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async is not zero-cost #1571

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@kpp

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@kpp

See my post https://www.reddit.com/r/rust/comments/bi9yzs/async_is_not_zerocost/.

Long story short, given the most simple function future::ready among all combinators:

pub async fn ready<T>(value: T) -> T {
    value
}

I benchmarked it against https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.15/futures/future/fn.ready.html. with:

executor::block_on(async {
   for _ in 1..n {
       await!(ready(42));
   }
});

Conclusion:

futures-rs is ~1.85 faster than async version of ready without lto and ~1.20 times faster with lto.

I hope the compiler and futures-rs teams will dig into what's going on and why async fn slows down the code a little bit.

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