Re: [boost] [variant] address of a variant type from the addressofitssub-object?

Subject: Re: [boost] [variant] address of a variant type from the addressofitssub-object?
From: Peter Dimov (lists_at_[hidden])
Date: 2016-04-14 11:47:28

Chris Glover wrote:
> > That's hard to believe. Copying a pointer should be insignificant 
> > compared to the cost of the dispatch. What does the profiler say?
>
> Which compiler? I have found that MSVC is extremely pessimistic with this 
> type of situation and generates unnecessary mov instructions for the 
> pointer in the struct.

It's inside a switch on the type index. A mov instruction or two shouldn't 
matter. The generated code should be something like

    switch( v.which() )
    {
    case 0:

        my_visitor( get<T1>(v) );
        break;

    case 1:

        my_visitor( get<T2>(v) );
        break;

    // and so on
    }

except with about seven layers of templates on top. I just don't see an 
extra pointer copy being of significance here. If the switch is compiled to 
a table you have an indirect jmp; if to a sequence of comparisons, would 
depend on how well they can be predicted.