Re: [review][LEAF] Review of LEAF starts today : May 22 - May 31

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2020-06-02 13:55:07

Am 02.06.20 um 15:38 schrieb Peter Dimov via Boost:
> Alexander Grund wrote:
>
>> With an API like `result<Foo> do() noexcept` instead of `Foo do()` 
>> you won't get (N)RVO when you return a `Foo` (in the happy case) as 
>> the standard only allows it when the types are exactly the same.
>
> This doesn't matter much nowadays. E.g. in 
> https://godbolt.org/z/zC-Cah if you remove x.f(), x goes away.
Not sure if that really proves the point. I think you need to compare a 
result<X> function to a function returning X directly. Like so: 
https://godbolt.org/z/JFmn_j

And for that there is a difference: There are 4 main memory accesses in 
the result<X> function and none in the "regular" one.

Writing and reading those values likely influences performance for the 
worse. However it looks like for large objects (like Y above) there 
seems to be no difference indeed and it very much looks like RVO is 
happening. Didn't expect that.

So there is only a difference for small and trivial types which will be 
passed through RAM instead of registers.