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From: rico@oscvax.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Caveat for Manx C (3.20 and 3.40)
Message-ID: <478@oscvax.UUCP>
Date: Tue, 24-Mar-87 10:28:51 EST
Article-I.D.: oscvax.478
Posted: Tue Mar 24 10:28:51 1987
Date-Received: Wed, 25-Mar-87 07:22:04 EST
References: <477@oscvax.UUCP> <210@rocky.STANFORD.EDU>
Reply-To: rico@oscvax.UUCP (Rico Mariani)
Organization: Ontario Science Centre, Toronto
Lines: 76
Keywords: Manx, unsigned, subscript, assembler, bug
Summary: Watch out for unsigned array subscripts...


Here's a little more elaboration on the problem that Tom Rockiki(sp?) pointed
out.  This only causes difficulty when using 16 bit integers as there is no
difference between signed and unsigned addition/indexing when you use the full
width of your registers. 

The following two expressions must generate slightly different code, I'm not
sure which one makes the wrong code because I can't remember whether indexing
is signed or unsigned on the 68000.  I think it's signed, which would mean
that the a[i] code is wrong...

Lattice probably isn't vulnerable to this bug because it uses only 32 bit int's

Here's the source:

char *a ;
unsigned int i ;
int j;

test() {
   a[i]  = 1;
   a[j]  = 2;
}

Aztec C 3.40a generates the following assembler:

;:ts=8
;char *a ;
	global	_a,4
;unsigned int i ;
	global	_i,2
;int j;
	global	_j,2
;
;test() {
	public	_test
_test:
	link	a5,#.2
	movem.l	.3,-(sp)
;   a[i]  = 1;
	move.w	_i,d0
	move.l	_a,a0
	move.b	#1,(a0,d0.w)	<--- wrongo!
;   a[j]  = 2;
	move.w	_j,d0
	move.l	_a,a0		<--- I would've been real impressed if this
	move.b	#2,(a0,d0.w)	     wasn't here... oh well maybe next time :-)
;}
.4
	movem.l	(sp)+,.3
	unlk	a5
	rts
.2	equ	0
.3	reg	
	public	.begin
	dseg
	end

The code that a[i] = 1 should generate is

	move.l	#0,d0		;this gets squeezed into a moveq automagically
	move.w	_j,d0
	move.l	_a,a0
	move.b	#1,(a0,d0.l)

Like I said... this is not a problem if you're using 32 bit integers...

	-Rico

P.S.
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