[v3] libstdc++/24469

[Paolo Carlini]

Hi Benjamin,

> 	// Counters used to keep track of the amount of blocks
> 	// that are on the freelist/used for a thread id and bin.
>-	size_t volatile		     	_M_free;
>-	size_t volatile			_M_used;
>+	_Atomic_word			_M_free;
>+	_Atomic_word			_M_used;
>  
>
my only immediate concern (I supposed we can tackle the performance 
issues with follow up refinements) with this approach is the size of 
_Atomic_word. Well, actually this concern of mine goes beyond 
mt_allocator... On 64-bit machines we have to make sure the counters are 
64 bit, or, alternately, watch for risks of overflows and do something 
special. I'm afraid that at least some 64-bit targets are not ok at the 
moment... What do you think?

Paolo.