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From: jmunkki@hila.hut.fi (Juri Munkki)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Pascal deficiency?
Message-ID: <1990Dec19.191939.21773@santra.uucp>
Date: 19 Dec 90 19:19:39 GMT
References: <13650001@hpspdra.HP.COM> <1990Dec18.042748.6765@d.cs.okstate.edu> <N67786.90Dec18174755@lehtori.tut.fi>
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Reply-To: jmunkki@hila.hut.fi (Juri Munkki)
Organization: Helsinki University of Technology, FINLAND
Lines: 70

n67786@lehtori.tut.fi (Nieminen Tero) writes:
>   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;

One thing to remember about the Think C compiler is that it doesn't
automatically allocate register variables. The Think Pascal examples
that we have seen here allocated a register variable and used that.
That's why they saved the register in the stack first and then
initialized it.

If you really want to write something as stupid as the subroutine
that has been discussed here, write it like this in Think C:
void	dotest()
{
	register int	i=1;
	
	i++;
}

This produces:
	move.l	D7,-(sp)
	moveq.l	#1,D7
	addq.w	#1,D7
	move.l	(sp)+,D7
	rts

Looks good to me. Of course Think C is not an optimizing compiler in the
strict sense of the word. IMHO, C is just a fancy assembler that produces
portable code...

Also, Think C 4.02 produces the following code, if I omit the register
allocation:

	link	A6,-2
	moveq.l	#1,D0
	move.w	D0,-2(A6)
	addq.l	#1,-2(A6)
	unlk	A6
	rts

Changing the initializer to something like 1024 will change the code
so that we have:

	link	A6,-2
	move.w	#1024,-2(A6)
	addq.l	#1,-2(A6)
	unlk	A6
	rts

To be honest, I don't find this thread useful. The best way to learn
about these things is to read a basic text on compilers or take a course
on compiler design. The reason why C doesn't produce all that good code
with i=i+1 is because it has i++ available, so programmers use that and
there is no need to waste time trying to optimize i=i+1, since it doesn't
usually occur in normal code. Pascal compilers run into i:=i+1 all the time,
so they have to watch for it.

   ____________________________________________________________________________
  / Juri Munkki	    /  Helsinki University of Technology   /  Wind  / Project /
 / jmunkki@hut.fi  /  Computing Center Macintosh Support  /  Surf  /  STORM  /
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