Re: [PATCH 3/3] bitmap: Use memcmp optimisation in more situations

[Andy Shevchenko]

On Thu, Jun 8, 2017 at 5:55 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Thu, Jun 08, 2017 at 04:48:04AM +0300, Andy Shevchenko wrote:
>> > Commit 7dd968163f ("bitmap: bitmap_equal memcmp optimization") was
>> > rather more restrictive than necessary; we can use memcmp() to implement
>> > bitmap_equal() as long as the number of bits can be proved to be a
>> > multiple of 8.  And architectures other than s390 may be able to make
>> > good use of this optimisation.
>>
>> > -       if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0)
>> > +       if (__builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8))
>> >                 return !memcmp(src1, src2, nbits / 8);
>>
>> I'm not sure this is a fully correct change.
>> What exactly ' & 7' part does?
>> For me looks like you may just drop it.
>
> We only need to know if the bottom 3 bits are 0 to apply this optimisation.
> For example, if we have a user which does this:
>
>         nbits = 8;
>         if (argle)
>                 nbits += 8;
>         if (bitmap_equal(ptr1, ptr2, nbits))
>                 blah();
>
> then we can use memcmp() because gcc can deduce that the bottom 3 bits
> are never set (try it!  it works!).  We don't need nbits as a whole to
> be const.

What I'm talking about is that by my opinion the both below are equivalent.
__builtin_constant_p(nbits)
__builtin_constant_p(nbits & 7)

Thus, again, what & 7 does there?

-- 
With Best Regards,
Andy Shevchenko