Restore member-of-union expressions in trace output

byte_extract operations precisely capture the semantics, but aren't
meaningful to users.

Author: tautschnig

regression/cbmc/xml-trace/test.desc

@@ -7,7 +7,7 @@ VERIFICATION FAILED
 <assignment assignment_type="state" base_name="u" display_name="test::u" hidden="false" identifier="test::u" mode="C" step_nr="\d+" thread="0">
 <location file=".*" function="test" line="\d+" working-directory=".*"/>
 <type>union myunion</type>
-<full_lhs>byte_extract_little_endian\(u, 0ll?, .*int.*\)</full_lhs>
+<full_lhs>u</full_lhs>
 <full_lhs_value binary="[01]+">\d+ll?</full_lhs_value>
 <value>\{ \.i=\d+ll? \}</value>
 <value_expression>

src/goto-programs/goto_trace.cpp

@@ -126,7 +126,7 @@ void goto_trace_stept::output(
       if(!comment.empty())
         out << "  " << comment << '\n';
 
-      out << "  " << format(pc->get_condition()) << '\n';
+      out << "  " << format(cond_expr) << '\n';
       out << '\n';
     }
   }
@@ -397,9 +397,7 @@ void show_compact_goto_trace(
 
         if(step.pc->is_assert())
         {
-          out << "  "
-              << from_expr(ns, step.function_id, step.pc->get_condition())
-              << '\n';
+          out << "  " << from_expr(ns, step.function_id, cond_expr) << '\n';
         }
 
         out << '\n';
@@ -524,9 +522,7 @@ void show_full_goto_trace(
 
         if(step.pc->is_assert())
         {
-          out << "  "
-              << from_expr(ns, step.function_id, step.pc->get_condition())
-              << '\n';
+          out << "  " << from_expr(ns, step.function_id, cond_expr) << '\n';
         }
 
         out << '\n';

src/goto-programs/rewrite_union.cpp

@@ -124,3 +124,51 @@ void rewrite_union(goto_modelt &goto_model)
 {
   rewrite_union(goto_model.goto_functions);
 }
+
+/// Undo the union access -> byte_extract replacement that rewrite_union did for
+/// the purpose of displaying expressions to users.
+/// \param expr: expression to inspect and possibly transform
+/// \param ns: namespace
+/// \return True if, and only if, the expression is unmodified
+static bool restore_union_rec(exprt &expr, const namespacet &ns)
+{
+  bool unmodified = true;
+
+  Forall_operands(it, expr)
+    unmodified &= restore_union_rec(*it, ns);
+
+  if(expr.id() == byte_extract_id())
+  {
+    byte_extract_exprt &be = to_byte_extract_expr(expr);
+    if(
+      (be.op().type().id() == ID_union ||
+       be.op().type().id() == ID_union_tag) &&
+      be.offset().is_zero())
+    {
+      const union_typet &union_type = to_union_type(ns.follow(be.op().type()));
+
+      for(const auto &comp : union_type.components())
+      {
+        if(be.type() == comp.type())
+        {
+          expr = member_exprt{be.op(), comp.get_name(), be.type()};
+          return false;
+        }
+      }
+    }
+  }
+
+  return unmodified;
+}
+
+/// Undo the union access -> byte_extract replacement that rewrite_union did for
+/// the purpose of displaying expressions to users.
+/// \param expr: expression to inspect and possibly transform
+/// \param ns: namespace
+void restore_union(exprt &expr, const namespacet &ns)
+{
+  exprt tmp = expr;
+
+  if(!restore_union_rec(tmp, ns))
+    expr.swap(tmp);
+}

src/goto-programs/rewrite_union.h

@@ -24,4 +24,6 @@ void rewrite_union(goto_functionst::goto_functiont &);
 void rewrite_union(goto_functionst &);
 void rewrite_union(goto_modelt &);
 
+void restore_union(exprt &, const namespacet &);
+
 #endif // CPROVER_GOTO_PROGRAMS_REWRITE_UNION_H

src/goto-symex/build_goto_trace.cpp

@@ -22,6 +22,8 @@ Author: Daniel Kroening
 
 #include <solvers/decision_procedure.h>
 
+#include <goto-programs/rewrite_union.h>
+
 #include "partial_order_concurrency.h"
 
 static exprt build_full_lhs_rec(
@@ -378,6 +380,7 @@ void build_goto_trace(
             SSA_step.ssa_full_lhs),
           ns);
         replace_nondet_in_type(goto_trace_step.full_lhs, decision_procedure);
+        restore_union(goto_trace_step.full_lhs, ns);
       }
 
       if(SSA_step.ssa_full_lhs.is_not_nil())
@@ -405,6 +408,7 @@ void build_goto_trace(
       if(SSA_step.is_assert() || SSA_step.is_assume() || SSA_step.is_goto())
       {
         goto_trace_step.cond_expr = SSA_step.cond_expr;
+        restore_union(goto_trace_step.cond_expr, ns);
 
         goto_trace_step.cond_value =
           decision_procedure.get(SSA_step.cond_handle).is_true();