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From: Linus Torvalds <torvalds@transmeta.com>
To: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
Cc: dphillips@sistina.com, <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] Faster generic_fls
Date: Wed, 30 Apr 2003 09:16:04 -0700 (PDT)	[thread overview]
Message-ID: <Pine.LNX.4.44.0304300911420.16712-100000@home.transmeta.com> (raw)
In-Reply-To: <87el3kt1kt.fsf@student.uni-tuebingen.de>


On 30 Apr 2003, Falk Hueffner wrote:
> Linus Torvalds <torvalds@transmeta.com> writes:
> 
> > There is _never_ any excuse to use a lookup table for something that
> > can be calculated with a few simple instructions. That's just
> > stupid.
> 
> Well, the "few simple instructions" are 28 instructions on Alpha for
> example, including 6 data-dependent branches. So I don't think it's
> *that* stupid.

You're comparing apples to oranges.

Clearly you're not going to make _one_ load to get fls, since having a 
4GB lookup array for a 32-bit fls would be "somewhat" wasteful.

So the lookup table would probably look up just the last 8 bits.

So the lookup table version is several instructions in itself, doing about
half of what the calculating version needs to do _anyway_. Including those
data-dependent branches.

		Linus


  reply	other threads:[~2003-04-30 16:02 UTC|newest]

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-04-30  2:46 [RFC][PATCH] Faster generic_fls Daniel Phillips
2003-04-30  7:19 ` Willy Tarreau
2003-04-30  8:36   ` Aaron Lehmann
2003-04-30  8:43     ` Aaron Lehmann
2003-04-30  8:52     ` Aaron Lehmann
2003-04-30  8:51       ` P
2003-04-30 13:51   ` Daniel Phillips
2003-04-30 11:14 ` Falk Hueffner
2003-04-30 13:03   ` Daniel Phillips
2003-04-30 13:17     ` Falk Hueffner
2003-04-30 14:07       ` Daniel Phillips
2003-04-30 14:11   ` Linus Torvalds
2003-04-30 14:53     ` Falk Hueffner
2003-04-30 15:28       ` Linus Torvalds
2003-04-30 16:03         ` Falk Hueffner
2003-04-30 16:16           ` Linus Torvalds [this message]
2003-04-30 16:43             ` Falk Hueffner
2003-04-30 20:25             ` Alan Cox
2003-04-30 21:59               ` Falk Hueffner
2003-04-30 22:22                 ` Alan Cox
2003-04-30 23:41               ` Linus Torvalds
2003-04-30 22:47                 ` Alan Cox
2003-05-01  0:12                 ` Falk Hueffner
2003-05-01  1:07                   ` Linus Torvalds
2003-04-30 19:15     ` Daniel Phillips
2003-04-30 20:59       ` Willy Tarreau
2003-05-01  1:02         ` Daniel Phillips
2003-05-01  9:19           ` Willy Tarreau
2003-05-01 16:02             ` Linus Torvalds
2003-04-30 20:55 ` Andrew Morton
2003-05-01  5:03   ` hugang
2003-05-01  5:11     ` Andrew Morton
2003-05-01  5:33       ` hugang
2003-05-01  7:05         ` hugang
2003-05-01 13:52           ` Willy TARREAU
2003-05-01 14:14             ` Falk Hueffner
2003-05-01 14:26               ` Willy TARREAU
2003-05-01 14:53             ` hugang
2003-05-01 15:54               ` Thomas Schlichter
2003-05-02  0:33                 ` hugang
2003-05-01 17:16               ` Willy TARREAU
2003-05-01 23:27                 ` Thomas Schlichter
2003-05-02  0:10                   ` Willy Tarreau
2003-05-02  0:43                     ` Daniel Phillips
2003-05-02  0:54                       ` Andrew Morton
2003-05-02 12:21                         ` Daniel Phillips
2003-05-02  1:47                       ` Thomas Schlichter
2003-05-02 13:24                         ` Willy Tarreau
2003-05-02  0:13                   ` Daniel Phillips
2003-05-02  0:13             ` Lincoln Dale
2003-05-01  5:16   ` hugang
2003-05-01 10:22     ` Willy TARREAU
2003-05-01 11:17       ` hugang
2003-05-01 11:45         ` Willy TARREAU
     [not found] <87d6j34jad.fsf@student.uni-tuebingen.de.suse.lists.linux.kernel>
     [not found] ` <Pine.LNX.4.44.0304301801210.20283-100000@home.transmeta.com.suse.lists.linux.kernel>
2003-05-01  1:46   ` Andi Kleen
2003-05-01  4:40     ` Linus Torvalds
2003-05-01 12:00       ` David S. Miller
2003-05-02  5:14         ` Linus Torvalds
2003-05-01 13:31 linux
2003-05-01 17:15 Chuck Ebbert
2003-05-02  8:50 Chuck Ebbert
2003-05-02  9:37 ` Peter Osterlund
2003-05-02 10:04 Chuck Ebbert
2003-05-02 22:55 ` David S. Miller
2003-05-03  2:21 Chuck Ebbert

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