linux-kernel - Re: [PATCH 6/6] lib: add fast path for find_first_*_bit() and find_last

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Message-ID: <YAlaFJpV5Jd9VN2S@smile.fi.intel.com>
Date:   Thu, 21 Jan 2021 12:40:20 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, 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>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Subject: Re: [PATCH 6/6] lib: add fast path for find_first_*_bit() and
 find_last_bit()

On Wed, Jan 20, 2021 at 04:06:30PM -0800, Yury Norov wrote:
> Similarly to bitmap functions, users will benefit if we'll handle
> a case of small-size bitmaps that fit into a single word.
> 
> While here, move the find_last_bit() declaration to bitops/find.h
> where other find_*_bit() functions sit.

...

> +static inline
> +unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
> +{
> +	if (small_const_nbits(size)) {
> +		unsigned long idx;
> +
> +		if (!*addr)
> +			return size;
> +
> +		idx = __ffs(*addr);

> +		return idx < size ? idx : size;

But can't we mask it first, then check for 0 (no bit set) otherwise return the
result of __ffs directly?

Same comment for other similar places.

> +	}
> +
> +	return _find_first_bit(addr, size);
> +}


-- 
With Best Regards,
Andy Shevchenko