     [file gcc-common.texi not included]

This file documents the internals of the GNU Fortran compiler, (gfortran).

Published by the Free Software Foundation 51 Franklin Street, Fifth Floor Boston, MA 02110-1301 USA

Copyright © 2007-2008 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License'' and ``Funding Free Software'', the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below).  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software.  Copies published by the Free Software Foundation raise funds for GNU development.

Introduction
************

This manual documents the internals of gfortran, the GNU Fortran compiler.

Introduction
************

At present, this manual is very much a work in progress, containing miscellaneous notes about the internals of the compiler.  It is hoped that at some point in the future it will become a reasonably complete guide; in the interim, GNU Fortran developers are strongly encouraged to contribute to it as a way of keeping notes while working on the compiler.

Code that Interacts with the User
*********************************

Command-Line Options
====================

Command-line options for gfortran involve four interrelated pieces within the Fortran compiler code.

The relevant command-line flag is defined in lang.opt, according to the documentation in Options (GNU Compiler Collection Internals).  This is then processed by the overall GCC machinery to create the code that enables gfortran and gcc to recognize the option in the command-line arguments and call the relevant handler function.

This generated code calls the gfc_handle_option code in options.c with an enumerator variable indicating which option is to be processed, and the relevant integer or string values associated with that option flag.  Typically, gfc_handle_option uses these arguments to set global flags which record the option states.

The global flags that record the option states are stored in the gfc_option_t struct, which is defined in gfortran.h. Before the options are processed, initial values for these flags are set in gfc_init_option in options.c; these become the default values for the options.

Error Handling
==============

The GNU Fortran compiler's parser operates by testing each piece of source code against a variety of matchers.  In some cases, if these matchers do not match the source code, they will store an error message in a buffer.  If the parser later finds a matcher that does correctly match the source code, then the buffered error is discarded.  However, if the parser cannot find a match, then the buffered error message is reported to the user.  This enables the compiler to provide more meaningful error messages even in the many cases where (erroneous) Fortran syntax is ambiguous due to things like the absence of reserved keywords.

As an example of how this works, consider the following line:

     IF = 3

Hypothetically, this may get passed to the matcher for an IF statement.  Since this could plausibly be an erroneous IF statement, the matcher will buffer an error message reporting the absence of an expected ( following an IF.  Since no matchers reported an error-free match, however, the parser will also try matching this against a variable assignment.  When IF is a valid variable, this will be parsed as an assignment statement, and the error discarded.  However, when IF is not a valid variable, this buffered error message will be reported to the user.

The error handling code is implemented in error.c.  Errors are normally entered into the buffer with the gfc_error function. Warnings go through a similar buffering process, and are entered into the buffer with gfc_warning.  There is also a special-purpose function, gfc_notify_std, for things which have an error/warning status that depends on the currently-selected language standard.

The gfc_error_check function checks the buffer for errors, reports the error message to the user if one exists, clears the buffer, and returns a flag to the user indicating whether or not an error existed.  To check the state of the buffer without changing its state or reporting the errors, the gfc_error_flag_test function can be used.  The gfc_clear_error function will clear out any errors in the buffer, without reporting them.  The gfc_warning_check and gfc_clear_warning functions provide equivalent functionality for the warning buffer.

Only one error and one warning can be in the buffers at a time, and buffering another will overwrite the existing one.  In cases where one may wish to work on a smaller piece of source code without disturbing an existing error state, the gfc_push_error, gfc_pop_error, and gfc_free_error mechanism exists to implement a stack for the error buffer.

For cases where an error or warning should be reported immediately rather than buffered, the gfc_error_now and gfc_warning_now functions can be used.  Normally, the compiler will continue attempting to parse the program after an error has occurred, but if this is not appropriate, the gfc_fatal_error function should be used instead.  For errors that are always the result of a bug somewhere in the compiler, the gfc_internal_error function should be used.

The syntax for the strings used to produce the error/warning message in the various error and warning functions is similar to the printf syntax, with %-escapes to insert variable values.  The details, and the allowable codes, are documented in the error_print function in error.c.

Frontend Data Structures
************************

This chapter should describe the details necessary to understand how the various gfc_* data are used and interact.  In general it is advisable to read the code in dump-parse-tree.c as its routines should exhaust all possible valid combinations of content for these structures.

gfc_code
========

The executable statements in a program unit are represented by a nested chain of gfc_code structures.  The type of statement is identified by the op member of the structure, the different possible values are enumerated in gfc_exec_op.  A special member of this enum is EXEC_NOP which is used to represent the various END statements if they carry a label. Depending on the type of statement some of the other fields will be filled in.  Fields that are generally applicable are the next and here fields.  The former points to the next statement in the current block or is NULL if the current statement is the last in a block, here points to the statement label of the current statement.

If the current statement is one of IF, DO, SELECT it starts a block, i.e. a nested level in the program.  In order to represent this, the block member is set to point to a gfc_code structure whose block member points to the block in question.  The SELECT and IF statements may contain various blocks (the chain of ELSE IF and ELSE blocks or the various CASEs, respectively).

The LibGFortran Runtime Library
*******************************

Symbol Versioning
=================

In general, this capability exists only on a few platforms, thus there is a need for configure magic so that it is used only on those targets where it is supported.

The central concept in symbol versioning is the so-called map file, which specifies the version node(s) exported symbols are labeled with. Also, the map file is used to hide local symbols.

Some relevant references:

  * GNU ld manual (http://www.gnu.org/software/binutils/manual/ld-2.9.1/html_node/ld_25.html)

  * ELF Symbol Versioning - Ulrich Depper (http://people.redhat.com/drepper/symbol-versioning)

  * How to Write Shared Libraries - Ulrich Drepper (see Chapter 3) (http://people.redhat.com/drepper/dsohowto.pdf)

If one adds a new symbol to a library that should be exported, the new symbol should be mentioned in the map file and a new version node defined, e.g., if one adds a new symbols foo and bar to libgfortran for the next GCC release, the following should be added to the map file:

     GFORTRAN_1.1 {
         global:
             foo;
             bar;
     } GFORTRAN_1.0;

where GFORTRAN_1.0 is the version node of the current release, and GFORTRAN_1.1 is the version node of the next release where foo and bar are made available.

If one wants to change an existing interface, it is possible by using some asm trickery (from the ld manual referenced above):

     __asm__(".symver original_foo,foo@");
     __asm__(".symver old_foo,foo@VERS_1.1");
     __asm__(".symver old_foo1,foo@VERS_1.2");
     __asm__(".symver new_foo,foo@VERS_2.0");

In this example, foo@ represents the symbol foo bound to the unspecified base version of the symbol. The source file that contains this example would define 4 C functions: original_foo, old_foo, old_foo1, and new_foo.

In this case the map file must contain foo in VERS_1.1 and VERS_1.2 as well as in VERS_2.0.

     [file fdl.texi not included]