linux-kernel - Re: [PATCH v11 1/4] bitops: Introduce the for_each_set

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Message-ID: <CACG_h5raZ5T3X2xHbB5NnPaRS0aqmFDigtjtdmFkmh94qCdNDg@mail.gmail.com>
Date:   Fri, 16 Oct 2020 17:15:48 +0530
From:   Syed Nayyar Waris <syednwaris@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 1/4] bitops: Introduce the for_each_set_clump macro

On Fri, Oct 16, 2020 at 2:46 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Fri, Oct 16, 2020 at 04:23:05AM +0530, Syed Nayyar Waris wrote:
> > On Tue, Oct 6, 2020 at 4:56 PM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > > On Tue, Oct 06, 2020 at 02:52:16PM +0530, Syed Nayyar Waris wrote:
>
> ...
>
> > > > +             return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> > >
> > > Have you considered to use rather BIT{_ULL}(nbits) - 1?
> > > It maybe better for code generation.
> >
> > Yes I have considered using BIT{_ULL} in earlier versions of patchset.
> > It has a problem:
> >
> > This macro when used in both bitmap_get_value and
> > bitmap_set_value functions, it will give unexpected results when nbits or clump
> > size is BITS_PER_LONG (32 or 64 depending on arch).
> >
> > Actually when nbits (clump size) is 64 (BITS_PER_LONG is 64, for example),
> > (BIT(nbits) - 1)
> > gives a value of zero and when this zero is ANDed with any value, it
> > makes it full zero. This is unexpected, and incorrect calculation occurs.
> >
> > What actually happens is in the macro expansion of BIT(64), that is 1
> > << 64, the '1' overflows from leftmost bit position (most significant
> > bit) and re-enters at the rightmost bit position (least significant
> > bit), therefore 1 << 64 becomes '0x1', and when another '1' is
> > subtracted from this, the final result becomes 0.
> >
> > This is undefined behavior in the C standard (section 6.5.7 in the N1124)
>
> I see, indeed, for 64/32 it is like this.
>
> ...
>
> > Yes I have incorporated your suggestion to use the '<<' operator. Thank You.
>
> One side note, consider the use round_up() vs. roundup(). I don't remember
> which one is optimized to divisor being power of 2.

Yes. changed 'roundup' to 'round_up'. 'round_up' is optimized for
power-of-2. Thank you.

Syed Nayyar Waris