Le 12/09/2011 05:42, Eric Niebler a écrit :
On 9/11/2011 9:20 PM, Joel de Guzman wrote:
On 9/12/2011 1:44 AM, Joel Falcou wrote:
- fusion is also a big hitter : lots of PP and lots of forced instanciation instead of lazy specialization.
PP: right, this can be fixed.
lots of forced instanciation: I don't know what you mean. Can you please be more specific?
I can't speak for Joel F. here, but consider the templates instantiated simply to access the Nth element of a fusion vector. From a cursory inspection of sequence/intrinsic/at.hpp, the following call:
at_c<N>(v);
where v is a fusion vector instantiates:
lazy_disable_if is_const result_of::at_c result_of::at mpl::int_ detail::tag_of extension::at_impl extension::at_impl::apply mpl::at detail::ref_result add_reference
I believe that it also must compute the return type of the const overload of at_c in order to do overload resolution, so that many of the above templates must be instantiated twice: once for a const vector and once for non-const, and throw in an additional add_const. (And come to think of it, Proto probably suffers from this problem too!)
That's a lot of templates for a simple element access. I didn't chase the template breadcrumbs into mpl, so there may be more.
^ This and the fact that the _impl struct are all made like : template<class Tag> struct at_impl; template<> struct at_impl<some_tag> instead of a more CT friendly template<class Tag, class Dummy=void> struct at_impl; template<class Dummy> struct at_impl<some_tag,Dummy> I think heller started played with that and got some measurable CT performance increase THe C++11 rewrite is obviously a long term project, MPL has to go this way too (My secret dream is to merge Fusion and MPL and have MPL be deltype over Fusion calls) and I think at some point we should start thinking of doign it. Fusion laready have a 0x implementation in the SOC sandbox folder but I think it can be pushed a bit more but it'll require us to have access to a couple of strong C++11 enabled compilers. The CT performances of our infrastructure trifecta (Fusion/MPL/Proto) should become target #1 at some point.