linux-kernel - Re: [RFC PATCH 1/6] kenrel.h: add ALIGN_OF_LAST

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Date:	Tue, 20 Mar 2012 09:20:58 -0500
From:	Peter Seebach <peter.seebach@...driver.com>
To:	Michal Nazarewicz <mina86@...a86.com>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 1/6] kenrel.h: add ALIGN_OF_LAST_BIT()

On Tue, 20 Mar 2012 12:32:14 +0100
Michal Nazarewicz <mina86@...a86.com> wrote:

> >+#define ALIGN_OF_LAST_BIT(x)	((((x)^((x) - 1))>>1) + 1)  
> 
> Wouldn't ALIGNMENT() be less confusing? After all, that's what this
> macro is calculating, right? Alignment of given address.

Why not just LAST_BIT(x)?  It's not particularly specific to pointer
alignment, even though that's the context in which it apparently came
up.  So far as I can tell, this isn't even meaningfully defined on
pointer types as such; you'd have to convert.  So the implications for
alignment seem a convenient side-effect, really.

It might be instructive to see some example proposed uses; the question
of why I'd care what alignment something had, rather than whether it
was aligned for a given type, is one that will doubtless keep me awake
nights.

I guess this feels like it answers a question that is usually the wrong
question.  Imagine if you will a couple-page block of memory, full of
unsigned shorts.  Iterate through the array, calculating
ALIGN_OF_LAST_BIT(&a[i]).  Do we really *care* that it's PAGE_SIZE for
some i, and 2 (I assume) for other i, and PAGE_SIZE*2 for either i==0 or
i==PAGE_SIZE?  (Apologies if this is a silly question; maybe this is
such a commonly-needed feature that it's obvious.)

-s
-- 
Listen, get this.  Nobody with a good compiler needs to be justified.
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