linux-kernel - Re: [PATCH] Bitmap: Optimized division operation to shift operation

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Date:   Wed, 15 Apr 2020 10:56:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wang Qing <wangqing@...o.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dennis Zhou <dennis@...nel.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        David Sterba <dsterba@...e.com>,
        Josef Bacik <josef@...icpanda.com>,
        Stefano Brivio <sbrivio@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Yury Norov <yury.norov@...il.com>,
        linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH] Bitmap: Optimized division operation to shift operation

On Wed, Apr 15, 2020 at 03:27:40PM +0800, Wang Qing wrote:
> On some processors, the / operate will call the compiler`s div lib,
> which is low efficient. Bitmap is performance sensitive, We can
> replace the / operation with shift.
> 
> Signed-off-by: Wang Qing <wangqing@...o.com>
> ---
>  include/linux/bitmap.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 99058eb..85ff982 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -337,7 +337,7 @@ static inline int bitmap_equal(const unsigned long *src1,
>  		return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
>  	if (__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
>  	    IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
> -		return !memcmp(src1, src2, nbits / 8);
> +		return !memcmp(src1, src2, nbits >> 3);
>  	return __bitmap_equal(src1, src2, nbits);
>  }

If your compiler gets this wrong, set it on fire and scatter its remains.