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

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Message-ID: <CAHp75VfQ7RwBm1SO83BxEU6oDj_zyKZ_yDvRGv8tbYn0e1xpxw@mail.gmail.com>
Date:   Thu, 8 Jun 2017 15:31:23 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 3/3] bitmap: Use memcmp optimisation in more situations

On Thu, Jun 8, 2017 at 5:55 AM, Matthew Wilcox <willy@...radead.org> 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