memmove broken

[Joakim Tjernlund]

> Joakim Tjernlund writes:
>
> > I just realized that there is something strange going on in zlib.
> > zlib_inflate_fast() does a standard byte copy:
> > do{
> >   *q++ = *r++;
> > while(--c); /* c >= 3 */
> >
> > q is always greater than r and this works. This is something else than a
> > regular mem copy. I havn't figured out yet what is happening here but I don't
> > think there is a problem with backwards_memcpy()
>
> Ah. Yes.  This is actually quite clever.  If you have for example 256
> zero bytes to compress, zlib will output a single literal zero byte
> and then a block reference of length 255 and offset -1, saying that
> the block from 1..255 is the same as the block from 0..254.  So the
> fact that it copies forwards byte by byte is essential.  So you
> couldn't use memcpy() either since it copies in larger chunks than 1
> byte.

I see, thanks.

Currenly this works, but it is PPC specific:
static inline void memcpy_update(Byte **dest, Byte **src, size_t *n)
{
  if(*dest - *src <= 8){ /* memcpy on PPC will do 8 bytes in a chunk */
    **dest = **src;
    --(*n);
    do {
      *++(*dest) = *++(*src);
    } while (--(*n));
    (*dest)++;  (*src)++;
  } else {
    memcpy(*dest, *src, *n);
    *dest += *n;
    *src += *n;
    *n = 0;
  }

Jocke


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