linux-kernel - Re: [RFC][PATCH] introduce ptr

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Message-ID: <20100820125745.GA1771@one.firstfloor.org>
Date:	Fri, 20 Aug 2010 14:57:45 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andi Kleen <andi@...stfloor.org>,
	Namhyung Kim <namhyung@...il.com>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] introduce ptr_diff()

> I'm not sure the price is so high.  I googled around and came across
> http://mdfs.net/Docs/Comp/ARM/Cookbook/cook3 (section 3.3).  Dividing by
> 2^n, (2^n + 2^m) or (2^n - 2^m) can be done using a small series of adds
> and subtractions (important on ARM as it had no divide instruction at
> the time).  Most structures are going to be of one of these sizes ...
> and in particular, struct page is 56 bytes in my config, which is 64 - 8.
> Maybe sparse needs to be taught that dividing by 2^n [+-] 2^m is cheap
> enough to not warn about.

Usually you can just use x/n = x*(1/n) and generate *(1/n) with 1/n 
being computed at compile time. Multiplications are fast in hardware.

gcc only does that when -Os is not set though (friends don't let friends
compile with -Os...)

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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