linux-kernel - Re: [PATCH] Add a rounddown_pow_of

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Message-Id: <0A70B252-C11C-4A5C-93B3-AEF820C9D284@mac.com>
Date:	Fri, 26 Jan 2007 10:57:30 -0500
From:	Kyle Moffett <mrmacman_g4@....com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	Andrew Morton <akpm@...l.org>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	dhowells@...hat.com
Subject: Re: [PATCH] Add a rounddown_pow_of_two() macro to log2.h.

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@...dspring.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

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