Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!uflorida!haven!mimsy!dftsrv!ames!pasteur!helios.ee.lbl.gov!nosc!humu!uhccux!mikem
From: mikem@uhccux.uhcc.hawaii.edu (Mike Morton)
Newsgroups: comp.sys.m68k
Subject: Re: addq.w #n,sp and a pop quiz
Keywords: subx
Message-ID: <2840@uhccux.uhcc.hawaii.edu>
Date: 16 Dec 88 22:15:53 GMT
References: <5005@bsu-cs.UUCP> <5460@cbmvax.UUCP> <1100@ncar.ucar.edu> <191@spt.entity.com>
Distribution: na
Organization: University of Hawaii
Lines: 29



Gail Zacharias recently contributed this 4-instruction signum function, for
which I've counted cycles.

                    word                 long
    add Dx,Dx       4                    8  ; shift sign bit -> carry
                                            ; set Z bit only for Dx = 0, 0x8000
    subx Dx,Dx      4                    8  ; set Dx = 0xffff if Dx was < 0
                                            ;    (and set C bit)
                                            ; set Dx = 0 if sign Dx was >= 0
                                            ;    (and clear C bit)
                                            ;    (and leave Z bit alone)
    bls .+4               10/8 (taken/not)  ; branch on C bit (Dx < 0)
                                            ;    (Dx now holds -1)
                                            ; ... or on Z bit (Dx = 0, 0x8000)
                                            ;    (Dx now holds 0)
    moveq #1,Dx     4                    4  ; none of the above: return 1
                 ----                 ----
                18/20                26/28

Very elegant!  It uses only one register and is faster than the
Superoptimizer's solution for the longword case (30 cycles).  However,
the SO's solution is faster for the word case (16 cycles), mostly because
it avoids the conditional branch.  Takers?

 -- Mike Morton // P.O. Box 11378, Honolulu, HI  96828, (808) 676-6966 HST
      Internet: msm@ceta.ics.hawaii.edu
    (anagrams): Mr. Machine Tool; Ethical Mormon; Chosen Immortal; etc.