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   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. _basic-data-structures:
   7
   8 Basic data structures
   9 *********************
  10
  11 Gfortran creates GENERIC as an intermediate language for the
  12 middle-end. Details about GENERIC can be found in the GCC manual.
  13
  14 The basic data structure of GENERIC is a ``tree``. Everything in
  15 GENERIC is a ``tree``, including types and statements.  Fortunately
  16 for the gfortran programmer, ``tree`` variables are
  17 garbage-collected, so doing memory management for them is not
  18 necessary.
  19
  20 ``tree`` expressions are built using functions such as, for
  21 example, ``fold_build2_loc``.  For two tree variables ``a`` and
  22 ``b``, both of which have the type ``gfc_arry_index_type``,
  23 calculation ``c = a * b`` would be done by
  24
  25 .. code-block:: c++
  26
  27   c = fold_build2_loc (input_location, MULT_EXPR,
  28                        gfc_array_index_type, a, b);
  29
  30 The types have to agree, otherwise internal compiler errors will occur
  31 at a later stage.  Expressions can be converted to a different type
  32 using ``fold_convert``.
  33
  34 Accessing individual members in the ``tree`` structures should not
  35 be done. Rather, access should be done via macros.
  36
  37 One basic data structure is the ``stmtblock_t`` struct. This is
  38 used for holding a list of statements, expressed as ``tree``
  39 expressions.  If a block is created using ``gfc_start_block``, it
  40 has its own scope for variables; if it is created using
  41 ``gfc_init_block``, it does not have its own scope.
  42
  43 It is possible to
  44
  45 * Add an expression to the end of a block using ``gfc_add_expr_to_block``
  46
  47 * Add an expression to the beginning of a block using ``void gfc_prepend_expr_to_block``
  48
  49 * Make a block into a single ``tree`` using
  50   ``gfc_finish_block``.  For example, this is needed to put the
  51   contents of a block into the ``if`` or ``else`` branch of
  52   a ``COND_EXPR``.
  53
  54 Variables are also ``tree`` expressions, they can be created using
  55 ``gfc_create_var``. Assigning to a variable can be done with
  56 ``gfc_add_modify``.
  57
  58 An example: Creating a default integer type variable in the current
  59 scope with the prefix 'everything' in the ``stmt_block``
  60 ``block`` and assigning the value 42 would be
  61
  62 .. code-block:: c++
  63
  64   tree var, *block;
  65   /* Initialize block somewhere here.  */
  66   var = gfc_create_var (integer_type_node, "everything");
  67   gfc_add_modify (block, var, build_int_cst (integer_type_node, 42));