Re: [PATCH 3/3] Do not use std::move when assigning an anonymous object to a unique_ptr.

Simon Marchi
2016-11-24T00:08:14Z

On 2016-11-23 18:31, John Baldwin wrote:
> On Wednesday, November 23, 2016 04:19:29 PM Simon Marchi wrote:
>> On 2016-11-23 15:06, John Baldwin wrote:
>> > Using std::move forces an extra copy of the object. These changes fix
>> > -Wpessimizing-move warnings from clang.
>>
>> For those who, like me, do not quite understand what is happening
>> here,
>> I suggest the following read:
>>
>> https://www.ibm.com/developerworks/community/blogs/5894415f-be62-4bc0-81c5-3956e82276f3/entry/RVO_V_S_std_move?lang=en
>
> My head also hurts. I think what clang is warning about is that the
> std::move() in these lines breaks RVO for the function being called,
> not the function that the modified line belongs to. That is:
>
> foo = bar ();
>
> Is able to do RVO if bar() does the right things for RVO to work.
> However:
>
> foo = std::move (bar ());
>
> forces an extra copy of the object since the return value of bar
> can't use the storge of 'foo' directly, it has to be copied into
> an anonymous object (I think) for std::move to consume.
>
> The commit log for the warning is here:
>
> http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20150427/128053.html
>
> I think these instances fall under the "using a move to create a new
> object
> from a temporary object" case.

That's what I understand. Without the move, the object is constructed
directly in the caller's stack, so no move/copy is required at all. It
seems like the warning works as intended and is useful.