string-opt-16.c

gcc-you can use this code to learn something about gcc, and inquire further into linux,

/* Copyright (C) 2001  Free Software Foundation.

   Ensure that builtin memcmp operations when all three arguments
   are constant is optimized and performs correctly.  Taken from
   PR optimize/3508.

   Written by Roger Sayle, 12/26/2001.  */

extern void abort (void);
extern void link_error (void);

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

int
main (int argc)
{
  if (memcmp ("abcd", "efgh", 4) >= 0)
     link_error ();
  if (memcmp ("abcd", "abcd", 4) != 0)
     link_error ();
  if (memcmp ("efgh", "abcd", 4) <= 0)
     link_error ();
  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 void *s1, const void *s2, size_t len)
{
  abort ();
}
#else
/* When not optimizing, the above tests may generate references to
   the function link_error, but should never actually call it.  */
static void
link_error ()
{
  abort ();
}
#endif