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 10:17:09 -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:

> On FRV, for example, I don't want to wrap fls() because the code for ilog2()
> can be shorter and simpler.
> 
> If I did fls() as a wrapper around ilog2() then it would have to involve a
> conditional jump because the compiler can't alter the inline asm of ilog2() to
> turn the SCAN instruction into CSCAN (which is a conditionally executed
> version of SCAN).

Hmmm.. Why not? If you can define fls on a per arch basis then that should 
be possible? You can tell the compiler to produce the correct version of 
the scan instructions. We do that frequently on IA64.
 
> (I have defined ilog2(n) as returning an undefined value if n < 1).

Undefined values are bad. Could you produce a runtime error instead?

> > >  (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).
> 
> Not quite.  I think (4) might be sufficiently achievable with an inline
> function, but (5) is definitely not.

With the appropriate per arch modification of fls() this should also be 
possible within the existing framework.

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