linux-kernel - Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

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Message-ID: <878ufvowcd.fsf@mail.parknet.co.jp>
Date:	Thu, 19 Feb 2015 04:25:54 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Alexander Kuleshov <kuleshovmail@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

Alexander Kuleshov <kuleshovmail@...il.com> writes:

>  static inline unsigned char fat_checksum(const __u8 *name)
>  {
> +	u8 i;
>  	unsigned char s = name[0];
> -	s = (s<<7) + (s>>1) + name[1];	s = (s<<7) + (s>>1) + name[2];
> -	s = (s<<7) + (s>>1) + name[3];	s = (s<<7) + (s>>1) + name[4];
> -	s = (s<<7) + (s>>1) + name[5];	s = (s<<7) + (s>>1) + name[6];
> -	s = (s<<7) + (s>>1) + name[7];	s = (s<<7) + (s>>1) + name[8];
> -	s = (s<<7) + (s>>1) + name[9];	s = (s<<7) + (s>>1) + name[10];
> +
> +	for (i = 1; i < 11; i++)
> +		s = (s << 7) + (s >> 1) + name[i];
> +
>  	return s;
>  }

When I wrote this, IIRC, there was measurable performance
difference. All major gcc versions are enough smart now to optimize this?
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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