From: cinap_lenrek@felloff.net
To: 9fans@9fans.net
Subject: [9fans] subtracting pointers on amd64 6c
Date: Tue, 5 Jan 2016 07:36:34 +0100 [thread overview]
Message-ID: <8edba5d726ee6a032d8521c7de33f499@felloff.net> (raw)
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
next reply other threads:[~2016-01-05 6:36 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-05 6:36 cinap_lenrek [this message]
2016-01-05 7:53 ` cinap_lenrek
2016-01-05 10:17 ` Charles Forsyth
2016-01-05 10:15 ` Charles Forsyth
2016-01-05 10:28 ` Charles Forsyth
2016-01-05 19:01 ` Devon H. O'Dell
2016-01-05 19:46 ` Charles Forsyth
2016-01-05 20:40 ` erik quanstrom
2016-01-05 21:53 ` Devon H. O'Dell
2016-01-05 23:27 ` erik quanstrom
2016-01-05 23:37 ` Dave Eckhardt
2016-01-05 21:03 ` erik quanstrom
2016-01-05 22:32 ` cinap_lenrek
2016-01-05 22:57 ` Devon H. O'Dell
2016-01-05 23:17 ` Devon H. O'Dell
2016-01-05 23:14 ` erik quanstrom
2016-01-06 6:58 ` cinap_lenrek
2016-01-06 19:10 ` erik quanstrom
2016-01-07 1:08 ` cinap_lenrek
2016-01-08 22:23 ` erik quanstrom
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