Re: [boost] Enabling fast swap optimization for Boost types VC9 SP1.

From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-08-26 15:21:18

Niels Dekker - mail address until 2008-12-31 wrote:
> Michael Marcin wrote:
>> I think the only real way to get it right in every case is
>> to have the author of the type specify the proper traits.
> 
> Okay, I give up. A generic way to tweak MSVC's "Swaptimization" just 
> doesn't seem feasable without adding a /lot/ of template specializations.
> 
> See also the reply by Stephan T. Lavavej on my questions at
> http://blogs.msdn.com/vcblog/archive/2008/08/11/tr1-fixes-in-vc9-sp1.aspx
> 
> So any type for which default-construction + swap outperforms 
> copy-construction should preferably have "Swaptimization" enabled. Let's 
> call such a type "Swaptimizable". So should we add a specialization of 
> std::_Move_operation_category to each Boost HPP file that defines a 
> "Swaptimizable" type?  Or would it be preferable to have a single header 
> file, containing std::_Move_operation_category specializations of for 
> all "Swaptimizable" Boost types?
> 
> Note that for some templates whether an instantiation is "Swaptimizable" 
> depends upon its template argument.  For example, "Swaptimization" 
> should be applied to boost::array<std::string>, but not to 
> boost::array<int>.

I think that each library including the proper specializations makes the 
most sense and supports the weakest coupling.

Some utility, maybe in config, should probably be created so that 
libraries can easily enable these swaptimzations for their types. If it 
is not easy and clean people won't do it and that would be a shame 
because the VC team has provided us with a customization point to easily 
achieve some large performance gains.