Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK

[Wei Wang]

On 07/26/2018 08:10 PM, Yury Norov wrote:
> On Thu, Jul 26, 2018 at 06:15:59PM +0800, Wei Wang wrote:
> > External Email
> >
> > On 07/26/2018 05:37 PM, Yury Norov wrote:
> > > On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote:
> > > > The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if nbits is
> > > > 0. This patch changes the macro to return 0 when there is no bit needs to
> > > > be masked.
> > > I think this is intentional behavour. Previous version did return ~0UL
> > > explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h: improve
> > > BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus.
> > Yes, I saw that. But it seems confusing for the corner case that nbits=0
> > (no bits to mask), the macro returns with all the bits set.
> >
> >
> > > Introducing conditional branch would affect performance. All existing
> > > code checks nbits for 0 before handling last word where needed
> > > explicitly. So I think we'd better change nothing here.
> > I think that didn't save the conditional branch essentially, because
> > it's just moved from inside this macro to the caller as you mentioned.
> > If callers missed the check for some reason and passed 0 to the macro,
> > they will get something unexpected.
> >
> > Current callers like __bitmap_weight, __bitmap_equal, and others, they have
> >
> > if (bits % BITS_PER_LONG)
> >      w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
> >
> > we could remove the "if" check by "w += hweight_long(bitmap[k] &
> > BITMAP_LAST_WORD_MASK(bits % BITS_PER_LONG));" the branch is the same.
> But your patch doesn't remove external conditional, and it fact
> introduces overhead, right? Also, in some cases it's not so trivial to
> remove it. Consider __bitmap_intersects() for example.
>
> Anyway, this patch changes the very basic API. In that case you should
> check every user of the macro to be safe against the change, including
> possible performance downsides.
>
> If you find this corner case behavior of macro confusing, I think that
> the better option would be introducing detailed comment to the
> BITMAP_LAST_WORD_MASK(), or writing wrapper around it that handles
> nbits == 0 as you expect.

OK. Thanks Yury and Andy for the discussion. It seems the more preferred 
way is just to add comments as a note. Agree with that.

Best,
Wei