Function memcpy

freebsd/libkern/bcopy.c:71–147

* Copy a block of memory, handling overlap. * This is the routine that actually implements * (the portable versions of) bcopy, memcpy, and memmove. */

 * (the portable versions of) bcopy, memcpy, and memmove.
 */
void *
memcpy(void *dst0, const void *src0, size_t length)
{
	char		*dst;
	const char	*src;
	size_t		t;

	dst = dst0;
	src = src0;

	if (length == 0 || dst == src) {	/* nothing to do */
		goto done;
	}

	/*
	 * Macros: loop-t-times; and loop-t-times, t>0
	 */
#define	TLOOP(s) if (t) TLOOP1(s)
#define	TLOOP1(s) do { s; } while (--t)

	if ((unsigned long)dst < (unsigned long)src) {
		/*
		 * Copy forward.
		 */
		t = (size_t)src;	/* only need low bits */

		if ((t | (uintptr_t)dst) & wmask) {
			/*
			 * Try to align operands.  This cannot be done
			 * unless the low bits match.
			 */
			if ((t ^ (uintptr_t)dst) & wmask || length < wsize) {
				t = length;
			} else {
				t = wsize - (t & wmask);
			}

			length -= t;
			TLOOP1(*dst++ = *src++);
		}
		/*
		 * Copy whole words, then mop up any trailing bytes.
		 */
		t = length / wsize;
		TLOOP(*(word *)dst = *(const word *)src; src += wsize;
		    dst += wsize);
		t = length & wmask;
		TLOOP(*dst++ = *src++);
	} else {
		/*
		 * Copy backwards.  Otherwise essentially the same.
		 * Alignment works as before, except that it takes
		 * (t&wmask) bytes to align, not wsize-(t&wmask).
		 */
		src += length;
		dst += length;
		t = (uintptr_t)src;