Re: notes on current atomics config and usage

[Paolo Carlini <pcarlini at suse dot de>]

Benjamin Kosnik wrote:

> One of the weaknesses of the above three-part plan is that with this in
> place, and somebody who builds libstdc++ with CXXFLAGS="-march=i686",
> then they get the builtin atomics. Cool: this is probably exactly what
> they were expecting.
>
> However, if they install this as a system lib then all users are going
> to have to use the -march flag exactly as configured or else they'll
> get synthesized calls to non-existent builtin atomics functions. (since
> without the -march flag the default arch for this platform is i386,
> with no builtin atomics.)
>  
I see this point in a slightly different way. It all depends on whether 
the atomics (implemented in terms of underlying builtins) are part of 
the library, i.e., exported, or not. If they are part of the library, 
then the user code, irrespective of the -march, will call into the 
library and all will be fine. If, on the other hand, the atomics are 
inlined, then the issue is for our *headers* to recognize the actual 
-march (via __GCC_ATOMIC_BUILTINS, I can't see any other way...) and do 
the right thing.

Of course, I'm not considering *static* linking, which would give us 
troubles anyway, I think it *already* does if one builds a program on a 
machine using the atomics in config/cpu/i486 and then tries to use it on 
an i386...

Paolo.