[v3] libstdc++/51798

[Benjamin Kosnik]

> The patch uses the weak version of compare_exchange universally, which
> is incorrect in a number of cases.  You wouldn't see this on x86_64;
> you'd have to use a ll/sc target such as powerpc.
> 
> In addition to changing several uses to strong compare_exchange, I
> also optimize the idiom
> 
> 	do
> 	  {
>             var = *m;
> 	    newval = ...;
> 	  }
> 	while (!atomic_compare_exchange(m, &var, newval, ...));
> 
> With the new builtins, VAR is updated with the current value of the 
> memory (regardless of the weak setting), so the initial read from *M
> can be hoisted outside the loop.

nice!

> 
> Ok?
 
cool, thanks for reviewing this. 

I fixed up the line numbers for the header file edits.

-benjamin
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