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Date: Fri, 24 Sep 2010 14:56:29 +0200
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Subject: [Qemu-devel] Re: [PATCH 1/7] qcow2: Make get_bits_from_size() common
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Christoph Hellwig <hch@lst.de>

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.

Paolo