Re: [boost] Re: [shared_ptr] Interlocked* - possible bug?

From: val salamakha (v_s_ha_at_[hidden])
Date: 2004-10-03 08:52:21

Peter,

--- Peter Dimov <pdimov_at_[hidden]> wrote:

> Ben Hutchings wrote:
> > Peter Dimov <pdimov_at_[hidden]> wrote:
> >> Alexander Terekhov wrote:
> >>> C'mon, volatile is brain-dead.
> >>
> >> Nobody's arguing otherwise. ;-) But a nop it
> isn't.
> >
> > I think Alexander is arguing that without a clear
> definition of what
> > it means for a memory access to be "observable",
> 
> A memory access is observable if and only if the
> variable is volatile. 
> 1.9/6.
> 
> > the fact that
> > volatile memory accesses are "observable
> behaviour" doesn't prevent
> > them from being optimised away under the as-if
> rule.
> 
> A compiler is not allowed to alter the observable
> behavior under the "as if" 
> rule. 1.9/1. 
> 

I recently read the article "Double-Checked Locking,
Threads, Compiler Optimizations, and More" of Scott
Meyers where he touched the Sequence Points and
Observable Behavior, Compiler Optimizations and
Instruction Reodering. He is more convincing than
Alexander; "road to the thread-safe code isn't paved
with volatile". 
(the presentation is available from
http://www.nwcpp.org/Downloads/2004/DCLP_notes.pdf)

Regards

Valery Salamakha

                
_______________________________
Do you Yahoo!?
Declare Yourself - Register online to vote today!
http://vote.yahoo.com