memcpy code

#include "string.h"

/*
 * sizeof(word) MUST BE A POWER OF TWO
 * SO THAT wmask BELOW IS ALL ONES
 */
typedef    int word;        /* "word" used for optimal copy speed */

#define    wsize    sizeof(word)
#define    wmask    (wsize - 1)

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

__private_extern__
void * memcpy(void *dst0, const void *src0, size_t length)
{
    char *dst = dst0;
    const char *src = src0;
    size_t t;
    
    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 = (uintptr_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 = *(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;
        if ((t | (uintptr_t)dst) & wmask) {
            if ((t ^ (uintptr_t)dst) & wmask || length <= wsize)
                t = length;
            else
                t &= wmask;
            length -= t;
            TLOOP1(*--dst = *--src);
        }
        t = length / wsize;
        TLOOP(src -= wsize; dst -= wsize; *(word *)dst = *(word *)src);
        t = length & wmask;
        TLOOP(*--dst = *--src);
    }
done:
    return (dst0);
}

__private_extern__ void *
memmove(void *s1, const void *s2, size_t n)
{
    return memcpy(s1, s2, n);
}

__private_extern__ void
bcopy(const void *s1, void *s2, size_t n)
{
    memcpy(s2, s1, n);