linux-kernel - Re: [PATCH 1/1] GCD: add binary GCD algorithm

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Message-ID: <20140815201601.2497.qmail@ns.horizon.com>
Date:	15 Aug 2014 16:16:01 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	peterz@...radead.org, zhaoxiu.zeng@...il.com
Cc:	akpm@...ux-foundation.org, dhowells@...hat.com, jwboyer@...hat.com,
	linux-kernel@...r.kernel.org, linux@...izon.com, mingo@...nel.org,
	takahiro.akashi@...aro.org
Subject: Re: [PATCH 1/1] GCD: add binary GCD algorithm

> So I looked at wikipedia (I wasn't aware of this algorithm, clever
> though), and am still somewhat puzzled by your 'r'.
> 
> What's 'wrong' with their iterative version, or what's better about your
> 'r' stuff?

I need to look more carefully, but it looks nifty.

Basically, it's avoiding the usual issue of counting the lsbits to factor
out a power of 2.

Rather than taking out the common factors of 2 a bit at a time, to
reduce to the case where one is even and one is odd (i.e. ((a ^ b) & 1)
== 1), just find the low-order differing bit and use that instead of
"1" in the algorithm.  I.e. ((1 ^ b) & r) == r.

I have to see if there's a nicer way to arrange things, but it looks good.

What I'd like is a better way to automatically configure "divide is
slow" from an architecture.
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