From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Sat,  6 Aug 2011 11:58:31 -0400
To: 9fans@9fans.net
Message-ID: <fa8af6523e976ddf9d157c693e5a1497@brasstown.quanstro.net>
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
Subject: [9fans] c compiler bug, and fix
Topicbox-Message-UUID: 0bf71e18-ead7-11e9-9d60-3106f5b1d025

here's the test case.

	#include <u.h>
	#include <libc.h>

	void
	main(void)
	{
		int i;

		i = 0;
		i /= 1 / 100;
	}

which generates this output
	minooka; 8c ccbug.c
	8c 117738: suicide: sys: trap: divide error pc=0x33c0a
the real code used an
enumerated type instead of 100, making the
bug a bit hard to spot.

i've attached a diff and a testcase.  note that the divisor is set to something
bogus (but not zero) to allow additional diagnostics to be issued.  since diag()
prevents code generation, no bogus code will be emitted.

diffy -c /sys/src/cmd/cc/com.c
/n/dump/2011/0806/sys/src/cmd/cc/com.c:921,926 - /sys/src/cmd/cc/com.c:921,935
  	case OASADD:
  		ccom(l);
  		ccom(r);
+ 		if(n->op == OASMOD || n->op == OASLMOD || n->op == OASDIV || n->op == OASLDIV)
+ 		if(r->op == OCONST)
+ 		if(r->vconst == 0) {
+ 			if(n->op == OASMOD || n->op == OASLMOD)
+ 				diag(n, "modulo by zero %d", n->op);
+ 			else
+ 				diag(n, "divide by zero");
+ 			r->vconst = ~0;
+ 		}
  		if(n->op == OASLSHR || n->op == OASASHR || n->op == OASASHL)
  		if(r->op == OCONST) {
  			t = n->type->width * 8;	/* bits per byte */

#include <u.h>
#include <libc.h>

void
main(void)
{
	int i;
	ulong u;
	uvlong v;

	i = 0;
	i /= 1 / 100;
	i %= 1 / 100;

	u = 0;
	u /= 1 / 100;
	u %= 1 / 100;

	v = 0;
	v /= 1 / 100;
	v %= 1/100;
}

- erik