Re: [boost] [sp_counted_base] Why does sp_counted_base.hpp notuseatomic_count for gcc environments?

From: Jerry Lawson (Jerry.Lawson_at_[hidden])
Date: 2007-01-11 14:33:57

Thanks for the response. I will patch my boost with those files.

Just for my education, however, I'd like to understand what was missing from
the gcc primitives that you needed for the sp_counted_base required
implementation? Furthermore, should it be okay for me to continue to use
atomic_count_gcc.hpp's pre-increment and post-increment overloaded operators
as the implementation for my specialization of intrusive_ptr_add_ref(T*) and
intrusive_ptr_add_release(T*) free functions?

Thanks.

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Peter Dimov
> Sent: Thursday, January 11, 2007 11:50 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] [sp_counted_base] Why does 
> sp_counted_base.hpp notuseatomic_count for gcc environments?
> 
> Jerry Lawson wrote:
> > On sparc/gcc environment, boost/detail/sp_counted_base.hpp will use 
> > sp_counted_base_pt.hpp, which of course implements counter 
> protection 
> > via pthread_mutex operations. Other gcc-environments (x86, 
> x64, ia64,
> > ppc) have the concept of using asm directives for atomic 
> increment & 
> > decrement, which obviously is more efficient than the mutex 
> > operations.
> >
> > The boost/detail/atomic_count_gcc.hpp implementation seems 
> like a more 
> > efficient mechanism for incrementing/decrementing the shared_ptr 
> > reference counters. In fact, the shared_ptr_nmt.hpp implementation 
> > does use boost::detail::atomic_count for the reference 
> count (albeit 
> > directly, rather than through the shared_count template).
> >
> > I am using atomic_count as the basis for my internal reference 
> > counting for my intrusive_ptr-based objects and am happy with this.
> >
> > So I'm curious as to why sp_counted_base.hpp does not use 
> the atomic 
> > increment/decrement model for sparc/gcc that it does for other gcc 
> > environments?
> 
> The primitives supplied by g++ were not enough to support 
> sp_counted_base (although this is going to change in 4.2, I 
> think). There is an assembly language version scheduled for 
> 1.35 in the CVS. You can play with it by downloading
> 
> http://boost.cvs.sourceforge.net/*checkout*/boost/boost/boost/
detail/sp_counted_base_gcc_sparc.hpp
> 
> and applying the patch
> 
> http://boost.cvs.sourceforge.net/boost/boost/boost/detail/sp_c
> ounted_base.hpp?r1=1.8&r2=1.9
> 
> There is currently a problem with 64-bit SPARCs in 32-bit 
> mode, where the default architecture isn't v9 (g++ doesn't 
> support the notion of v8+). We're looking into fixing that on 
> the bjam side. 
> 
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