[Qemu-devel] Re: [PATCH v2 1/7] qcow2: Make get_bits_from_size() common

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 10/08/2010 10:48 AM, 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.
>
> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>    

Acked-by: Anthony Liguori <aliguori@us.ibm.com>

Regards,

Anthony Liguori

> ---
>   block/qcow2.c |   22 ----------------------
>   cutils.c      |   18 ++++++++++++++++++
>   qemu-common.h |    1 +
>   3 files changed, 19 insertions(+), 22 deletions(-)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index ee3481b..6e25812 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -794,28 +794,6 @@ static int qcow2_change_backing_file(BlockDriverState *bs,
>       return qcow2_update_ext_header(bs, backing_file, backing_fmt);
>   }
>
> -static int get_bits_from_size(size_t size)
> -{
> -    int res = 0;
> -
> -    if (size == 0) {
> -        return -1;
> -    }
> -
> -    while (size != 1) {
> -        /* Not a power of two */
> -        if (size&  1) {
> -            return -1;
> -        }
> -
> -        size>>= 1;
> -        res++;
> -    }
> -
> -    return res;
> -}
> -
> -
>   static int preallocate(BlockDriverState *bs)
>   {
>       uint64_t nb_sectors;
> diff --git a/cutils.c b/cutils.c
> index 5883737..6c32198 100644
> --- a/cutils.c
> +++ b/cutils.c
> @@ -283,3 +283,21 @@ int fcntl_setfl(int fd, int flag)
>   }
>   #endif
>
> +/**
> + * Get the number of bits for a power of 2
> + *
> + * The following is true for powers of 2:
> + *   n == 1<<  get_bits_from_size(n)
> + */
> +int get_bits_from_size(size_t size)
> +{
> +    if (size == 0 || (size&  (size - 1))) {
> +        return -1;
> +    }
> +
> +#if defined(_WIN32)&&  defined(__x86_64__)
> +    return __builtin_ctzll(size);
> +#else
> +    return __builtin_ctzl(size);
> +#endif
> +}
> diff --git a/qemu-common.h b/qemu-common.h
> index 81aafa0..e0ca398 100644
> --- a/qemu-common.h
> +++ b/qemu-common.h
> @@ -153,6 +153,7 @@ time_t mktimegm(struct tm *tm);
>   int qemu_fls(int i);
>   int qemu_fdatasync(int fd);
>   int fcntl_setfl(int fd, int flag);
> +int get_bits_from_size(size_t size);
>
>   /* path.c */
>   void init_paths(const char *prefix);
>