Re-implementing memcpy
Not very fast
Your memcpy() implementation is not really better than a standard byte by byte copy.  Even though you attempt to copy more bytes at a time, the limiting factor isn't actually the number of bytes you copy per instruction.
If you research the various memcpy() implementations there are for x86 targets, you will find a wealth of information about how to get faster speeds.  I think the simplest thing for you to do is to just use the simple &quot;rep movsb&quot; implementation.
My own benchmarks
I ran your version against the following two versions.  One is a straightforward byte by byte copy, and the other is just using &quot;rep movsb&quot;, which on modern processors is highly optimized:
void *memcpy2(void *dst, const void *src,size_t n)
{
    size_t i;

    for (i=0;i&lt;n;i++)
        *(char *) dst++ = *(char *) src++;
    return dst;
}

void *memcpy3(void *dst, const void *src, size_t n)
{
    void *ret = dst;
    asm volatile(&quot;rep movsb&quot; : &quot;+D&quot; (dst) : &quot;c&quot;(n), &quot;S&quot;(src) : &quot;cc&quot;, &quot;memory&quot;);
    return ret;
}

My results for copying 2GB (32-bit host):
OP's function  : 3.74 sec
memcpy2 (naive): 3.74 sec
memcpy3 (movsb): 2.96 sec