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

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Date:	Fri, 22 Aug 2014 14:31:17 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, George Spelvin <linux@...izon.com>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Josh Boyer <jwboyer@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] GCD: add binary GCD algorithm

On Fri, 15 Aug 2014 20:49:16 +0800 Zhaoxiu Zeng <zhaoxiu.zeng@...il.com> wrote:

> Because some architectures (alpha, armv6, etc.) don't provide hardware division,
> the mod operation is slow! Binary GCD algorithm uses simple arithmetic operations,
> it replaces division with arithmetic shifts, comparisons, and subtraction.

I had a look around and it seems that most (all?) gcd() and lcd()
callers are on initialization-time slowpaths.

Do you know of a workload which will significantly benefit from this
change?  If so, by how much?

Otherwise I don't think we can justify the additional maintenance
cost/risk, sorry.

And if we *do* decide to proceed with this patch, we should include a
patch which enables it on as many architectures as possible, so it gets
runtime tested.

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