linux-kernel - Re: [PATCH 1/4] LOG2: Implement a general integer log2 facility in the kernel [try #4]

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Date:	Mon, 9 Oct 2006 04:06:41 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	David Howells <dhowells@...hat.com>
cc:	Matthew Wilcox <matthew@....cx>, torvalds@...l.org, akpm@...l.org,
	sfr@...b.auug.org.au, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/4] LOG2: Implement a general integer log2 facility in
 the kernel [try #4] 

On Mon, 9 Oct 2006, David Howells wrote:

> There are a number of reasons:
> 
>  (1) There are a bunch of independent log2 implementations lying around in the
>      code.  It'd be nice to just have one set that anyone can use.

True. So wrap around what is there but do not add gazillion of definitions 
to all arches.

>  (2) Not everyone realises that fls() can be used to do log2().

So this is a case for a wrapper.

>  (3) ilog2(n) != fls(n)
> 
>      This means that the asm-optimised version for one might be less optimal
>      for the other (for example, ilog2() produces an undefined result if n <=
>      1, fls() must return 0).

Ok these are boundary checks that are easily coded around. Some 
variations on fls even exist that also do various flavors of end case 
handling.

>  (4) There are occasions when you might want to take a log2 of a constant.
>      With the totally inline asm approach, it would always execute some code,
>      though it should be unnecessary.  What I've done permits you to avoid that
>      as the answer is always going to be the same.

Good stuff. I have always wanted that. The wrapper could check for a 
constant.

>  (5) fls() and fls64() can't be used to initialise a variable at compile time,
>      ilog2() can.

Well that is the same issue as (4).
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