[libvirt] [PATCH 2/4] fixup? util: Optimize virBitmapUnion()

[Ján Tomko]

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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