From: Wolfgang Denk <wd@denx.de>
To: "Joakim Tjernlund" <Joakim.Tjernlund@lumentis.se>
Cc: "Marc Singer" <elf@buici.com>, linux-mtd@lists.infradead.org
Subject: Re: crc32() optimization
Date: Sun, 10 Nov 2002 22:00:03 +0100 [thread overview]
Message-ID: <20021110210008.7CFF210162@denx.denx.de> (raw)
In-Reply-To: Your message of "Sun, 10 Nov 2002 21:05:11 +0100." <006901c288f4$79e2ce20$0200a8c0@telia.com>
In message <006901c288f4$79e2ce20$0200a8c0@telia.com> you wrote:
>
> What's "Duff's Device"?
It's a tricky way to implement general loop unrolling directly in C.
Applied to your problem, code that looks like this (instead of 8 any
other loop count may be used, but you need to adjust the "case"
statements then):
register int n = (len + (8-1)) / 8;
switch (len % 8) {
case 0: do { val = crc32_table ... ;
case 7: val = crc32_table ... ;
case 6: val = crc32_table ... ;
case 5: val = crc32_table ... ;
case 4: val = crc32_table ... ;
case 3: val = crc32_table ... ;
case 2: val = crc32_table ... ;
case 1: val = crc32_table ... ;
} while (--n > 0);
}
BTW: this is strictly legal ANSI C!
For an explanation see http://www.lysator.liu.se/c/duffs-device.html
Best regards,
Wolfgang Denk
--
Software Engineering: Embedded and Realtime Systems, Embedded Linux
Phone: (+49)-8142-4596-87 Fax: (+49)-8142-4596-88 Email: wd@denx.de
See us @ electronica 2002 in Munich, Nov 12-15, Hall A3, Booth A3.325
next prev parent reply other threads:[~2002-11-10 20:30 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <F122OgRGkQ6mBySsxVY00000854@hotmail.com>
[not found] ` <24987.1036797874@passion.cambridge.redhat.com>
2002-11-10 15:28 ` crc32() optimization Joakim Tjernlund
2002-11-10 18:43 ` Marc Singer
2002-11-10 19:25 ` Wolfgang Denk
2002-11-10 20:05 ` Joakim Tjernlund
2002-11-10 21:00 ` Wolfgang Denk [this message]
2002-11-10 21:22 ` Joakim Tjernlund
2002-11-10 22:35 ` Joakim Tjernlund
2002-11-10 22:41 ` Wolfgang Denk
2002-11-10 23:00 ` Joakim Tjernlund
2002-11-10 23:56 ` Eric W. Biederman
2002-11-11 1:31 ` Marc Singer
2002-11-11 1:37 ` Wolfgang Denk
2002-11-11 4:42 ` Marc Singer
2002-11-25 15:55 ` Herman Oosthuysen
2002-11-25 16:12 ` Joakim Tjernlund
2002-11-11 0:50 ` Marc Singer
2002-11-10 20:04 ` Joakim Tjernlund
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