Re: [libvirt] [PATCH 2/4] fixup? util: Optimize virBitmapUnion()
On Mon, 2019-06-03 at 16:43 +0200, Ján Tomko wrote:
On Mon, Jun 03, 2019 at 04:31:51PM +0200, Andrea Bolognani wrote:
> On Mon, 2019-06-03 at 14:10 +0200, Ján Tomko wrote:
> > if (b->nbits &&
> > a->nbits < b->nbits &&
> > virBitmapExpand(a, b->nbits -1) < 0) {
>
> Yeah, you're right, we need to account for the zero-indexing of bits.
> I mean, it's not like the resulting bitmap would be incorrect either
> way, but we might end up allocating more memory than it's actually
> required.
Actually I do think it would be incorrect. The bitmap size is as
important as its values - e.g. if the resulting bitmap had 4 set bits,
a size of 4 vs 5 would alter the result of virBitmapIsAllSet.
Oh, right, that's something that I hadn't considered. This is pretty
much exactly why I didn't trust my implementation to be correct O:-)
> The first check seems unnecessary, though: the only case in
which the
> argument to virBitmapExpand() would be incorrect is b->nbits == 0,
> but we know that both a->nbits and b->nbits are >= 0 and we also just
> verified that a->nbits < b->nbits, so b->nbits must be >= 1 and the
> argument to virBitmapExpand() will always be correct. Or am I missing
> something?
Right, it will work unless someone changes nbits from size_t to double.
So ACK to drop the first check while adopting your version of that
hunk?
--
Andrea Bolognani / Red Hat / Virtualization