gcc/testsuite/gcc.dg/duff-3.c

/* Duff's device is legal C; test to make sure the compiler
   doesn't complain about it.

   Jason Thorpe <thorpej@wasabisystems.com>
   Derived from Tom Duff's original usenet message about the device.  */

/* { dg-do run } */
/* { dg-options "-O2" } */

extern void abort (void);
extern void exit (int);

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

void
duffcpy (char *dst, const char *src, unsigned long size)
{
  unsigned long n = (size + 7) / 8;

  switch (size % 8)
    {
    case 0:	do {	*dst++ = *src++;
    case 7:		*dst++ = *src++;
    case 6:		*dst++ = *src++;
    case 5:		*dst++ = *src++;
    case 4:		*dst++ = *src++;
    case 3:		*dst++ = *src++;
    case 2:		*dst++ = *src++;
    case 1:		*dst++ = *src++;
		} while (--n > 0);
    }
}

const char testpat[] = "The quick brown fox jumped over the lazy dog.";

int
main()
{
  char buf[64];

  duffcpy (buf, testpat, sizeof (testpat));
  if (memcmp (buf, testpat, sizeof (testpat)) != 0)
    abort ();

  exit (0);
}