Only expand lhs EVS if rhs is not initializer

This replaces the previous logic involving `lhs_written`, which wasn't
doing quite the right thing.

Author: Owen Jones

regression/LVSA/TestPreciseAccessPaths/test_precise_access_paths.py

@@ -86,7 +86,6 @@ def test_get_list_node(tmpdir):
     value_set_expectation.check_contains_external_value_set()
 
 
-@pytest.mark.xfail(strict=True)
 def test_check_using_imprecise_access_paths(tmpdir):
     # This test should change when precise access paths have been implemented
     lvsa_driver = LvsaDriver(tmpdir, folder_name).with_test_function('check_using_imprecise_access_paths')

src/pointer-analysis/local_value_set_analysis.cpp

@@ -173,12 +173,9 @@ void local_value_set_analysist::transform_function_stub(
   std::map<exprt, local_value_sett::object_mapt> pre_call_rhs_value_sets;
   const std::string per_field_evs_prefix="external_objects";
 
-  std::set<std::pair<std::string, std::string> > lhs_written;
-
   for(const auto &assignment : assignments)
   {
     ++nstub_assignments;
-    const auto &lhs_fieldname=assignment.first;
     const auto &rhs_expr=assignment.second;
     if(pre_call_rhs_value_sets.count(rhs_expr))
       continue;
@@ -191,11 +188,6 @@ void local_value_set_analysist::transform_function_stub(
       const std::string &evse_label=
         id2string(to_constant_expr(evse.label()).get_value());
 
-      // Differentiate external-set entries that only contain
-      // their initialiser from ones that have been written:
-      if(!evse.is_initializer())
-        lhs_written.insert({lhs_fieldname.base_name, lhs_fieldname.field_name});
-
       if(has_prefix(evse_label, per_field_evs_prefix))
       {
         // This external value set denotes either all pointers stored in a
@@ -275,7 +267,6 @@ void local_value_set_analysist::transform_function_stub(
       // Ordinary value set member (not an external value set);
       // just add to the RHS map.
       valuesets.insert(rhs_map, rhs_expr);
-      lhs_written.insert({lhs_fieldname.base_name, lhs_fieldname.field_name});
     }
   }
 
@@ -295,12 +286,21 @@ void local_value_set_analysist::transform_function_stub(
       const auto find_pair=std::make_pair(
         lhs_fieldname.base_name,
         lhs_fieldname.field_name);
-      if(lhs_written.count(find_pair))
+
+      const bool rhs_is_initializer_per_field_evs =
+        assignment.second.id() == ID_external_value_set &&
+        to_external_value_set(assignment.second)
+            .get_external_value_set_type() ==
+          external_value_set_typet::PER_FIELD &&
+        to_external_value_set(assignment.second).is_initializer();
+      if(!rhs_is_initializer_per_field_evs)
+      {
         get_all_field_value_sets(
           lhs_fieldname.field_name,
           lhs_fieldname.declared_on_type,
           valuesets,
           lhs_entries);
+      }
 
       // Also write to the external value set itself, representing writes
       // whose effects are external to this context too: