[PATCH] kernel: rounddown helper function

[PATCH] kernel: rounddown helper function

[Eric Paris]

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;		\

Re: [PATCH] kernel: rounddown helper function

[Randy Dunlap]

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).


---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***

Re: [PATCH] kernel: rounddown helper function

[Andrew Morton]

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.

Re: [PATCH] kernel: rounddown helper function

[Eric Paris]

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@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.

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

Re: [PATCH] kernel: rounddown helper function

[Andrew Morton]

On Thu, 05 Aug 2010 13:56:36 -0400
Eric Paris <eparis@redhat.com> wrote:

> > > 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)); })

looks good.

> If that's what you are hoping for, would you also like to see a patch
> doing the same thing for roundup() ?

Sure.  I doubt if anything accidentally depends on the curent behavior,
although that would be amusing.

> > 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
> 

OK, thanks for checking.