Basic data structures#

Gfortran creates GENERIC as an intermediate language for the middle-end. Details about GENERIC can be found in the GCC manual.

The basic data structure of GENERIC is a tree. Everything in GENERIC is a tree, including types and statements. Fortunately for the gfortran programmer, tree variables are garbage-collected, so doing memory management for them is not necessary.

tree expressions are built using functions such as, for example, fold_build2_loc. For two tree variables a and b, both of which have the type gfc_arry_index_type, calculation c = a * b would be done by

c = fold_build2_loc (input_location, MULT_EXPR,
                     gfc_array_index_type, a, b);

The types have to agree, otherwise internal compiler errors will occur at a later stage. Expressions can be converted to a different type using fold_convert.

Accessing individual members in the tree structures should not be done. Rather, access should be done via macros.

One basic data structure is the stmtblock_t struct. This is used for holding a list of statements, expressed as tree expressions. If a block is created using gfc_start_block, it has its own scope for variables; if it is created using gfc_init_block, it does not have its own scope.

It is possible to

  • Add an expression to the end of a block using gfc_add_expr_to_block

  • Add an expression to the beginning of a block using void gfc_prepend_expr_to_block

  • Make a block into a single tree using gfc_finish_block. For example, this is needed to put the contents of a block into the if or else branch of a COND_EXPR.

Variables are also tree expressions, they can be created using gfc_create_var. Assigning to a variable can be done with gfc_add_modify.

An example: Creating a default integer type variable in the current scope with the prefix ‘everything’ in the stmt_block block and assigning the value 42 would be

tree var, *block;
/* Initialize block somewhere here.  */
var = gfc_create_var (integer_type_node, "everything");
gfc_add_modify (block, var, build_int_cst (integer_type_node, 42));