Add default body for some subprograms

and that to those that have identity-like type: A -> A. The default body is
exactly that identity. And their name starts with "system".

Author: petr-bauch

gnat2goto/driver/goto_utils.adb

@@ -153,6 +153,7 @@ package body GOTO_Utils is
 
    procedure New_Subprogram_Symbol_Entry (Subprog_Name : Symbol_Id;
                                           Subprog_Type : Irep;
+                                          Subprog_Body : Irep;
                                           A_Symbol_Table : in out Symbol_Table)
    is
       Subprog_Symbol : Symbol;
@@ -162,7 +163,7 @@ package body GOTO_Utils is
       Subprog_Symbol.PrettyName := Subprog_Name;
       Subprog_Symbol.SymType    := Subprog_Type;
       Subprog_Symbol.Mode       := Intern ("C");
-      Subprog_Symbol.Value      := Make_Nil (No_Location);
+      Subprog_Symbol.Value      := Subprog_Body;
 
       A_Symbol_Table.Insert (Subprog_Name, Subprog_Symbol);
    end New_Subprogram_Symbol_Entry;
@@ -363,6 +364,23 @@ package body GOTO_Utils is
                                         A_Symbol_Table => A_Symbol_Table);
    end Build_Function;
 
+   function Build_Identity_Body (Parameters : Irep) return Irep
+   is
+      Parameter_List : constant Irep_List := Get_Parameter (Parameters);
+      First_Cursor : constant List_Cursor := List_First (Parameter_List);
+      Body_Block : constant Irep :=
+        Make_Code_Block (Source_Location => No_Location,
+                         I_Type          => Make_Nil_Type);
+   begin
+      pragma Assert (List_Has_Element (Parameter_List, First_Cursor));
+      Append_Op (Body_Block,
+                 Make_Code_Return (Return_Value    =>
+                    Param_Symbol (List_Element (Parameter_List, First_Cursor)),
+                                   Source_Location => No_Location,
+                                   I_Type          => Make_Nil_Type));
+      return Body_Block;
+   end Build_Identity_Body;
+
    function Build_Index_Constant (Value : Int; Source_Loc : Source_Ptr)
                                   return Irep
    is

gnat2goto/driver/goto_utils.ads

@@ -63,8 +63,10 @@ package GOTO_Utils is
 
    procedure New_Subprogram_Symbol_Entry (Subprog_Name : Symbol_Id;
                                           Subprog_Type : Irep;
+                                          Subprog_Body : Irep;
                                           A_Symbol_Table : in out Symbol_Table)
-   with Pre => Kind (Subprog_Type) = I_Code_Type;
+     with Pre => Kind (Subprog_Type) = I_Code_Type
+     and Kind (Subprog_Body) in I_Nil | I_Code_Block;
    --  Insert the subprogram specification into the symbol table
 
    procedure New_Type_Symbol_Entry (Type_Name : Symbol_Id; Type_Of_Type : Irep;
@@ -116,6 +118,10 @@ package GOTO_Utils is
                   and then Kind (Func_Params) = I_Parameter_List
                   and then Kind (FBody) in Class_Code);
 
+   function Build_Identity_Body (Parameters : Irep) return Irep
+     with Pre => Kind (Parameters) = I_Parameter_List,
+     Post => Kind (Build_Identity_Body'Result) = I_Code_Block;
+
    function Build_Array_Size (Array_Comp : Irep) return Irep
      with Pre => Kind (Array_Comp) in Class_Expr,
      Post => Kind (Build_Array_Size'Result) = I_Op_Add;

gnat2goto/driver/tree_walk.adb

@@ -5458,9 +5458,17 @@ package body Tree_Walk is
         Do_Subprogram_Specification (N);
       Subprog_Name : constant Symbol_Id :=
         Intern (Unique_Name (Defining_Unit_Name (N)));
+      Default_Body : Irep := Make_Nil (Sloc (N));
    begin
+      if List_Length (Parameter_Specifications (N)) = 1 and
+        not (Kind (Get_Return_Type (Subprog_Type)) = I_Void_Type) and
+        Is_Prefix ("system", Unique_Name (Defining_Unit_Name (N)))
+      then
+         Default_Body := Build_Identity_Body (Get_Parameters (Subprog_Type));
+      end if;
       New_Subprogram_Symbol_Entry (Subprog_Name   => Subprog_Name,
                                    Subprog_Type   => Subprog_Type,
+                                   Subprog_Body   => Default_Body,
                                    A_Symbol_Table => Global_Symbol_Table);
    end Register_Subprogram_Specification;