Purpose of USE_ATOMIC_COMPILER_BUILTINS

[Florian Weimer]

On 11/16/2015 07:02 PM, Joseph Myers wrote:
> On Mon, 16 Nov 2015, Florian Weimer wrote:
> 
>> What is the USE_ATOMIC_COMPILER_BUILTINS?  Is it still needed now that
>> we require GCC 4.7 or later to compile glibc?
> 
> Yes.  glibc can't use out-of-line functions from libatomic, since 
> libatomic can only be built after glibc.  (Out-of-line functions from 
> libgcc are OK if it's considered they don't involve performance issues on 
> a particular processor.  Built-in functions might generate calls to either 
> libgcc or libatomic.)  The macro says whether it's known for a given 
> architecture that the __atomic_* built-in functions are suitable both 
> regarding correctness (not generating calls to libatomic functions for the 
> glibc use cases) and performance.

Thanks for the explanation, but I can't say I like this situation.

What do I have to do if I want to add atomic sub-and-fetch with
acquire-release semantics?  Should I simply use CAS instead?

If this is too much work, would it be easier to compile libatomic before
glibc instead?

Florian