Re: [Boost-users] boost.proto optimization on MSVC (2005/2008)

From: Eric Niebler (eric_at_[hidden])
Date: 2007-12-08 17:30:39

AJD wrote:
> "Eric Niebler" <eric_at_[hidden]> wrote in message 
> news:475AEB69.9070304_at_boost-consulting.com...
>> I'm confused. So your vector2<> type is intended as a wrapper for any
>> kind of terminal, not arrays?
> 
> vector2<> doesn't wrap terminals, it just wraps regular types to provide 
> uniform access to them with a vector (math vector) interface. This way 
> vector2<float[3]>, vector2<float(&)[3]>, and vector2<D3DXVECTOR> could all 
> be used with expressions regardless of their underlying type, and additional 
> types can be used just by specializing vector2<T> and providing the 
> subscript operator as well as some traits.

OK...

>> So how is your example different than any
>> of the examples in proto's docs?
> 
> I based it off of the lazyvector example, but the code does not optimize as 
> expected. (In fact, all I really did was swap out the vector type.)

Your code doesn't optimize because you replaced the terminal type with 
something that proto doesn't recognize as a terminal.

>> Have you tried using proto::extends<>
>> (or BOOST_PROTO_EXTENDS) to extend terminal<T>::type instead of
>> terminal<float[3]>::type ? You'll also need to patch up your grammar to
>> accept any kind of terminal.
> 
> I'm not sure that's the correct approach... proto should not have to know 
> about the internal type of vector2<> at all. I could do this with a helper 
> type where vector2 extends vector2_internal which contains all the 
> functionality, but then I'm faced with the problem that the many 
> specializations of vector2_internal<> could have different constructors 
> which would no longer be accessible through the vector2 interface.

Proto builds trees. Trees have terminals and non-terminals. Any type 
which proto doesn't recognize is assumed to be a terminal and is 
automatically wrapped. Types which proto recognizes are:

- proto::expr<>
- Anything that inherits from proto::extends<>
- Anything type that uses the BOOST_PROTO_EXTENDS macro

Your vector terminals must conform to one of those. Otherwise, proto 
will make it conform by wrapping it, and that makes your optimizer's job 
harder.

The code I sent has a specialization for vector2<float[3]>. Can't you 
just add additional specializations to handle the terminals you're 
interested in? Or define vector2 once as:

template< typename T, typename U = proto::is_proto_expr >
struct vector2
{
     BOOST_PROTO_EXTENDS(
         typename proto::terminal<T>::type
       , vector2<T>
       , VectorDomain
     )

     typedef typename vector_traits<T>::value_type value_type;

     value_type & operator [](std::size_t i)
     {
         return vector_traits<T>::index(proto::arg(*this), i);
     }

     value_type const & operator [](std::size_t i) const
     {
         return vector_traits<T>::index(proto::arg(*this), i);
     }
};

And then specialize vector_traits<>.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com