Skip phi assignment if one of the merged states has an uninitialised object

With level-2 counters incremented on declaration and non-deterministic
initialisation upon allocation, the only remaining sources are pointer
dereferencing, where uninitialised objects necessarily refer to invalid objects.

This is a cleaner implementation of 369f077. Removing only the code
introduced in 369f077 would yield a wrong result for
regression/cbmc/Local_out_of_scope3.

It also fixes the problem of extern symbols not having an initial value, and
thus being skipped.

Fixes: diffblue#1630

Author: tautschnig

regression/cbmc-concurrency/thread_local2/main.c

@@ -0,0 +1,11 @@
+int __CPROVER_thread_local thlocal = 4;
+
+int main()
+{
+  int loc;
+
+  loc = 123;
+
+__CPROVER_ASYNC_3:
+  thlocal = loc, __CPROVER_assert(thlocal == 123, "hello");
+}

regression/cbmc-concurrency/thread_local2/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring

regression/cbmc/extern3/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 
 ^EXIT=10$

src/goto-symex/symex_assign.cpp

@@ -218,17 +218,6 @@ void goto_symext::symex_assign_symbol(
   guardt &guard,
   assignment_typet assignment_type)
 {
-  // do not assign to L1 objects that have gone out of scope --
-  // pointer dereferencing may yield such objects; parameters do not
-  // have an L2 entry set up beforehand either, so exempt them from
-  // this check (all other L1 objects should have seen a declaration)
-  const symbolt *s;
-  if(!ns.lookup(lhs.get_object_name(), s) &&
-     !s->is_parameter &&
-     !lhs.get_level_1().empty() &&
-     state.level2.current_count(lhs.get_identifier())==0)
-    return;
-
   exprt ssa_rhs=rhs;
 
   // put assignment guard into the rhs

src/goto-symex/symex_goto.cpp

@@ -469,17 +469,23 @@ static void merge_names(
   //  1. Either guard is false, so we can't follow that branch.
   //  2. Either identifier is of generation zero, and so hasn't been
   //     initialized and therefore an invalid target.
-  if(dest_state.guard.is_false())
-    rhs = goto_state_rhs;
-  else if(goto_state.guard.is_false())
-    rhs = dest_state_rhs;
-  else if(goto_count == 0)
+  if(
+    dest_state.guard.is_false() ||
+    (dest_count == 0 && !symbol.value.is_not_nil()))
   {
-    rhs = dest_state_rhs;
+    if(goto_count == 0)
+      return;
+
+    rhs = goto_state_rhs;
   }
-  else if(dest_count == 0)
+  else if(
+    goto_state.guard.is_false() ||
+    (goto_count == 0 && symbol.value.is_not_nil()))
   {
-    rhs = goto_state_rhs;
+    if(dest_count == 0)
+      return;
+
+    rhs = dest_state_rhs;
   }
   else
   {