linux-kernel - Re: [PATCH 24/38] move round

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Message-ID: <4B5A05A1.3040901@zytor.com>
Date:	Fri, 22 Jan 2010 12:08:01 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Joe Perches <joe@...ches.com>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 24/38] move round_up/down to kernel.h

On 01/22/2010 11:28 AM, Joe Perches wrote:
> On Fri, 2010-01-22 at 08:52 -0600, Christoph Lameter wrote:
>> On Fri, 22 Jan 2010, Yinghai Lu wrote:
>>> +#define __round_mask(x,y) ((__typeof__(x))((y)-1))
>>> +#define round_up(x,y) ((((x)-1) | __round_mask(x,y))+1)
>>> +#define round_down(x,y) ((x) & ~__round_mask(x,y))
>> s/round/round_power2/
> 
> s/round/round_power_of_2/
> BUILD_BUG_ON_NOT_POWER_OF_2
> 

Uh... the whole reason why the classical rounding functions don't work
is that we're using it with non-constants, and therefore the compiler
doesn't know that we're restricted to powers of two (and therefore it
will generate divides.)

	-hpa
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