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

[Paolo Bonzini]

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