Template:GNU C Compiler Internals/Instrumenting an AST 3 4

The walk_tree callback function can instrument the AST. Functions build1() and build() construct new tree nodes. The former function takes one operand, the latter one takes more then one operand. The following code computes the address of the operand, same as '&' C operator:

  t = build1(ADDR_EXPR, TREE_TYPE(t), t);

The following example refers to an array element arr[0]:

  t = build(ARRAY_REF, integer_type_node, arr, integer_zero_node);

The following example builds an integer constant:

  t = build_int_2(value, 0);

Building string constant is more difficult. The following example demonstrates the idea:

  tree gem_build_string_literal(int len, const char *str) {
     tree t, elem, index, type;
     t = build_string (len, str);
     elem = build_type_variant (char_type_node, 1, 0);
     index = build_index_type (build_int_2(len-1, 0));
     type = build_array_type (elem, index);
     T_T(t) = type;
     TREE_READONLY(t)=1;
     TREE_STATIC(t)=1;
     TREE_CONSTANT(t)=1;
     type=build_pointer_type (type);
     t = build1 (ADDR_EXPR, type, t);
     t = build1 (NOP_EXPR, build_pointer_type(char_type_node), t);
     return t;
  }

To build a function call one needs to build the list of arguments. Then the CALL_EXPR is constructed:

  t_arg1 = build_tree_list(NULL_TREE, arg1);
  t_arg2 = build_tree_list(NULL_TREE, arg2);
  ...
  TREE_CHAIN(t_arg1)=t_arg2;
  ...
  TREE_CHAIN(t_argn)=NULL_TREE;
  t_call = build_function_call(t_func_decl, t_arg1);

The constructed tree node is added to the AST as a statement or as an expression. A statement is added to the linked list of statements using TREE_CHAIN:

  t_stmt=build_stmt (EXPR_STMT, t_call);
  TREE_CHAIN(t_cur)=t_stmt;

Adding a tree node as an expression is the same as using '()' C operator. The new expression is added as the first argument of the operator. The result of the operator is the result of its second argument:

  t_res = build(COMPOUND_EXPR, TREE_TYPE(t), t_call, t);

If you want to add the call after t then you need to build a compound statement. This is equivalent to using curly brackets in C:

  static tree gen_start_scope() {
    t_hdr = build_stmt (COMPOUND_STMT, NULL_TREE);
    ss = build_stmt (SCOPE_STMT, NULL_TREE);
    SCOPE_BEGIN_P(ss)=1;
    SCOPE_PARTIAL_P(ss)=0;
    TREE_CHAIN(t_hdr)=NULL_TREE;
    TREE_OPERAND(t_hdr, 0)=ss;
    return t_hdr;
  }

  static tree gen_end_scope() {
    ss = build_stmt (SCOPE_STMT, NULL_TREE);
    SCOPE_BEGIN_P(ss)=0;
    SCOPE_PARTIAL_P(ss)=0;
    return ss;
  }

  static tree scope_stmt(tree t) {
    t_res=gen_start_scope();
    TREE_CHAIN(TREE_OPERAND(t_res, 0)) = t;
    while (TREE_CHAIN(t)) t = TREE_CHAIN(t);
    TREE_CHAIN(t)=gen_end_scope();
    return t_res;
  }

Take home:

Use GCC's functions to build new nodes and add them to the AST, either as a statement or as an expression.