Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!uw-june!pardo
From: pardo@june.cs.washington.edu (David Keppel)
Newsgroups: comp.lang.c
Subject: pcc optimizer strangeness?
Keywords: pcc optimize assembly
Message-ID: <4061@june.cs.washington.edu>
Date: 24 Jan 88 17:01:52 GMT
Organization: U of Washington, Computer Science, Seattle
Lines: 39

I have a program that compiles some (to me) strange code under pcc even
with the (presumed peephole) optimzer turned on  (Ultrix).  I'm led to
wonder if this is even legal C.  Basically I'm trying to create an array
of range (-1..1) instead of (0..2):

	main()
	{
	    static int	foo[3];
	    static int	*bar = &foo;

	    bar[-1] =  1;
	    bar[ 0] =  0;
	    bar[ 1] = -1;

	    exit(bar[-1]+bar[1]);	/* wierd */
	}

For the /* wierd */ part, the resulting unoptimized code is:
	movl	L17,r0
	movl	L17,r1
	addl3	4(r1),-4(r0),r0
	pushl	r0

The resulting optimized code is:
	movl	L17,r0
	movl	r0,r1
	addl3	4(r1),-4(r0),-(sp)

And it all works just hunky-dory when I change it to
	movl	L17,r0
	addl3	4(r0),-4(r0),-(sp)

So my questions are:

(a) is this legal C?
(b) is this lack of optimization worth telling anybody about?

	;-D on  (I want a float-indexed array !-)  Pardo
	pardo@cs.washington.edu    ucbvax!uw-beaver!uw-june!pardo