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: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2016-04-15 04:10:37

2016-04-14 17:47 GMT+02:00 Peter Dimov <lists_at_[hidden]>:

> 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.
>

Thank you for putting it this way. Ultimately my goal is to compare two
ways of representing a (mathematical) expression tree. I tried one with
OO-based virtual dispatch:

```
struct AdditioNode : NodeInterface
{
  unique_ptr<NodeInterface> leftSubexpression, rightSubexpression;
  double eval() override;
};
// etc...
```

And storing variants in a vector and representing links to subnodes as
indexes:

```
struct Addition
{
  int leftSubexpression, rightSubexpression;
};

using Node = variant<Addition, ...>;
using ExpressionTree = vector<Node>;
```

I haven't done any profiling yet, but my benchmarks show, the former one
(with virtual interfaces) is about twice faster. I found it quite
surprising.

Regards,
&rzej