Message-ID: <1461864170.2307.19.camel@HansenPartnership.com>
Subject: Re: [patch V3] lib: GCD: add binary GCD algorithm
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: zengzhaoxiu@163.com, akpm@linux-foundation.org, linux@horizon.com,
        peterz@infradead.org
Cc: Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>, Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        Russell King <linux@arm.linux.org.uk>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        James Hogan <james.hogan@imgtec.com>, Michal Simek <monstr@monstr.eu>,
        Ralf Baechle <ralf@linux-mips.org>, Ley Foon Tan <lftan@altera.com>,
        Jonas Bonn <jonas@southpole.se>,
        "James E.J. Bottomley" <jejb@parisc-linux.org>,
        Helge Deller <deller@gmx.de>, Chen Liqin <liqin.linux@gmail.com>,
        Lennox Wu <lennox.wu@gmail.com>, Rich Felker <dalias@libc.org>,
        "David S. Miller" <davem@davemloft.net>, linux-kernel@vger.kernel.org,
        linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        uclinux-h8-devel@lists.sourceforge.jp, linux-m68k@vger.kernel.org,
        linux-metag@vger.kernel.org, linux-mips@linux-mips.org,
        nios2-dev@lists.rocketboards.org, linux@lists.openrisc.net,
        linux-parisc@vger.kernel.org, linux-sh@vger.kernel.org,
        sparclinux@vger.kernel.org
Date: Thu, 28 Apr 2016 10:22:50 -0700
In-Reply-To: <1461843824-19853-1-git-send-email-zengzhaoxiu@163.com>
References: <1461843824-19853-1-git-send-email-zengzhaoxiu@163.com>
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On Thu, 2016-04-28 at 19:43 +0800, zengzhaoxiu@163.com wrote:
> From: Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>
> 
> 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 subtractions.
> 
> I have compiled successfully with x86_64_defconfig and 
> i386_defconfig.

What's the reason for wanting to optimize this and thus have to
maintain (and test) two separate code paths, which is a significant
expense? As far as I can see, gcd() is mosly used in finding optimal
clocks for operations, which is usually done at start of day and not
time critical.

James