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

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-10-01 05:33:37

Peter Dimov <pdimov_at_[hidden]> wrote:
> Alexander Terekhov wrote:
> > Peter Dimov wrote:
<snip> 
> >> The observable behavior is
> >> 
> >> write volatile @a 1
> > 
> >  nop
> > 
> >> read volatile @a x
> > 
> >  nop
> > 
> >> write volatile @a x-1
> > 
> >  nop
> > 
> >> exit(x-1)
> > 
> >  push    0
> >  call    _exit
> > 
> >> 
> >> in the first case, and
> > 
> > Happy now (debugger notwithstanding)?
> 
> No. A conforming compiler is not allowed to do that.
> 
> > 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", the fact that
volatile memory accesses are "observable behaviour" doesn't prevent
them from being optimised away under the as-if rule.

I expect most implementations treat "observable" as "observable by
a software or hardware debugger", but of course the standard doesn't
and can't say that.