Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ucsd!nosc!humu!uhccux!cs661s01
From: cs661s01@uhccux.UUCP (Cs661s01)
Newsgroups: comp.sys.m68k
Subject: Re: 68000 Tricks
Message-ID: <1629@uhccux.UUCP>
Date: 3 Mar 88 05:36:27 GMT
References: <326@rose3.rosemount.com> <1200@microsoft.UUCP>
Reply-To: cs661s01@uhccux.UUCP (Cs661s01)
Distribution: na
Organization: U. of Hawaii, Manoa (Honolulu)
Lines: 30
Keywords: 68000 Tricks Speed bitcount strlen

Odd that Kent Schnaith would bring up bit-counting.  I spent part of
the weekend doing a bit-counter.  Of course, I cheated and used a 64K-byte
table, so there's not much point in including the code here.

In Kent's strlen, shouldn't that "add.w #$ffff, d0" be just "subq.w #1, d0"?
And while we're shaving cycles, doesn't
        neg.w d0
        subq.w #1, d0
just become "not.w d0"?  (Take the hint from his comment "d0 = $FFFF - d0").

Here's a puzzle, unsolved as far as I know -- I need a fast way to compare
two bitmaps, to see which has more bits set.  Is there a faster way than
counting each one's bits and comparing?  If it helps, the two maps have
no 1's in common.  (This is to evaluate an endgame in Othello.)  A 68000
solution is preferable, but anything is interesting...

Jan Gray's strlen seems preferable, especially since it looks right for
strings longer than 64K.  Has it actually been timed on an '020?  Seems
like a 7-instruction loop for every four bytes isn't ALL that much
better than a 2-instruction loop for every byte...  I like the simpler
solution adapted to just compare pointers, so the loop is just:
    loop:   tst.b (An)+
            bne.s loop
...and then the final pointer has the initial one subtracted (probably
give or take 1).  Good especially if strings are likely to be short.

 -- Mike Morton // P.O. Box 11378, Honolulu, HI  96828, (808) 456-8455 HST
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