From: "George Spelvin" <linux@horizon.com>
To: linux@horizon.com, mpn@google.com, vda.linux@googlemail.com
Cc: hughd@google.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] lib: vsprintf: Optimize division by 10000
Date: 24 Sep 2012 08:16:14 -0400 [thread overview]
Message-ID: <20120924121614.5422.qmail@science.horizon.com> (raw)
In-Reply-To: <xa1twqzk640j.fsf@mina86.com>
> 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?
The following 32-bit processors have 32x32->64-bit multiply:
x86
ARM (as of ARMv4 = ARM7TDMI, the lowest version in common use)
SPARCv7, SPARCv8
MIPS32
MC68020
PA-RISC 1.1 (XMPYU)
avr32
PowerPC (MULHWU)
VAX (EMUL)
I could keep going through the full list of architectures in arch/,
but it's starting to get slow and I haven't hit one *without* a widening
multiply yet. (And if it doesn't have hardware divide, I expect the
multiply is still faster.)
Ah! Found one! ColdFire MCF5272 has 32/32-bit divide, but only 32x32->32
multiply. However, DIVU takes 20 or 35 cycles, which is pretty close to the
time to synthesize the multiply out of 4 16x16->32 pieces (4 cycles each).
I could do some Kconfig hacking and make the code path architecture-dependent.
Do you think it's worth it?
next prev parent reply other threads:[~2012-09-24 12:16 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-03 5:21 [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers George Spelvin
2012-08-03 5:21 ` [PATCH 2/4] lib: vsprintf: Optimize division by 10000 George Spelvin
2012-09-23 17:30 ` Michal Nazarewicz
2012-09-24 12:16 ` George Spelvin [this message]
2012-09-24 12:41 ` Michal Nazarewicz
2012-09-24 13:56 ` George Spelvin
2012-09-24 15:14 ` Geert Uytterhoeven
2012-09-24 15:48 ` George Spelvin
2012-09-24 9:03 ` Denys Vlasenko
2012-09-24 12:35 ` George Spelvin
2012-09-24 15:02 ` Denys Vlasenko
2012-08-03 5:21 ` [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8 George Spelvin
2012-09-23 14:18 ` Rabin Vincent
2012-09-24 11:13 ` George Spelvin
2012-09-24 14:33 ` George Spelvin
2012-09-24 14:53 ` Michal Nazarewicz
2012-09-24 14:57 ` Michal Nazarewicz
2012-09-23 18:22 ` Michal Nazarewicz
2012-09-24 11:46 ` George Spelvin
2012-09-24 12:29 ` Michal Nazarewicz
2012-09-24 13:49 ` George Spelvin
2012-09-24 15:06 ` Michal Nazarewicz
2012-09-25 11:44 ` George Spelvin
2012-09-25 13:00 ` Denys Vlasenko
2012-08-03 5:21 ` [PATCH 4/4] lib: vsprintf: Fix broken comments George Spelvin
2012-09-23 17:22 ` [PATCH 1/4] lib: vsprintf: Optimize division by 10 for small integers Michal Nazarewicz
2012-09-24 14:18 ` George Spelvin
2012-09-24 9:06 ` Denys Vlasenko
2012-09-24 11:27 ` George Spelvin
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