How to use __sync_bool_compare_and_swap_16 on x86_64

[Jeff Donner]

Hi,

  __sync_bool_compare_and_swap(int64_t*, int64_t, int64_t) and the
equivalent __sync_bool_compare_and_swap_8(...) work with 4.4.0 (and
trunk),
and __sync_bool_compare_and_swap_16 will /compile/ when you pass
128-byte structs by value, strangely. However, this won't link.
Is it possible to use __sync_bool_compare_and_swap_16 from GCC 4.4.+
at all? And if not, could it be added? I can see where the lack of any
128-byte native type to put in the signature makes a problem.

Thanks,
Jeff

#include <assert.h>
#include <iostream>

using namespace std;


struct Storage128 {
   Storage128(int* ptr_, int refs_)
   : ptr(ptr_)
   , refs(refs_)
   , remnant(0)
   {}

   int* ptr; // 64bits
   int refs;  // 32
   int remnant; // 32
};


int main()
{
   int three = 3;
   int five = 5;
   Storage128 victim(&three, 0);
   Storage128 old(&three, 0);
   Storage128 novo(&five, 0);

   assert(sizeof(victim) == 16);

   bool swapped = __sync_bool_compare_and_swap(&victim, three, five);
   assert(swapped);
   cout << *victim.ptr << endl;
}