Alexey Kardashevskiy <a...@ozlabs.ru> writes:
> On 02/24/2015 12:59 AM, Markus Armbruster wrote:
>> Alexey Kardashevskiy <a...@ozlabs.ru> writes:
>>
>>> This adds a helper to get closest bigger power-of-two value.
>>>
>>> Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
>>> ---
>>> Changes:
>>> v2:
>>> * s/up_pow_of_two/pow2ceil/
>>> ---
>>> include/qemu-common.h | 2 ++
>>> util/cutils.c | 9 +++++++++
>>> 2 files changed, 11 insertions(+)
>>>
>>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>>> index 644b46d..ae29748 100644
>>> --- a/include/qemu-common.h
>>> +++ b/include/qemu-common.h
>>> @@ -417,6 +417,8 @@ static inline bool is_power_of_2(uint64_t value)
>>>
>>> /* round down to the nearest power of 2*/
>>> int64_t pow2floor(int64_t value);
>>> +/* round up to the nearest power of 2*/
>>> +int64_t pow2ceil(int64_t value);
>>>
>>> #include "qemu/module.h"
>>>
>>> diff --git a/util/cutils.c b/util/cutils.c
>>> index dbe7412..ecaa440 100644
>>> --- a/util/cutils.c
>>> +++ b/util/cutils.c
>>> @@ -483,6 +483,15 @@ int64_t pow2floor(int64_t value)
>>> return value;
>>> }
>>>
>>> +/* round up to the nearest power of 2*/
>>> +int64_t pow2ceil(int64_t value)
>>> +{
>>> + if (!is_power_of_2(value)) {
>>> + value = 0x8000000000000000ULL >> (clz64(value) - 1);
>>> + }
>>> + return value;
>>> +}
>>> +
>>> /*
>>> * Implementation of ULEB128 (http://en.wikipedia.org/wiki/LEB128)
>>> * Input is limited to 14-bit numbers
>>
>> pow2ceil(INT64_MIN) = INT64_MIN. Should be 1.
>>
>> pow2ceil(INT64_MAX) = INT64_MIN. Garbage.
>>
>> Related: "round down to the nearest power of 2" is defined only for x >
>> 0, but our pow2floor(x) happily returns garbage then.
>>
>> In particular we return 0x8000000000000000ULL >> 64 when value is 0,.
>> Undefined behavior.
>>
>> Here's how I'd do these functions:
>>
>> int64_t pow2floor(int64_t value)
>> {
>> assert(value > 0);
>> return 0x8000000000000000u >> clz64(value);
>> }
>>
>> int64_t pow2ceil(int64_t value)
>> {
>> assert(value <= 0x4000000000000000)
>> if (value <= 1)
>> return 1;
>> return 0x8000000000000000u >> (clz64(value - 1) - 1);
>> }
>>
>
>
> I've read the whole thread, that was cool :)
I indulged in a bit of language lawyering, glad you liked it.
> Do you want me to repost your versions as a patch? I do not feel I
> should since it is totally yours but I can :)
I'll post a patch fixing the existing pow2floor(). I prefer not to add
pow2ceil() without a user. If you have one, I suggest you do your
patches on top of mine.
