notes on current atomics config and usage

[Paolo Carlini]

Paolo Carlini wrote:

> Paolo Carlini wrote:
>
>>> It does allow for CXXFLAGS="-march=native" to D.T.R.T., as discussed 
>>> earlier today.  
>>
>> I particularly like the idea that if one makes sure to have CXXFLAGS 
>> set when configure is run, then the atomic builtins are automatically 
>> picked by the configure test. This is a nice advantage (unrelated to 
>> making maintainance easier) wrt the hardcoded approach in place 
>> before. Makes perfect sense, indeed, and is already used outside 
>> libstdc++.
>
> As a side note, I want to add that this possibility should be 
> exploited only by careful people: if one configures and builds with 
> CXXFLAGS="-march=i686" then _GLIBCXX_ATOMIC_BUILTINS is defined and 
> afterwards, if the user code is not *always* compiled using the same 
> settings the __sync_fetch_and_add calls remain unresolved. Maybe at 
> some point we want to add a short note to the docs about this...

Never mind: I see only now that in this conservative version of the 
patch you can only force the library to use the i386 configuration 
(passing i386), you cannot force it to use the atomic builtins, because 
you didn't remove the cpu/i486 config and the autoconf test is used only 
when cpu/generic would be otherwise selected. Yes, for now at least 
better this safe, conservative change ...

Paolo.