Re: [Qemu-devel] [PATCH] block/io: optimize bdrv_co_pwritev for small requests

[Paolo Bonzini]

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;
> +        }