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

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-10-01 07:47:38

Peter Dimov 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.

Yes, I understand what is observable (extensional definition).  Yet the 
real meaning of "observable" (intensional definition) is vague (and 
understandably so).

>> 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.

Yet there is no statement of *how* the observable behaviour can be 
observed.  Observation is outside the program and outside the standard 
and so practically implementation-defined.  However, I do feel that 
Alexander's interpretation of volatile memory access is perverse.

Ben.