Re: [Qemu-devel] [PATCH v2 3/3] host-utils: Simplify pow2ceil()

[Eric Blake]

On 07/27/2017 04:46 AM, Markus Armbruster wrote:
> Cc: Radim Krčmář <address@hidden>
> Signed-off-by: Markus Armbruster <address@hidden>
> ---
>  include/qemu/host-utils.h | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 

>  
> -/* round up to the nearest power of 2 (0 if overflow) */
> +/*
> + * Return @value rounded up to the nearest power of two modulo 2^64.
> + * This is *zero* for @value > 2^63, so be careful.
> + */
>  static inline uint64_t pow2ceil(uint64_t value)
>  {
> -    uint8_t nlz = clz64(value);
> +    int n = clz64(value - 1);
>  
> -    if (is_power_of_2(value)) {
> -        return value;
> +    if (!n) {
> +        /*
> +         * @value - 1 has no leading zeroes, thus @value - 1 >= 2^63
> +         * Therefore, either @value == 0 or @value > 2^63.
> +         * If it's 0, return 1, else return 0.
> +         */
> +        return !value;

Tricky, but the comment is worth its weight in gold.

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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