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

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Date:   Wed, 7 Jun 2017 19:55:56 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
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 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.