string-opt-15.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

/* Copyright (C) 2001  Free Software Foundation.

   Ensure that short builtin memcmp are optimized and perform correctly.
   On architectures with a cmpstrsi instruction, this test doesn't determine
   which optimization is being performed, but it does check for correctness.

   Written by Roger Sayle, 12/02/2001.  */

extern void abort (void);
typedef __SIZE_TYPE__ size_t;
extern int memcmp (const void *, const void *, size_t);
extern char *strcpy (char *, const char *);

int
main ()
{
  char str[8];

  strcpy (str, "3141");

  if ( memcmp (str, str+2, 0) != 0 )
    abort ();
  if ( memcmp (str+1, str+3, 0) != 0 )
    abort ();

  if ( memcmp (str+1, str+3, 1) != 0 )
    abort ();
  if ( memcmp (str, str+2, 1) >= 0 )
    abort ();
  if ( memcmp (str+2, str, 1) <= 0 )
    abort ();

  return 0;
}

#ifdef __OPTIMIZE__
/* When optimizing, all the above cases should be transformed into
   something else.  So any remaining calls to the original function
   should abort.  */
static int
memcmp (const char *p1, const char *p2, size_t len)
{
  abort ();
}
#endif