);
        });
    }

    #[bench]
    fn bench_trait_object_borrowed(b: &mut Bencher) {
        let a: Vec<i64> = (1..=N).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &a {
                sum += trait_object(i);  // implicit reborrow occurs here
            }
            black_box(sum);
        });
    }

    #[bench]
    fn bench_trait_object_borrowed_2(b: &mut Bencher) {
        let a: Vec<i64> = (1..=N).collect();
        let p: Vec<&dyn Double> = a.iter().map(|i| &*i as &dyn Double).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &p {
                sum += trait_object(*i);
            }
            black_box(sum);
        });
    }

    #[bench]
    fn bench_trait_object_owned(b: &mut Bencher) {
        let a: Vec<Box<i64>> = (1..=N).map(Box::new).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &a {
                sum += trait_object(&**i);
            }
            black_box(sum);
        });
    }

    #[bench]
    fn bench_generic(b: &mut Bencher) {
        let a: Vec<i64> = (1..=N).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &a {
                sum += generic(i);
            }
            black_box(sum);
        });
    }
    
    #[bench]
    fn bench_universal_impl_trait(b: &mut Bencher) {
        let a: Vec<i64> = (1..=N).collect();
        b.iter(|| {
            let mut sum = 0;
            for i in &a {
                sum += universal_impl_trait(i);
            }
            black_box(sum);
        });
    }
}

result1

$ cargo bench
   Compiling …… v0.1.0 (/……/……/……)
    Finished release [optimized] target(s) in 4.45s
     Running target/release/deps/…………

running 6 tests
test bench::bench_generic                 ... bench:       2,363 ns/iter (+/- 2,325)
test bench::bench_plain                   ... bench:       2,658 ns/iter (+/- 1,556)
test bench::bench_trait_object_borrowed   ... bench:      21,093 ns/iter (+/- 19,475)
test bench::bench_trait_object_borrowed_2 ... bench:      25,183 ns/iter (+/- 13,294)
test bench::bench_trait_object_owned      ... bench:      17,931 ns/iter (+/- 12,724)
test bench::bench_universal_impl_trait    ... bench:       2,258 ns/iter (+/- 1,042)

test result: ok. 0 passed; 0 failed; 0 ignored; 6 measured; 0 filtered out

result2

running 6 tests
test bench::bench_generic                 ... bench:       2,443 ns/iter (+/- 2,948)
test bench::bench_plain                   ... bench:       2,221 ns/iter (+/- 1,221)
test bench::bench_trait_object_borrowed   ... bench:      21,436 ns/iter (+/- 18,166)
test bench::bench_trait_object_borrowed_2 ... bench:      24,242 ns/iter (+/- 16,131)
test bench::bench_trait_object_owned      ... bench:      16,996 ns/iter (+/- 4,563)
test bench::bench_universal_impl_trait    ... bench:       2,223 ns/iter (+/- 1,232)

test result: ok. 0 passed; 0 failed; 0 ignored; 6 measured; 0 filtered out

result3

running 6 tests
test bench::bench_generic                 ... bench:       2,352 ns/iter (+/- 656)
test bench::bench_plain                   ... bench:       2,540 ns/iter (+/- 14,684)
test bench::bench_trait_object_borrowed   ... bench:      21,055 ns/iter (+/- 12,510)
test bench::bench_trait_object_borrowed_2 ... bench:      25,084 ns/iter (+/- 9,762)
test bench::bench_trait_object_owned      ... bench:      16,135 ns/iter (+/- 5,920)
test bench::bench_universal_impl_trait    ... bench:       2,427 ns/iter (+/- 1,112)

test result: ok. 0 passed; 0 failed; 0 ignored; 6 measured; 0 filtered out