Question about The modified-GNU algorithm

Question

I read the article Optimizing Memcpy improves speed, I have a question about the modified-GNU algorithm, I get an error running the code the src & 0xFFFFFFFC , src is a void pointer, can it be a left operand for ‘&’ ? Did I miss anything here?

Thanks

Error 1 error C2296: '&' : illegal, left operand has type 'const void *’

void * memcpy(void * dst, void const * src, size_t len)
{
    long * plDst = (long *) dst;
    long const * plSrc = (long const *) src;
    if (!(src & 0xFFFFFFFC) && !(dst & 0xFFFFFFFC))
    {
        while (len >= 4)
    {
            *plDst++ = *plSrc++;
            len -= 4;
        }
    }
    char * pcDst = (char *) plDst;
    char const * pcSrc = (char const *) plSrc;
    len += 4;
    while (len--)
    {
        *pcDst++ = *pcSrc++;
    }
    return (dst);
}

Answer 1

You can bitmask pointers - it is a legitimate operation if you know what you are doing.

First off, make sure your compiler is not in C++ mode (since it appears you are using MSVC++) - this should be a warning in C, not an error.

Second, 0xFFFFFFFC is an integer - you should make the appropriate cast for the operation in question (make it a pointer type).

Third, I would expect your compiler/libc already has a fantastic implementation of memcpy - use the built in one, not your own version. Compilers do perform some trickery with C and standard library functions, and may even inline and unroll memcpy loops. When you make your own memcpy, this trickery is usually not done.