linux-kernel - Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}

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Date:   Thu, 26 Aug 2021 15:57:13 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
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        kvm@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Alexander Lobakin <alobakin@...me>,
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        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 11/17] find: micro-optimize for_each_{set,clear}_bit()

On Sat 2021-08-14 14:17:07, Yury Norov wrote:
> The macros iterate thru all set/clear bits in a bitmap. They search a
> first bit using find_first_bit(), and the rest bits using find_next_bit().
> 
> Since find_next_bit() is called shortly after find_first_bit(), we can
> save few lines of I-cache by not using find_first_bit().

Is this only a speculation or does it fix a real performance problem?

The macro is used like:

	for_each_set_bit(bit, addr, size) {
		fn(bit);
	}

IMHO, the micro-opimization does not help when fn() is non-trivial.


> --- a/include/linux/find.h
> +++ b/include/linux/find.h
> @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
>  #endif
>  
>  #define for_each_set_bit(bit, addr, size) \
> -	for ((bit) = find_first_bit((addr), (size));		\
> +	for ((bit) = find_next_bit((addr), (size), 0);		\
>  	     (bit) < (size);					\
>  	     (bit) = find_next_bit((addr), (size), (bit) + 1))
>  

It is not a big deal. I just think that the original code is slightly
more self-explaining.

Best Regards,
Petr