linux-kernel - Re: [PATCH 5/8] bitsperlong.h: introduce SMALL

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Date:   Mon, 1 Feb 2021 23:10:02 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-SH <linux-sh@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Arnd Bergmann <arnd@...db.de>,
        Dennis Zhou <dennis@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        David Sterba <dsterba@...e.com>,
        Stefano Brivio <sbrivio@...hat.com>,
        "Ma, Jianpeng" <jianpeng.ma@...el.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH 5/8] bitsperlong.h: introduce SMALL_CONST() macro

On Mon, Feb 1, 2021 at 5:45 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Sat, Jan 30, 2021 at 11:17:16AM -0800, Yury Norov wrote:
> > Many algorithms become simpler if they are passed with relatively small
> > input values. One example is bitmap operations when the whole bitmap fits
> > into one word. To implement such simplifications, linux/bitmap.h declares
> > small_const_nbits() macro.
> >
> > Other subsystems may also benefit from optimizations of this sort, like
> > find_bit API in the following patches. So it looks helpful to generalize
> > the macro and extend it's visibility.
>
> Hmm... Are we really good to allow 0 as a parameter to it? I remember we had
> a thread at some point where Rasmus explained why 0 is excluded.

Now we pass (nbits - 1) instead of nbits, which is ULONG_MAX in case
of nbits == 0

> > --- a/tools/include/asm-generic/bitsperlong.h
> > +++ b/tools/include/asm-generic/bitsperlong.h
>
> Tools part in a separate patch?
>
> --
> With Best Regards,
> Andy Shevchenko
>
>