Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Understanding variations in Dhrystone performance
Message-ID: <1989May16.172354.1417@utzoo.uucp>
Organization: U of Toronto Zoology
References: <474@estevax.UUCP> <1989May15.173631.3029@utzoo.uucp>
Date: Tue, 16 May 89 17:23:54 GMT

In article <1989May15.173631.3029@utzoo.uucp> I wrote:
>	(((x & ~0x80808080) - 0x01010101) & 0x80808080)
>
>The result is nonzero if, and only if, there was a NUL byte in x...

Oops, my mistake, it does get a false alarm on an 0x80.  So you do end
up needing a false-alarm filter.  There are ways around this, but they
add their own overheads.  Nevertheless, alignment permitting, using a
fast filter like this is a considerable win if you're scanning big
chunks of text.  Now, how worthwhile *any* of this is for typical C
strings is a different question -- it's hard to amortize any significant
setup overhead over the short strings typically found in real code.
-- 
Subversion, n:  a superset     |     Henry Spencer at U of Toronto Zoology
of a subset.    --J.J. Horning | uunet!attcan!utzoo!henry henry@zoo.toronto.edu