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

[Juan Quintela <quintela@redhat.com>]

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.

Later, Juan.