On 24/05/2016 15:39, Peter Lieven wrote:
> bytes += offset & (align - 1);
> offset = offset & ~(align - 1);
Because the low bits have been masked away from offset and added to bytes,
> +
> + /* if head and tail fall into the same alignment
> + * we can omit the second read as it would read
> + * the same block again */
> + if ((offset + bytes) & (align - 1) &&
... the first part is just "bytes & (align - 1)"...
> + offset / align == (offset + bytes) / align) {
... and the second part is just "bytes < align" (you can distribute
division over addition because offset / align has no reminder, and
simplify to "0 == bytes / align").
Putting it together, it becomes "bytes > 0 && bytes < align", or even
"bytes < align".
Thanks,
Paolo
> + size_t tail_offs;
> + tail_offs = (offset + bytes) & (align - 1);
> + qemu_iovec_add(&local_qiov, head_buf + tail_offs,
> + align - tail_offs);
> + bytes += align - tail_offs;
> + }
