Re: [boost] [Smart Ptr] make_shared slower than shared_ptr(new) on VC++9 (and 10) with fix

Subject: Re: [boost] [Smart Ptr] make_shared slower than shared_ptr(new) on VC++9 (and 10) with fix
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-04-26 13:41:02

on Thu Apr 26 2012, "Stephan T. Lavavej" <stl-AT-exchange.microsoft.com> wrote:

> [STL]
>> Each is optimally sized (in particular, make_shared/allocate_shared
>> implement the "we know where you live" optimization that I have
>> previously described
>
> [Dave Abrahams]
>> I can't find that description.  Pointer please?
>
> See
> http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/STL11-Magic-Secrets
> (which also has links to my slides - viewable online even without
> PowerPoint), in particular Slide 6. 

9:30 or so in the video.

> I described the optimization in detail, but without code, so you can
> pick it up without reading my sources. Basically, the "traditional"
> control blocks need to keep a pointer to the object so they can delete
> it (because of conversions, the shared_ptr's own pointer cannot be
> used for this purpose - it could have the wrong address with no way to
> restore it at runtime). But if you're willing to write dedicated
> control blocks for make_shared/allocate_shared (instead of trying to
> stuff the object in a special "deleter"), then they can just destroy
> the object in place. "We know where you live", so we don't need to
> store a pointer to the object.
>
> I was surprised when I learned that I was apparently the first one to
> implement this - I just assumed that everyone would write it this
> way. 

Me too :-)

> See slide 7 for measurements - picking up this optimization should
> save you 8 bytes on x86 and 16 bytes (!!!) on x64. That's per object,
> so if you have a lot of them, it adds up.

Yep.

> Consider it a gift - my thanks for all of the wonderful things that
> Boost has given TR1/C++11.

Thanks kindly.  

>> You don't need a special case for emptiness if you make them base
>> classes when they're classes (a.k.a. use boost::compressed_pair)
>
> I should look into implementing that from scratch in VC12. Sometimes I
> dream about making the STL dependent on Boost, and causing the
> universe to recursively implode. It would be fun!

Yeah, who needs an LHC to destroy the universe as we know it?

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com