linux-kernel - Re: [PATCH 0/5] x86 cleanup: convert round

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Date:	Tue, 29 Jul 2008 10:02:51 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 0/5] x86 cleanup: convert round_up() to roundup()

On Mon, Jul 28, 2008 at 11:41:27PM -0700, Yinghai Lu wrote:
> On Sat, Jul 26, 2008 at 6:39 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Joerg Roedel <joerg.roedel@....com> wrote:
> >
> >> The x86 architecture declares its own round_up macro. But there is a
> >> generic one in <linux/kernel.h> which can also be used. This patchset
> >> replaces the x86 round_up() macro with the generic roundup() one where
> >> possible. The patches have been compile-tested for 32 and 64 bit and
> >> boot-tested for 64 bit. If we can fix the other places too we can get
> >> rid of the x86 specific round_up() in the future.
> >
> > applied to tip/x86/cleanups - thanks Joerg!
> >
> these patches looks like some backward, aka wrong direction.
> 
> #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> 
> #define round_up(x, y) (((x) + (y) - 1) & ~((y) - 1))
> #define round_down(x, y) ((x) & ~((y) - 1))
> 
> round_up looks more efficient.

It may be more efficient depending on the compiler optimizations. The
other problem with the x86 round_up is that it only works when rounding to
multiples of power of 2. This behavior is not represented in the macro
name and also the generic roundup macro does not have this problem.

Joerg

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