Symex: shortcut the common case of whole-struct initialisation

Rather than assign a temporary symbol and then expand it, directly extract the fields
and assign them. This cuts the time taken on one initialisation-heavy benchmark from 3.6s
to 3.2s.

Author: smowton

jbmc/regression/jbmc-generics/constant_propagation/test.desc

@@ -3,7 +3,6 @@ Test.class
 --function Test.main --show-vcc
 ^EXIT=0$
 ^SIGNAL=0$
-^\{-\d+\} symex_dynamic::dynamic_object1#2 = \{ \{ \{ "java::GenericSub" \}, NULL, 0 \} \}$
 ^\{-\d+\} symex_dynamic::dynamic_object1#2\.\.@Generic\.\[email protected]\.\.@class_identifier = "java::GenericSub"$
 ^\{-\d+\} symex_dynamic::dynamic_object1#2\.\.@Generic\.\.key = NULL$
 ^\{-\d+\} symex_dynamic::dynamic_object1#3\.\.@Generic\.\.x = 5$

src/goto-symex/goto_symex.h

@@ -499,6 +499,13 @@ class goto_symext
     const exprt &rhs,
     exprt::operandst &,
     assignment_typet);
+  void symex_assign_from_struct(
+    statet &,
+    const ssa_exprt &lhs,
+    const exprt &full_lhs,
+    const struct_exprt &rhs,
+    exprt::operandst &,
+    assignment_typet);
   void symex_assign_symbol(
     statet &,
     const ssa_exprt &lhs,

src/goto-symex/symex_assign.cpp

@@ -364,6 +364,50 @@ static void shift_indexed_access_to_lhs(
   }
 }
 
+/// Assign a struct expression to a symbol. If \ref symex_assign_symbol was used
+/// then we would assign the whole symbol, before extracting its components,
+/// with results like `x = {1, 2}; x..field1 = x.field1; x..field2 = x.field2;`
+/// This abbreviates the process, directly producing
+/// `x..field1 = 1; x..field2 = 2;`
+/// \param state: goto-symex state
+/// \param lhs: symbol to assign
+/// \param full_lhs: expression skeleton corresponding to \p lhs, to be included
+///   in the result trace
+/// \param rhs: struct expression to assign to \p lhs
+/// \param guard: guard conjuncts that must hold for this assignment to be made
+/// \param assignment_type: assignment type (see
+///   \ref symex_targett::assignment_typet)
+void goto_symext::symex_assign_from_struct(
+  statet &state,
+  const ssa_exprt &lhs, // L1
+  const exprt &full_lhs,
+  const struct_exprt &rhs,
+  exprt::operandst &guard,
+  assignment_typet assignment_type)
+{
+  const struct_typet &type = to_struct_type(ns.follow(lhs.type()));
+  const struct_union_typet::componentst &components = type.components();
+
+  for(std::size_t i = 0; i < components.size(); ++i)
+  {
+    const auto &comp = components[i];
+    exprt lhs_field = member_exprt(lhs, comp.get_name(), comp.type());
+    state.field_sensitivity.apply(ns, state, lhs_field, true);
+    INVARIANT(
+      lhs_field.id() == ID_symbol,
+      "member of symbol should be susceptible to field-sensitivity");
+
+    const exprt &rhs_field = rhs.operands()[i];
+    symex_assign_symbol(
+      state,
+      to_ssa_expr(lhs_field),
+      full_lhs,
+      rhs_field,
+      guard,
+      assignment_type);
+  }
+}
+
 void goto_symext::symex_assign_symbol(
   statet &state,
   const ssa_exprt &lhs, // L1
@@ -372,6 +416,14 @@ void goto_symext::symex_assign_symbol(
   exprt::operandst &guard,
   assignment_typet assignment_type)
 {
+  // Shortcut the common case of a whole-struct initializer:
+  if(rhs.id() == ID_struct)
+  {
+    symex_assign_from_struct(
+      state, lhs, full_lhs, to_struct_expr(rhs), guard, assignment_type);
+    return;
+  }
+
   exprt ssa_rhs=rhs;
 
   // put assignment guard into the rhs