Date: Sun, 29 Dec 2002 00:25:49 +0100 (MET) From: Anders Gavare <g@dd.chalmers.se> To: freebsd-alpha@freebsd.org Subject: faster strlen() using longs (?) Message-ID: <Pine.GSO.4.44.0212290009180.18017-100000@elros.dd.chalmers.se>
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Hi.
I'm using FreeBSD 4.5 on an Alpha, and I noticed that strlen() isn't
implemented using words, but using chars.
I've experimented with several different variations of using longs, and
this is the fastest one I've come up with. It's a quick hack, I know,
and it's a bit hard coded, so it would have to be placed in the
alpha-specific part of libc. It is 2.8 times faster than the default
strlen() in libc.
size_t my_strlen3(char *databuf)
{
long data;
long *lp;
size_t len = 0;
/* Count non-aligned chars: */
while (((size_t) databuf) & (sizeof(long)-1)) {
if (!*databuf++)
return len;
len++;
}
lp = (long *) databuf;
/* Loop through full 'long' words: */
for (;;) {
/* See comment (START) */
data = *lp++;
if ( ((data & 0xff) == 0) ||
((data & 0xff00) == 0) ||
((data & 0xff0000) == 0) ||
((data & 0xff000000) == 0) ||
((data & 0xff00000000) == 0) ||
((data & 0xff0000000000) == 0) ||
((data & 0xff000000000000) == 0) ||
((data & 0xff00000000000000) == 0)
)
break;
len += sizeof(long);
/* See comment (END) */
}
/* Return the actual length: */
if (!(data & 0xff)) return len;
if (!(data & 0xff00)) return len + 1;
if (!(data & 0xff0000)) return len + 2;
if (!(data & 0xff000000)) return len + 3;
data >>= 32; len += 4;
if (!(data & 0xff)) return len;
if (!(data & 0xff00)) return len + 1;
if (!(data & 0xff0000)) return len + 2;
return len + 3;
}
It is sometimes faster when compiled with -O than with -O3 (!), but this
depends on which compiler is used. The stuff between (START) and (END)
can be #defined and then included multiple times. That way, there will
be multiple word tests before the jump back to the start of the for
loop. This give a small performance gain using some compilers /
compiler options.
If this is not an issue anymore with newer releases of FreeBSD on Alpha,
then just ignore this mail :-)
Anders
PS. I'm not on the list, so please CC me if you feel like replying. DS.
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