linux-kernel - Re: [PATCH] x86 rwsem optimization extreme

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Message-ID: <4B7C6DCE.3080609@zytor.com>
Date:	Wed, 17 Feb 2010 14:29:34 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Zachary Amsden <zamsden@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH] x86 rwsem optimization extreme

On 02/17/2010 02:10 PM, Linus Torvalds wrote:
> 
> 
> On Wed, 17 Feb 2010, Zachary Amsden wrote:
>>
>> The x86 instruction set provides the ability to add an additional
>> bit into addition or subtraction by using the carry flag.
>> It also provides instructions to directly set or clear the
>> carry flag.  By forcibly setting the carry flag, we can then
>> represent one particular 64-bit constant, namely
>>
>>    0xffffffff + 1 = 0x100000000
>>
>> using only 32-bit values.  In particular we can optimize the rwsem
>> write lock release by noting it is of exactly this form.
> 
> Don't do this.
> 
> Just shift the constants down by two, and suddenly you don't need any 
> clever tricks, because all the constants fit in 32 bits anyway, 
> regardless of sign issues.
> 

Why bother at all?  I thought it mattered when I saw __downgrade_write()
as an inline, but in fact it is only ever used inside the
downgrade_write() out-of-line function, so we're talking about saving
*five bytes* across the whole kernel in the best case.  I vote for
leaving it the way it is, and get the very slight extra readability.
There is no point in moving bits around, either.

	-hpa
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