gcc/fortran/doc/gfc-internals/generating-the-intermediate-language-for-later-stages/converting-expressions-to-tree.rst

   1 ..
   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 .. _converting-expressions:
   7
   8 Converting Expressions to tree
   9 ******************************
  10
  11 Converting expressions to ``tree`` is done by functions called
  12 ``gfc_conv_*``.
  13
  14 The central data structure for a GENERIC expression is the
  15 ``gfc_se`` structure.  Its ``expr`` member is a ``tree`` that
  16 holds the value of the expression.  A ``gfc_se`` structure is
  17 initialized using ``gfc_init_se`` ; it needs to be embedded in an
  18 outer ``gfc_se``.
  19
  20 Evaluating Fortran expressions often require things to be done before
  21 and after evaluation of the expression, for example code for the
  22 allocation of a temporary variable and its subsequent deallocation.
  23 Therefore, ``gfc_se`` contains the members ``pre`` and
  24 ``post``, which point to ``stmt_block`` blocks for code that
  25 needs to be executed before and after evaluation of the expression.
  26
  27 When using a local ``gfc_se`` to convert some expression, it is
  28 often necessary to add the generated ``pre`` and ``post`` blocks
  29 to the ``pre`` or ``post`` blocks of the outer ``gfc_se``.
  30 Code like this (lifted from :samp:`trans-expr.cc`) is fairly common:
  31
  32 .. code-block:: c++
  33
  34   gfc_se cont_se;
  35   tree cont_var;
  36
  37   /* cont_var = is_contiguous (expr); .  */
  38   gfc_init_se (&cont_se, parmse);
  39   gfc_conv_is_contiguous_expr (&cont_se, expr);
  40   gfc_add_block_to_block (&se->pre, &(&cont_se)->pre);
  41   gfc_add_modify (&se->pre, cont_var, cont_se.expr);
  42   gfc_add_block_to_block (&se->pre, &(&cont_se)->post);
  43
  44 Conversion functions which need a ``gfc_se`` structure will have a
  45 corresponding argument.
  46
  47 ``gfc_se`` also contains pointers to a ``gfc_ss`` and a
  48 ``gfc_loopinfo`` structure.  These are needed by the scalarizer.