Re: [Qemu-devel] [PATCH 1/2] bitmap.h: add comments to BITMAP_LAST_WORD_MASK

[Wei Wang <wei.w.wang@intel.com>]

On 07/30/2018 09:19 PM, Juan Quintela wrote:
> Wei Wang <wei.w.wang@intel.com> wrote:
>> On 07/30/2018 03:13 PM, Wei Wang wrote:
>>> This macro was ported from Linux and we've reached an aggreement there
>>> that the corner case "nbits = 0" is not applicable to this macro, because
>>> when "nbits = 0", which means no bits to mask, this macro is expected to
>>> return 0, instead of 0xffffffff. This patch simply adds a comment above
>>> the macro as a note to users about the corner case.
>>>
>>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
>>> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> CC: Juan Quintela <quintela@redhat.com>
>>> CC: Peter Xu <peterx@redhat.com>
>>> ---
>>>    include/qemu/bitmap.h | 1 +
>>>    1 file changed, 1 insertion(+)
>>>
>>> diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
>>> index 509eedd..f53c640 100644
>>> --- a/include/qemu/bitmap.h
>>> +++ b/include/qemu/bitmap.h
>>> @@ -60,6 +60,7 @@
>>>     */
>>>      #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) &
>>> (BITS_PER_LONG - 1)))
>>> +/* "nbits = 0" is not applicable to this macro. Callers should avoid that. */
>>>    #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
>>>      #define DECLARE_BITMAP(name,bits)                  \
>> A better fix would be to directly change the macro to:  nbits ? (~0UL
>>>> (-(nbits) & (BITS_PER_LONG - 1))) : 0,
>> so that we don't need to fix other callers like bitmap_full,
>> bitmap_intersects.
>>
>> So just post this out for a discussion whether it's preferred to just
>> adding note comments as we did for linux or fixing the macro directly.
>>
>> Best,
>> Wei
> On one hand:
> a - we have copied it form linux, we don't want to diverge
> On the other hand:
> b - it is much easier to use if we change the macro
>
> So, it is a tought one.
>
> I slightly preffer b), but will not object to a either.  As you are the
> one doing the patch, your choice.

Thanks Juan. I plan to choose b - fixing the macro directly.

Best,
Wei