Re: Use C++11 direct init in __debug::forward_list

[Jonathan Wakely <jwakely at redhat dot com>]

On 11/10/18 22:46 +0200, François Dumont wrote:
> This patch makes extensive use of C++11 direct init in 
> __debug::forward_list.
>
> Doing so I also try to detect useless creation of safe iterators in 
> debug implementation. In __debug::forward_list there are severals but 
> I wonder if it is worth fixing those. Most of them are like this:
>
>       void
>       splice_after(const_iterator __pos, forward_list& __list)
>       { splice_after(__pos, std::move(__list)); }
>
> __pos is copied.
>
> Do you think I shouldn't care, gcc will optimize it ?

I think the _Safe_iterator construction/destruction is too complex to
be optimised away (it locks a mutex, doesn't it?).

Normally I'd say you could use std::move(__pos) but IIRC that's even
more expensive than a copy, as it locks two mutexes.

> I wonder if it would be ok in debug implementation to use this kind of 
> signature:
>
> void splice_after(const const_iterator& __pos, forward_list& __list)
>
> Iterator taken as rvalue reference ?
>
> I guess it is not Standard conformant so not correct but maybe I could 
> add a private _M_splice_after with this signature.

It doesn't seem worthwhile to me.