LLVM 3.7, x86_64-pc-linux-gnu. The following program fails to link (using libstdc++ from GCC 5.2.0):

--- begin ---
constexpr int memory_order_seq_cst = 5;

struct /* alignas(8) */ Term
{
    int a;
    int b;
};

class Atomic
{
    alignas(8) Term mm;
    
    public:
    
    Term load()
    {
        Term tmp;
        __atomic_load(&mm, &tmp, memory_order_seq_cst);
        return tmp;
    }
};

int main(int argc, char **argv)
{
    Atomic a;
    a.load();
    return 0;
}
--- end ---

Error at link is:
$ clang++ -std=c++11  clangatomic.cc 
/tmp/clangatomic-3242b4.o: In function `Atomic::load()':
clangatomic.cc:(.text._ZN6Atomic4loadEv[_ZN6Atomic4loadEv]+0x16): undefined reference to `__atomic_load_8'
clang-3.7: error: linker command failed with exit code 1 (use -v to see invocation)
$ 


If the 'alignas(8)' for struct Term is uncommented, the code compiles and links ok. IIUC this shouldn't be required because there is an alignas(...) on the 'mm' member.

This may be related to: https://llvm.org/bugs/show_bug.cgi?id=23262
(but that is marked INVALID due to a bug in libstdc++. The example above however does not use libstdc++).

This problem prevents using clang++ + libstdc++ with atomic operations.