From: Pavel Machek <pavel@elf.ucw.cz>
To: Alexander Kjeldaas <astor@guardian.no>
Cc: Philip Blundell <pb@nexus.co.uk>,
David Woodhouse <Dave@imladris.demon.co.uk>,
Meelis Roos <mroos@tartu.cyber.ee>,
linux-kernel@vger.rutgers.edu
Subject: Re: faster strcpy()
Date: Sun, 26 Apr 1998 17:17:46 +0200 [thread overview]
Message-ID: <19980426171746.22129@Elf.mj.gts.cz> (raw)
In-Reply-To: <19980426013108.48608@lucifer.guardian.no>; from Alexander Kjeldaas on Sun, Apr 26, 1998 at 01:31:08AM +0200
Hi!
> > I'm not sure why Mycroft used '& ~(w)' in the above. Unless something
> > obvious escapes me, you can optimize the above down to 3
> > instructions. Both codepaths have a dependency chain of 3
> > instructions.
> >
> > #define word_has_nullbyte(w) ((((w) - 0x01010101) ^ (w)) & 0x80808080)
> >
>
> Hmm.. two things obviously escaped me:
>
> 1) The above fails horribly and detects 0x80 as a null.
It does not matter too much: you usually want
word_has_probably_nullbyte(), since you usually want to know where the
nullbyte is and copy rest byte-by-byte.
Pavel
--
I'm really pavel@atrey.karlin.mff.cuni.cz. Pavel
Look at http://atrey.karlin.mff.cuni.cz/~pavel/ ;-).
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prev parent reply other threads:[~1998-04-26 17:22 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <E0ySfV5-00058I-00@imladris.demon.co.uk>
1998-04-24 10:26 ` Philip Blundell
1998-04-25 22:44 ` Alexander Kjeldaas
[not found] ` <19980426013108.48608@lucifer.guardian.no>
1998-04-26 15:17 ` Pavel Machek [this message]
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