On 06/28/2012 06:06 AM, Max Filippov wrote: > On Wed, Jun 27, 2012 at 8:55 PM, Eric Blake <ebl...@redhat.com> wrote: >> On 06/27/2012 04:34 AM, Orit Wasserman wrote: >
>> if (!is_power_of_2(num_pages)) {
>> num_pages |= num_pages >> 1;
>> num_pages |= num_pages >> 2;
>> num_pages |= num_pages >> 4;
>> num_pages |= num_pages >> 8;
>> num_pages |= num_pages >> 16;
>> num_pages |= num_pages >> 32;
>> num_pages -= num_pages / 2;
>> }
>
> Or
>
> if (!is_power_of_2(num_pages)) {
> num_pages = 0x8000000000000000ULL >> clz64(num_pages);
> }
Indeed. I knew about the gcc builtin for clz, but didn't suggest it
because it is not standardized, and didn't realize that qemu had already
wrapped it in a nice function (and one which is portable even when not
compiling with gcc). It's a shame that libc provides ffs() but not
clz() (since the two operations are rather symmetric (just differing in
which direction they find the first bit), and both useful in
bit-twiddling operations.
--
Eric Blake ebl...@redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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