Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!uokmax!d.cs.okstate.edu!minich
From: minich@d.cs.okstate.edu (Robert Minich)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Pascal deficiency?
Message-ID: <1990Dec18.042748.6765@d.cs.okstate.edu>
Date: 18 Dec 90 04:27:48 GMT
References: <13650001@hpspdra.HP.COM>
Organization: Oklahoma State University
Lines: 117

by stevewi@hpspdra.HP.COM (Steve Witten):
| You can use the 'succ' and 'pred' operators/functions in Pascal to
| accomplish the same function as '++' and '--'.  So:
| 
| 	integer i,j,k;
| 
| 	   ...
| 
| 	j := succ(i);  { same as 'j=i++' }
| 	k := pred(i);  { same as 'k=i--' }
| 
| 	   ...
| 
| However, you have to remember that '++/--' are positional as well.
| You cannot recreate this behavior.  You also cannot recreate the
| behavior of 'x++' without an assignment in Pascal.

  Do you not use C very often or are you VERY tired? The above Pascal
statements are equivalent to

	j := i + 1;
	k := i - 1;

whereas the C translates to Pascal as

j = i++;         | j := j + i;
                 | i := i + 1;
k = i--;         | k := i - 1;
                 | i := i - 1;

The pred() and succ() functions are really used to get increment ordinal
types whereas C doesn't care too much about types and will increment,
say, a char by 1 without blinking.

  Someone has already posted MPW C's output so I thought I'd see what
THINK C 4.0.2 does with the following:

	alpha() { int x;   x = 1; x = x + 1; }
	beta () { int x;   x = 1; x++; }
	main () { alpha(); beta(); }

  ALPHA
     +0000  LINK       A6,#$FFFE
     +0004  MOVEQ      #$01,D0           | tmp = 1;
     +0006  MOVE.W     D0,-$0002(A6)     | x   = tmp;     <== ick
     +000A  MOVE.W     -$0002(A6),D0     | tmp = x;       <==
     +000E  ADDQ.W     #$1,D0            | tmp = tmp + 1;
     +0010  MOVE.W     D0,-$0002(A6)     | x   = tmp;
     +0014  UNLK       A6
     +0016  RTS
  BETA
     +0000  LINK       A6,#$FFFE
     +0004  MOVEQ      #$01,D0           | tmp = 1;
     +0006  MOVE.W     D0,-$0002(A6)     | x   = tmp;
     +000A  ADDQ.W     #$1,-$0002(A6)    | x   = x + 1;
     +000E  UNLK       A6
     +0010  RTS

  I also tried {x += 1;} and it produced identical asm to BETA, which is
OK. What I don't understand is what the heck is going on around what I
marked "ick." Surely such a minute optimisation as not reading back what
you just wrote is not too difficult to code. (Of course, I don't write
compilers, either. :-) However, I think the ideal would be to replace
ALPHA's

     +0004  MOVEQ      #$01,D0           | tmp = 1;
     +0006  MOVE.W     D0,-$0002(A6)     | x   = tmp;     <== ick
     +000A  MOVE.W     -$0002(A6),D0     | tmp = x;       <==
     +000E  ADDQ.W     #$1,D0            | tmp = tmp + 1;
     +0010  MOVE.W     D0,-$0002(A6)     | x   = tmp;

with

            MOVEQ.W    #$1,-$0002(A6)    | x = 1;
            ADDQ.W     #$1,-$0002(A6)    | x = x + 1;

and be done with it. BETA is almost that ideal but Th C insists on using
a register temp for x=1. I guess I can live with it if only because I
don't like modifying asm by hand on a regular basis. :-) Now a really
fancy optimizer would figure out that x always ends up as 2 or even that
x is never used outside this scope and so forget about the calling ALPHA
altogether. 

  For the heck of it, I looked at what THINK Pascal 2.02 generated for
the program below. It looks like the old Th Pascal betters the current
Th C...

program Test;
 procedure alpha;
  var i: integer;
 begin
  i := 1;
  i := i + 1;
 end;
begin
 alpha;
end.

  ALPHA
     +0000  2840EA     LINK       A6,#$FFEE
     +0004  2840EE     MOVE.L     D7,-(A7)  push(D7)
     +0006  2840F0     MOVEQ      #$01,D7   i := 1;
     +0008  2840F2     ADDQ.W     #$1,D7    i := i + 1;
     +000A  2840F4     MOVE.L     (A7)+,D7  pop(D7)
     +000C  2840F6     UNLK       A6
     +000E  2840F8     RTS

  Here, I don't understand what the deal is with saving D7 before use
but I also don't remember Pascal calling conventions, so this may be
proper. If anyone would like to add Th Pascal 3.0 output, that would be
great, too. (I decided Pascal was getting on my nerves and the 3.0
upgrade was way too expensive for "just to have it around.")
-- 
|_    /| | Robert Minich            |
|\'o.O'  | Oklahoma State University| "I'm a newcomer here, but does the
|=(___)= | minich@d.cs.okstate.edu  |  net ever lay any argument to rest?"
|   U    | - Ackphtth               |                    -- dan herrick