Re: [Qemu-devel] [PATCH] qemu-ga: Add guest-getip command

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/17/2012 04:58 AM, Michal Privoznik wrote:
> On 16.02.2012 21:10, Michael Roth wrote:
>> On Thu, Feb 16, 2012 at 06:25:03PM +0100, Michal Privoznik wrote:
>>> This command returns an array of:
>>>
>>>   [ifname, ipaddr, ipaddr_family, prefix, hwaddr]
>>>
>>> for each interface in the system that has an IP address.
>>> Currently, only IPv4 and IPv6 are supported.
>>
>> Cool stuff, seems pretty useful. Some comments below:
>>
>>>
>>> Signed-off-by: Michal Privoznik<mprivozn@redhat.com>
>>> ---
>>>   qapi-schema-guest.json     |   16 +++++
>>>   qga/guest-agent-commands.c |  156 ++++++++++++++++++++++++++++++++++++++++++++
>>>   2 files changed, 172 insertions(+), 0 deletions(-)
>>>
>
>>> +
>>> +/* Count the number of '1' bits in @x */
>>> +static int32_t
>>> +count_one_bits(uint32_t x)
>>> +{
>>> +  x = ((x&  0xaaaaaaaaU)>>  1) + (x&  0x55555555U);
>>> +  x = ((x&  0xccccccccU)>>  2) + (x&  0x33333333U);
>>> +  x = (x>>  16) + (x&  0xffff);
>>> +  x = ((x&  0xf0f0)>>  4) + (x&  0x0f0f);
>>> +  return (x>>  8) + (x&  0x00ff);
>>> +}
>>
>> I think my brain is too small for this. Why not just:
>>
>> for (i = 0, count = 0; i<  32; i++) {
>>      if (x&  (1<<  i)) {
>>          count++;
>>      }
>> }
>> return count;
>>
>> ?
>
> That algorithm I've used computes what is known as Hamming weight:
>
> http://en.wikipedia.org/wiki/Hamming_weight#Efficient_implementation
>
> It has advantage of being constant in time. However, I must admit, it is
> not easy readable so I'd better use something more obvious.
>
> Thanks for review, I'll sent v2.

__builtin_popcount() will do this as a single instruction on many architectures.

Regards,

Anthony Liguori

>
> Michal
>
>