From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <8edba5d726ee6a032d8521c7de33f499@felloff.net>
Date: Tue,  5 Jan 2016 07:36:34 +0100
From: cinap_lenrek@felloff.net
To: 9fans@9fans.net
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Subject: [9fans] subtracting pointers on amd64 6c
Topicbox-Message-UUID: 7e2e5562-ead9-11e9-9d60-3106f5b1d025

in the function /sys/src/cmd/cc/sub.c:/^arith we emit code
that cast the 64-bit subtraction result to 32-bit LONG *before*
doing the division of the sizeof of the pointer type.

so when the pointers are more than 4gb apart, we will calculate
the wrong result, even tho the result would have fit into a
32-bit LONG *after* the division.

questions:

1) why do we cast to long?
2) if a pointer subtraction has to yield a long, why dont we cast *after* the division?

	if(n->op == OSUB)
	if(i == TIND && j == TIND) {
		w = n->right->type->link->width;
		if(w < 1 || n->left->type->link == T || n->left->type->link->width < 1)
			goto bad;
		n->type = types[ewidth[TIND] <= ewidth[TLONG]? TLONG: TVLONG];
		if(1 && ewidth[TIND] > ewidth[TLONG]){	<------- here
			n1 = new1(OXXX, Z, Z);
			*n1 = *n;
			n->op = OCAST;
			n->left = n1;
			n->right = Z;
			n->type = types[TLONG];
		}
		if(w > 1) {
			n1 = new1(OXXX, Z, Z);
			*n1 = *n;
			n->op = ODIV;
			n->left = n1;
			n1 = new1(OCONST, Z, Z);
			n1->vconst = w;
			n1->type = n->type;
			n->right = n1;
			w = vlog(n1);
			if(w >= 0) {
				n->op = OASHR;
				n1->vconst = w;
			}
		}
		return;
	}

--
cinap