[kazinator]

Right; I could do it using nothing but standard C features in C source files, by defining some C compatible assembly language routines. The compare_swap primitive can be an external function, e.g.:

  bool cmpswap64(uint64_t *location, uint64_t if_old, uint64_t then_new);

Code that relies on the header doesn't have to process anything compiler-specific. FFI could be used to bind to that function from non-C languages.