[PATCH] Add a rounddown_pow_of_two() macro to log2.h.

[PATCH] Add a rounddown_pow_of_two() macro to log2.h.

[Robert P. J. Day]

  In the same way that include/linux/log2.h defines the
roundup_pow_of_two() macro, define the rounddown_pow_of_two() macro so
peopls can stop re-implementing this operation using a loop.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>

---

  compile tested on x86 using "make allyesconfig", but there wasn't
much chance of the build failing anyway since the patch only adds the
macro definition, it doesn't change any existing code to use it.
those patches will be submitted later, bit by bit.

diff --git a/include/linux/log2.h b/include/linux/log2.h
index d02e1a5..6cf7081 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -52,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 	return 1UL << fls_long(n - 1);
 }

+/*
+ * round down to nearest power of two
+ */
+static inline __attribute__((const))
+unsigned long __rounddown_pow_of_two(unsigned long n)
+{
+	return 1UL << (fls_long(n) - 1);
+}
+
 /**
  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
  * @n - parameter
@@ -154,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 	__roundup_pow_of_two(n)			\
  )

+/**
+ * rounddown_pow_of_two - round the given value down to nearest power of two
+ * @n - parameter
+ *
+ * round the given value down to the nearest power of two
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define rounddown_pow_of_two(n)			\
+(						\
+	__builtin_constant_p(n) ? (		\
+		(n == 1) ? 0 :			\
+		(1UL << ilog2(n)) :		\
+	__rounddown_pow_of_two(n)		\
+ )
+
 #endif /* _LINUX_LOG2_H */

-- 
==============================================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://www.fsdev.dreamhosters.com/wiki/index.php?title=Kernel_Janitor%27s_Todo_List
==============================================================================================

Re: [PATCH] Add a rounddown_pow_of_two() macro to log2.h.

[Andrew Morton]

On Thu, 25 Jan 2007 04:32:12 -0500 (EST)
"Robert P. J. Day" <rpjday@mindspring.com> wrote:

> 
>   In the same way that include/linux/log2.h defines the
> roundup_pow_of_two() macro, define the rounddown_pow_of_two() macro so
> peopls can stop re-implementing this operation using a loop.
> 
> Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
> 
> ---
> 
>   compile tested on x86 using "make allyesconfig", but there wasn't
> much chance of the build failing anyway since the patch only adds the
> macro definition, it doesn't change any existing code to use it.
> those patches will be submitted later, bit by bit.
> 
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index d02e1a5..6cf7081 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -52,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n)
>  	return 1UL << fls_long(n - 1);
>  }
> 
> +/*
> + * round down to nearest power of two
> + */
> +static inline __attribute__((const))
> +unsigned long __rounddown_pow_of_two(unsigned long n)
> +{
> +	return 1UL << (fls_long(n) - 1);
> +}

So __rounddown_pow_of_two(16) returns 8?

>  /**
>   * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
>   * @n - parameter
> @@ -154,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
>  	__roundup_pow_of_two(n)			\
>   )
> 
> +/**
> + * rounddown_pow_of_two - round the given value down to nearest power of two
> + * @n - parameter
> + *
> + * round the given value down to the nearest power of two
> + * - the result is undefined when n == 0
> + * - this can be used to initialise global variables from constant data
> + */
> +#define rounddown_pow_of_two(n)			\
> +(						\
> +	__builtin_constant_p(n) ? (		\
> +		(n == 1) ? 0 :			\
> +		(1UL << ilog2(n)) :		\
> +	__rounddown_pow_of_two(n)		\
> + )

But (1UL << ilog2(16)) returns 16?


And, afiact, your __rounddown_pow_of_two() is basically equivalent to (1UL
<< ilog2(n)) anyway.  So a suitable (and less buggy) implementation might be

static inline unsigned long rounddown_pow_of_two(unsigned long n)
{
	return (n == 1) ? 0 : (1UL << ilog2(n));
}

But I'm not sure.  Please create a userspace test harness to test this
patch.

Re: [PATCH] Add a rounddown_pow_of_two() macro to log2.h.

