Sorry, didn't see this until now (out all day). Here is a very stupid example that uses `xxbrd` to byteswap a 64-bit quantity.

  #include <errno.h>
  #include <stdio.h>
  #include <stdint.h>
  #include <stdlib.h>
  
  int main(int argc, char **argv) 
        uint64_t v = 0;
        double o;
  
        if (argc != 2) { 
                fprintf(stderr, "usage: %s quantity\n", argv[0]);
                return 1;
        }       
  
        v = strtoull(argv[1], NULL, 0);
        if (errno == EINVAL || errno == ERANGE) {
                perror("strtoull");
                return 1;
        }       
  
        __asm__(
                "xxbrd %0, %1\n"
                :"=f"(o)
                :"f"(*(double *)&v)
        );      
        fprintf(stderr, "0x%lx\n", *(uint64_t *)&o);
        return 0;
  }
  
  % gcc -o xxbrd xxbrd.c
  % ./xxbrd 0x123456789abcdef
  0xefcdab8967452301