[boost] Re: Lock-free shared_ptr on g++/x86, test/review needed

From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2005-04-04 15:03:15

Peter Dimov wrote:
[...]
> xadd is branchless; it just returns the old value, whereas inc doesn't. MSVC
> always generates lock xadd, even for _InterlockedIncrement, BTW. 

Well, maybe. But you need value neither for increments nor decrements.
(I mean that for decrements you can simply rely on ZF flag). Oder?

[...] 
>     void release() // nothrow
>     {
>         if( atomic_exchange_and_add( &use_count_, -1 ) == 1 )
>         {
>             dispose();
> 
>             if( (long volatile&)weak_count_ == 1 ) // no weak ptrs
>             {
>                 destroy();
>             }
>             else
>             {
>                 weak_release();
>             }
>         }
>     }
> 
> ?

Nah. For the sake of killing C/C++ volatiles rather sooner than later,
I strongly suggest that you hide that load in asm. Just add load+cmp
followed by ZF branch prior to "lock dec" which also sets ZF, IIRC. 

regards,
alexander.