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

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Message-ID: <CAHp75VdzKn7Kg_vQodk-7Sqz0HjaHG-JxR_ce-rt8+cke7OerA@mail.gmail.com>
Date:   Thu, 8 Jun 2017 04:48:04 +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 Wed, Jun 7, 2017 at 5:29 PM, Matthew Wilcox <willy@...radead.org> 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.

-- 
With Best Regards,
Andy Shevchenko