linux-kernel - Re: [PATCH 0/6] lib/find_bit: fast path for small bitmaps

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Message-ID: <CAAH8bW8cZYG0HxqVAK4HxZDP3abxkHXsqfhSzJ-amQ_S6yDY_w@mail.gmail.com>
Date:   Fri, 29 Jan 2021 12:51:32 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-m68k@...ts.linux-m68k.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-sh@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     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>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.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>
Subject: Re: [PATCH 0/6] lib/find_bit: fast path for small bitmaps

On Fri, Jan 29, 2021 at 12:45 PM Yury Norov <yury.norov@...il.com> wrote:
>
> Bitmap operations are much simpler and faster in case of small bitmaps, whicn
> fit into a single word. In linux/bitmap.h we have a machinery that allows
> compiler to replace actual function call with a few instructions if bitmaps
> passed into the function is small and its size is known at compile time.
>
> find_*_bit() API lacks this functionality; despite users will benefit from it
> a lot. One important example is cpumask subsystem, when NR_CPUS <= BITS_PER_LONG.
> In the very best case, the compiler may replace a find_*_bit() call for such a
> bitmap with a single ffs or ffz instruction.
>
> Tools is synchronized with new implementation where needed.

Sorry for the broken enumeration . If it's too confusing, please let me know
and I'll resend.