Re: [boost] Boost.Threads, N2178, N2184, et al

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2007-03-27 08:39:53

Alexander Terekhov <terekhov_at_[hidden]> writes:

> Anthony Williams wrote:

>> Not according to the intel specs. 25366818.pdf (IA32 software developers
>> manual volume 3A), section 7.7.2:
>
> The thing is that x86 native doesn't have officially defined memory 
> model (Itanium mapping may well be stronger than x86 native).
>
> Note that what you quote below was written for testers with scopes on 
> "system bus".
>
>> 
>> "1. Reads can be carried out speculatively and in any order."
>
> However...
>
> http://www.well.com/~aleks/CompOsPlan9/0005.html 
>
> <quote author=an architect at Intel> 
>
> The PPro does speculative and out-of-order loads.  However, 
> it has a mechanism called the "memory order buffer" to ensure 
> that the above memory ordering model is not violated.  Load 
> and store instructions do not get retired until the processor 
> can prove there are no memory ordering violations in the actual 
> order of execution that was used.  Stores do not get sent to 
> memory until they are ready to be retired.  If the processor 
> detects a memory ordering violation, it discards all unretired 
> operations (including the offending memory operation) and 
> restarts execution at the oldest unretired instruction. 
>
> </quote> 

Thanks for the links. The "Writes by a single processor are observed in the
same order by all processors" from later in the same spec I quoted above seems
to be key here. This essentially requires that all stores have release
semantics, and that loads have acquire semantics with respect to data written
by a single CPU.

I'm not sure if there is some obscure example where full release/acquire
semantics aren't upheld, but I can't think of one just now.

Anthony
-- 
Anthony Williams
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