5.2 Converting Expressons to tree

Converting expressions to tree is done by functions called gfc_conv_*.

The central data structure for a GENERIC expression is the gfc_se structure. Its expr member is a tree that holds the value of the expression. A gfc_se structure is initialized using gfc_init_se; it needs to be embedded in an outer gfc_se.

Evaluating Fortran expressions often require things to be done before and after evaluation of the expression, for example code for the allocation of a temporary variable and its subsequent deallocation. Therefore, gfc_se contains the members pre and post, which point to stmt_block blocks for code that needs to be executed before and after evaluation of the expression.

When using a local gfc_se to convert some expression, it is often necessary to add the generated pre and post blocks to the pre or post blocks of the outer gfc_se. Code like this (lifted from trans-expr.c) is fairly common:

gfc_se cont_se;
tree cont_var;

/* cont_var = is_contiguous (expr); .  */
gfc_init_se (&cont_se, parmse);
gfc_conv_is_contiguous_expr (&cont_se, expr);
gfc_add_block_to_block (&se->pre, &(&cont_se)->pre);
gfc_add_modify (&se->pre, cont_var, cont_se.expr);
gfc_add_block_to_block (&se->pre, &(&cont_se)->post);

Conversion functions which need a gfc_se structure will have a corresponding argument.

gfc_se also contains pointers to a gfc_ss and a gfc_loopinfo structure. These are needed by the scalarizer.