ssa_exprt::get_object_name: add fast path

In many cases (without field sensitivity actually: always) the object will just
be the original expression, no further processing is required. Thus add a fast
path that handles this case directly, which reduces the cost from 967 seconds
across all of SV-COMP's ReachSafety-ECA down to 143 seconds.

Author: tautschnig

src/util/ssa_expr.h

@@ -37,8 +37,13 @@ class ssa_exprt:public symbol_exprt
 
   irep_idt get_object_name() const
   {
+    const exprt &original_expr = get_original_expr();
+
+    if(original_expr.id() == ID_symbol)
+      return to_symbol_expr(original_expr).get_identifier();
+
     object_descriptor_exprt ode;
-    ode.object()=get_original_expr();
+    ode.object() = original_expr;
     return to_symbol_expr(ode.root_object()).get_identifier();
   }