linux-kernel - Re: [PATCH] kernel: rounddown helper function

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Message-Id: <20100804143504.e7dfade1.akpm@linux-foundation.org>
Date:	Wed, 4 Aug 2010 14:35:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	selinux@...ho.nsa.gov, sds@...ho.nsa.gov, jmorris@...ei.org
Subject: Re: [PATCH] kernel: rounddown helper function

On Tue, 3 Aug 2010 11:23:54 -0700
Randy Dunlap <randy.dunlap@...cle.com> wrote:

> On Tue, 03 Aug 2010 14:16:07 -0400 Eric Paris wrote:
> 
> > The roundup() helper function will round a given value up to a multiple of
> > another given value.  aka  roundup(11, 7) would give 14 = 7 * 2.  This new
> > function does the opposite.  It will round a given number down to the
> > nearest multiple of the second number: rounddown(11, 7) would give 7.
> > 
> > I need this in some future SELinux code and can carry the macro myself, but
> > figured I would put it in the core kernel so others might find and use it
> > if need be.
> > 
> > Signed-off-by: Eric Paris <eparis@...hat.com>
> > ---
> > 
> >  include/linux/kernel.h |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 7d5b10f..d6092fd 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -59,6 +59,7 @@ extern const char linux_proc_banner[];
> >  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
> >  #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> >  #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> > +#define rounddown(x, y) ((x) - ((x) % (y)))
> >  #define DIV_ROUND_CLOSEST(x, divisor)(			\
> >  {							\
> >  	typeof(divisor) __divisor = divisor;		\
> > 
> > --
> 
> I'm more used to seeing it like
> 
> #define DIV_ROUND_DOWN(n, d)	(((n) / (d)) * (d))
> 
> but since multiply/divide/modulus are usually slower, your (SELinux) way is better,
> I suppose.
> 
> and the usual caveats apply:  don't use these macros with expressions (nor with y
> or d == 0).

Yes, it really shouldn't reference its argument twice.  And that's easy
to fix.

A fancy version would detect constant-power-of-two and do an `& (d - 1)'
instead of the modulus.  But probably the compiler does optimisatons in
that case - for unsigned types, at least.
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