Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!agate!shelby!neon!kaufman
From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Increment (Was Re: Pascal deficiency)?
Message-ID: <1990Dec18.015258.8631@Neon.Stanford.EDU>
Date: 18 Dec 90 01:52:58 GMT
References: <ewright.661465844@convex.convex.com> <1990Dec18.001753.3756@Neon.Stanford.EDU> <1990Dec18.004838.5623@Neon.Stanford.EDU>
Organization: Computer Science Department, Stanford University
Lines: 50

In article <1990Dec18.004838.5623@Neon.Stanford.EDU> philip@pescadero.stanford.edu writes:
>In article <1990Dec18.001753.3756@Neon.Stanford.EDU>, kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes:
|>> Well, since we ARE in a Mac group, lets just look at the code MPW C generates
|>> for just such constructs:
|>> 
|>> 	i = i+1;
|>> 	MOVE.L     i,D2
|>> 	ADDQ.L     #$1,D2
|>> 	MOVE.L     D2,i
|>> 
|>> 	i++;
|>> 	MOVE.L     i,D2
|>> 	ADDQ.L     #$1,i
|>> 
|>> 	++i;
|>> 	ADDQ.L     #$1,i
|>> 
|>> Behold.  The 68K does, indeed, have an instruction to increment variables in
|>> memory.

>Interesting - but remember looking at "toy" examples doesn't tell you much.
>In "real" code, where performance really matters, I would hope the compiler
>would have loaded the variable into a register for as long as possbile.

Well, to generate the above code, I declared 'external int i'.  The C
compiler DOES put things in registers.  However, MPW C still generates
pretty crufty code for i = i+1:

	MOVE.L	D2,D0
	ADDQ.L	#$1,D0
	MOVE.L	D0,D2

and this is with optimization ON!

The MOVE.L i,D2 in the i++ case is because the value of the expression (i++)
is (i) before the +1.  D2 is dead because we never use the expression value,
and its a shame that the compiler doesn't remove it.  I'm going to look at
gcc and see if doesn't generate better code.  With the complexity of today's
applications, every little 5% helps.

Marc Kaufman (kaufman@Neon.stanford.edu)
>Still, it's bad that the compiler doesn't pick up i=i+small constant as a
>special case - it would be even worse if the Pascal compiler also did this
>since (the original point) you have no option of asking for i++.
>
>Many of the programmer-directed "optimizations" in C, like register
>variables, ought to be unnecessary with a modern optimizing compiler.
>-- 
>Philip Machanick
>philip@pescadero.stanford.edu