Speed up the execution of exit_policy_is_general_exit_helper()

It isn't necessary to walk through all possible subnets when the policy
we're looking at doesn't touch that subnet.

src/or/policies.c

@@ -870,7 +870,7 @@ policies_set_router_exitpolicy_to_reject_all(routerinfo_t *r)
 static int
 exit_policy_is_general_exit_helper(smartlist_t *policy, int port)
 {
-  uint32_t j;
+  uint32_t mask, ip, i;
   /* Is this /8 rejected (1), or undecided (0)? */
   char subnet_status[256];
 
@@ -878,24 +878,30 @@ exit_policy_is_general_exit_helper(smartlist_t *policy, int port)
   SMARTLIST_FOREACH(policy, addr_policy_t *, p, {
     if (p->prt_min > port || p->prt_max < port)
       continue; /* Doesn't cover our port. */
-    for (j = 0; j < 256; ++j) {
+    mask = 0;
+    tor_assert(p->maskbits <= 32);
+
+    if (p->maskbits)
+      mask = UINT32_MAX<<(32-p->maskbits);
+    ip = tor_addr_to_ipv4h(&p->addr);
+
+    /* Calculate the first and last subnet that this exit policy touches
+     * and set it as loop boundaries. */
+    for (i = ((mask & ip)>>24); i <= (~((mask & ip) ^ mask)>>24); ++i) {
       tor_addr_t addr;
-      if (subnet_status[j] != 0)
+      if (subnet_status[i] != 0)
         continue; /* We already reject some part of this /8 */
-      tor_addr_from_ipv4h(&addr, j<<24);
+      tor_addr_from_ipv4h(&addr, i<<24);
       if (tor_addr_is_internal(&addr, 0))
         continue; /* Local or non-routable addresses */
-      if (tor_addr_compare_masked(&addr, &p->addr, p->maskbits,
-                                  CMP_EXACT) == 0) {
-        if (p->policy_type == ADDR_POLICY_ACCEPT) {
-          if (p->maskbits > 8)
-            continue; /* Narrower than a /8. */
-          /* We found an allowed subnet of at least size /8. Done
-           * for this port! */
-          return 1;
-        } else if (p->policy_type == ADDR_POLICY_REJECT) {
-          subnet_status[j] = 1;
-        }
+      if (p->policy_type == ADDR_POLICY_ACCEPT) {
+        if (p->maskbits > 8)
+          continue; /* Narrower than a /8. */
+        /* We found an allowed subnet of at least size /8. Done
+         * for this port! */
+        return 1;
+      } else if (p->policy_type == ADDR_POLICY_REJECT) {
+        subnet_status[i] = 1;
       }
     }
   });