Messages in this thread | | | Date | Wed, 4 Aug 2010 14:35:04 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] kernel: rounddown helper function |
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On Tue, 3 Aug 2010 11:23:54 -0700 Randy Dunlap <randy.dunlap@oracle.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@redhat.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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