value_set_abstract_object destructive merge

Generate two intervals covering the upper and lower part of the value
sets range of values, then merge values into those.

Author: jezhiggins

src/analyses/variable-sensitivity/value_set_abstract_object.cpp

@@ -350,6 +350,7 @@ static bool are_any_top(const abstract_object_sett &set)
 /////////////////
 static abstract_object_sett
 non_destructive_compact(const abstract_object_sett &values);
+static abstract_object_sett destructive_compact(abstract_object_sett values);
 static bool value_is_not_contained_in(
   const abstract_object_pointert &object,
   const std::vector<constant_interval_exprt> &intervals);
@@ -359,7 +360,13 @@ static abstract_object_sett compact_values(const abstract_object_sett &values)
   if(values.size() <= value_set_abstract_objectt::max_value_set_size)
     return values;
 
-  return non_destructive_compact(values);
+  auto compacted = non_destructive_compact(values);
+  if(compacted.size() <= value_set_abstract_objectt::max_value_set_size)
+    return compacted;
+
+  compacted = destructive_compact(values);
+
+  return compacted;
 }
 
 static abstract_object_sett
@@ -392,6 +399,32 @@ non_destructive_compact(const abstract_object_sett &values)
   return compacted;
 }
 
+exprt eval_expr(exprt e)
+{
+  auto dummy_symbol_table = symbol_tablet{};
+  auto dummy_namespace = namespacet{dummy_symbol_table};
+
+  return simplify_expr(e, dummy_namespace);
+}
+
+static abstract_object_sett destructive_compact(abstract_object_sett values)
+{
+  auto width = values.to_interval();
+  auto slice_width = eval_expr(div_exprt(
+    minus_exprt(width.get_upper(), width.get_lower()),
+    from_integer(3, width.type())));
+
+  auto lower_slice = constant_interval_exprt(
+    width.get_lower(), eval_expr(plus_exprt(width.get_lower(), slice_width)));
+  auto upper_slice = constant_interval_exprt(
+    eval_expr(minus_exprt(width.get_upper(), slice_width)), width.get_upper());
+
+  values.insert(interval_abstract_valuet::make_interval(lower_slice));
+  values.insert(interval_abstract_valuet::make_interval(upper_slice));
+
+  return non_destructive_compact(values);
+} // destructive_compact
+
 static bool value_is_not_contained_in(
   const abstract_object_pointert &object,
   const std::vector<constant_interval_exprt> &intervals)

unit/analyses/variable-sensitivity/value_set_abstract_object/compacting.cpp

@@ -166,4 +166,34 @@ SCENARIO(
       }
     }
   }
+
+  GIVEN("compact values to create new intervals")
+  {
+    const exprt interval_1_4 = constant_interval_exprt(val1, val4);
+    const exprt interval_9_12 = constant_interval_exprt(val9, val12);
+
+    WHEN("compacting { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 }")
+    {
+      auto value_set = make_value_set(
+        {val1,
+         val2,
+         val3,
+         val4,
+         val5,
+         val6,
+         val7,
+         val8,
+         val9,
+         val10,
+         val11,
+         val12},
+        environment,
+        ns);
+      THEN("{ [1, 4], 5, 6, 7, 8, [9, 12] }")
+      {
+        EXPECT(
+          value_set, {val5, val6, val7, val8, interval_1_4, interval_9_12});
+      }
+    }
+  }
 }