Re: [libvirt] [PATCH 2/4] fixup? util: Optimize virBitmapUnion()
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:
> On Fri, May 31, 2019 at 05:22:00PM +0200, Andrea Bolognani wrote:
> > + if (a->nbits < b->nbits &&
> > + virBitmapExpand(a, b->nbits) < 0) {
>
> After this, 'b' can hold b->nbits and 'a' can hold b->nbits+1.
>
> 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.
It would only be a marginal over-allocation if we were dealing with
map_len.
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.
Jano
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