[Robert P. J. Day]

On Thu, 25 Jan 2007, Andrew Morton wrote:

> On Thu, 25 Jan 2007 04:32:12 -0500 (EST)
> "Robert P. J. Day" <rpjday@mindspring.com> wrote:

> > +/*
> > + * round down to nearest power of two
> > + */
> > +static inline __attribute__((const))
> > +unsigned long __rounddown_pow_of_two(unsigned long n)
> > +{
> > +	return 1UL << (fls_long(n) - 1);
> > +}
>
> So __rounddown_pow_of_two(16) returns 8?

it does?  but if that was true, so would 17, and 18, and 19 ...  i
didn't actually test this since it seemed so straightforward.
doesn't fls_long() return the most significant bit?  oh, wait ...
reading further ...
 >
> >  /**
> >   * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
> >   * @n - parameter
> > @@ -154,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
> >  	__roundup_pow_of_two(n)			\
> >   )
> >
> > +/**
> > + * rounddown_pow_of_two - round the given value down to nearest power of two
> > + * @n - parameter
> > + *
> > + * round the given value down to the nearest power of two
> > + * - the result is undefined when n == 0
> > + * - this can be used to initialise global variables from constant data
> > + */
> > +#define rounddown_pow_of_two(n)			\
> > +(						\
> > +	__builtin_constant_p(n) ? (		\
> > +		(n == 1) ? 0 :			\
> > +		(1UL << ilog2(n)) :		\
> > +	__rounddown_pow_of_two(n)		\
> > + )
>
> But (1UL << ilog2(16)) returns 16?
>
>
> And, afiact, your __rounddown_pow_of_two() is basically equivalent to (1UL
> << ilog2(n)) anyway.  So a suitable (and less buggy) implementation might be
>
> static inline unsigned long rounddown_pow_of_two(unsigned long n)
> {
> 	return (n == 1) ? 0 : (1UL << ilog2(n));
> }

i think you're right.  it's been a long day so give me a few minutes
to convince myself.

rday

-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://www.fsdev.dreamhosters.com/wiki/index.php?title=Main_Page
========================================================================

Re: [PATCH] Add a rounddown_pow_of_two() macro to log2.h.

[Kyle Moffett]

On Jan 26, 2007, at 02:24:35, Robert P. J. Day wrote:
> On Thu, 25 Jan 2007, Andrew Morton wrote:
>> On Thu, 25 Jan 2007 04:32:12 -0500 (EST)
>> "Robert P. J. Day" <rpjday@mindspring.com> wrote:
>>> +/*
>>> + * round down to nearest power of two
>>> + */
>>> +static inline __attribute__((const))
>>> +unsigned long __rounddown_pow_of_two(unsigned long n)
>>> +{
>>> +	return 1UL << (fls_long(n) - 1);
>>> +}
>>
>> So __rounddown_pow_of_two(16) returns 8?
>
> it does?  but if that was true, so would 17, and 18, and 19 ...  i  
> didn't actually test this since it seemed so straightforward.  
> doesn't fls_long() return the most significant bit?  oh, wait ...  
> reading further ...

The way that "ilog2" is defined this should be really  
straightforward, I dunno why those functions seem so
overly complicated:
   roundup_pow_of_two(x) := 1 << ilog2(2*x - 1)
   rounddown_pow_of_two(x) := 1 << ilog2(x)

Where ilog2(x) simply returns the first bit set in the word:
   ilog2(0) => undefined or -1 or something
   ilog2(1) => 0
   ilog2(2) => 1
   ilog2(3) => 1
   ilog2(4) => 2
   [...]

The results:
   roundup_pow_of_two(1) = 1    rounddown_pow_of_two(1) = 1
   roundup_pow_of_two(2) = 2    rounddown_pow_of_two(2) = 2
   roundup_pow_of_two(3) = 4    rounddown_pow_of_two(3) = 2
   roundup_pow_of_two(4) = 4    rounddown_pow_of_two(4) = 4
   roundup_pow_of_two(5) = 8    rounddown_pow_of_two(5) = 4
   [...]

Cheers,
Kyle Moffett