Re: [Qemu-devel] Re: [PATCH 1/7] qcow2: Make get_bits_from_size() common

[Stefan Hajnoczi]

On Fri, Sep 24, 2010 at 1:56 PM, Paolo Bonzini <address@hidden> wrote:
> On 09/24/2010 10:37 AM, Stefan Hajnoczi wrote:
>>
>> On Fri, Sep 24, 2010 at 01:50:39AM +0400, malc wrote:
>>>
>>> On Thu, 23 Sep 2010, Stefan Hajnoczi wrote:
>>>
>>>> The get_bits_from_size() calculates the log base-2 of a number.  This is
>>>> useful in bit manipulation code working with power-of-2s.
>>>>
>>>> Currently used by qcow2 and needed by qed in a follow-on patch.
>>>
>>> int ilog2 (size_t size)
>>> {
>>>     if (size&  (size - 1))
>>>         return -1;
>>>
>>>     return __builtin_ctzl (size);
>>> }
>>>
>>> Ifdef WIN64 omitted for brevity.
>>
>> What is the situation with WIN64?
>
> long is only 32 bit, so you need __builtin_ctzll.

Thanks Paolo and malc.  It should be easy enough to use the builtin instead.

Stefan