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Date:   Sat, 19 Jun 2021 17:24:15 +0100
Message-ID: <87bl81ddqo.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Lucas Stach <l.stach@pengutronix.de>,
        Russell King <linux+etnaviv@armlinux.org.uk>,
        Christian Gmeiner <christian.gmeiner@gmail.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jean Delvare <jdelvare@suse.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wei Yang <richard.weiyang@linux.alibaba.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Alexey Klimov <aklimov@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, etnaviv@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org, linux-hwmon@vger.kernel.org
Subject: Re: [PATCH 2/3] find: micro-optimize for_each_{set,clear}_bit()
In-Reply-To: <20210618195735.55933-3-yury.norov@gmail.com>
References: <20210618195735.55933-1-yury.norov@gmail.com>
        <20210618195735.55933-3-yury.norov@gmail.com>
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On Fri, 18 Jun 2021 20:57:34 +0100,
Yury Norov <yury.norov@gmail.com> 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().

Really?

> 
> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> ---
>  include/linux/find.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/find.h b/include/linux/find.h
> index 4500e8ab93e2..ae9ed52b52b8 100644
> --- 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);		\

On which architecture do you observe a gain? Only 32bit ARM and m68k
implement their own version of find_first_bit(), and everyone else
uses the canonical implementation:

#ifndef find_first_bit
#define find_first_bit(addr, size) find_next_bit((addr), (size), 0)
#endif

These architectures explicitly have different implementations for
find_first_bit() and find_next_bit() because they can do better
(whether that is true or not is another debate). I don't think you
should remove this optimisation until it has been measured on these
two architectures.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.