linux-kernel - Re: [PATCH 2/3]fs/inode: iunique() Optimize Performance

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Message-ID: <20091125144539.GA3520@hack>
Date:	Wed, 25 Nov 2009 22:45:39 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Liuweni <qingshenlwy@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	viro <viro@...iv.linux.org.uk>, akpm <akpm@...ux-foundation.org>,
	jack <jack@...e.cz>, npiggin <npiggin@...e.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	strongzgy <strongzgy@...il.com>, xgr178 <xgr178@....com>,
	Liu Hui <onlyflyer@...il.com>
Subject: Re: [PATCH 2/3]fs/inode: iunique() Optimize Performance

On Wed, Nov 25, 2009 at 07:17:28AM -0700, Matthew Wilcox wrote:
>On Wed, Nov 25, 2009 at 10:12:19PM +0800, Liuweni wrote:
>> @@ -605,8 +605,8 @@ static unsigned long hash(struct super_block *sb, unsigned long hashval)
>>  {
>>  	unsigned long tmp;
>>  
>> -	tmp = (hashval * (unsigned long)sb) ^ (GOLDEN_RATIO_PRIME + hashval) /
>> -			L1_CACHE_BYTES;
>> +	tmp = (hashval * (unsigned long)sb) ^ (GOLDEN_RATIO_PRIME + hashval) >>
>> +			L1_CACHE_SHIFT;
>>  	tmp = tmp ^ ((tmp ^ GOLDEN_RATIO_PRIME) >> I_HASHBITS);
>>  	return tmp & I_HASHMASK;
>>  }
>
>Have you compared the compiler output before/after your change?  I'd be
>amazed if GCC isn't able to optimise division-by-a-constant-power-of-two
>into shift-by-constant.

If a compiler can't do this nowadays, I'd consider it's a bug.

-- 
Live like a child, think like the god.
 
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