linux-kernel - Re: [PATCH 2/4] lib: vsprintf: Optimize division by 10000

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Message-ID: <xa1tfw67d24j.fsf@mina86.com>
Date:	Mon, 24 Sep 2012 14:41:32 +0200
From:	Michal Nazarewicz <mpn@...gle.com>
To:	George Spelvin <linux@...izon.com>, linux@...izon.com,
	vda.linux@...glemail.com
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] lib: vsprintf: Optimize division by 10000

On Mon, Sep 24 2012, George Spelvin wrote:
>> You are using a 64-bit multiply in a path that is designed for 32-bit
>> processors, which makes me feel that it will be slower.
>
> Slower than the divide it's replacing?

OK, granted, it might be faster after all. ;)  Still, I'd love to see
some benchmark.

> The following 32-bit processors have 32x32->64-bit multiply:
>
> x86
> ARM (as of ARMv4 = ARM7TDMI, the lowest version in common use)
> SPARCv7, SPARCv8

Didn't some SPARCs have 32x32->32 multiply?  I remember reading some
rant from a GMP developer about how SPARC is broken that way.

> MIPS32
> MC68020
> PA-RISC 1.1 (XMPYU)
> avr32
> PowerPC (MULHWU)
> VAX (EMUL)

> I could do some Kconfig hacking and make the code path
> architecture-dependent.  Do you think it's worth it?

Definitely not.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo--

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