linux-kernel - ALIGN via ilog2 without gccisms (Re: [PATCH] Fix get

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Message-Id: <E1HOybN-00031m-6M@flower>
Date:	Wed, 7 Mar 2007 17:02:53 +0100
From:	Oleg Verych <olecom@...wer.upol.cz>
To:	David Howells <dhowells@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@....linux.org.uk>, akpm@...ux-foundation.org,
	benh@...nel.crashing.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, johannes@...solutions.net
Subject: ALIGN via ilog2 without gccisms (Re: [PATCH] Fix get_order())

> From: David Howells
> Newsgroups: gmane.linux.kernel
> Subject: Re: [PATCH] Fix get_order()
> Date: Wed, 07 Mar 2007 11:43:06 +0000
>
[]
> Various archs (including i386, x86_64, powerpc and frv) have instructions that
> can be used to calculate integer log2(N).
>
Probably it can be used to get rid of gccisms and "type fluff" due to
bitwise arithmetics in ALIGN?
Here:

#define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
#define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))

like that:

#define ALIGN(x,y)		__ALIGN_MASK(x,ilog2(y))
#define __ALIGN_MASK(x,bits)	(((((x) >> (bits)) << 1) + 2) << (bits - 1))

Note side effect, that this one always yields even result. Maybe this is
good, due to avoidance of misaligned access.

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