On Wed, 2010-08-04 at 14:35 -0700, Andrew Morton wrote:
> On Tue, 3 Aug 2010 11:23:54 -0700
> Randy Dunlap <randy.dunlap@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >
> > > 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.
Are you suggesting something like
#define rounddown(n, d) ({ typeof(n) __n = (n); __n - (__n % (d)); })
If that's what you are hoping for, would you also like to see a patch
doing the same thing for roundup() ?
> 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.
I don't think we really need to. My quick test shows:
#define rounddown(n, d) ({typeof((n)) __n = (n); (__n - (__n % (d)));})
int round7(unsigned int a)
{
return rounddown(a, 7);
}
int round4(unsigned int a)
{
return rounddown(a, 4);
}
0000000000400504 <round7>:
400504: b9 07 00 00 00 mov $0x7,%ecx
400509: 89 f8 mov %edi,%eax
40050b: 31 d2 xor %edx,%edx
40050d: f7 f1 div %ecx
40050f: 89 f8 mov %edi,%eax
400511: 29 d0 sub %edx,%eax
400513: c3 retq
0000000000400514 <round4>:
400514: 89 f8 mov %edi,%eax
400516: 83 e0 fc and $0xfffffffffffffffc,%eax
400519: c3 retq
--
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