This is g77.info, produced by makeinfo version 4.2 from
/usr/src/gnu/usr.bin/gcc/gcc/f/g77.texi.

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   Permission is granted to copy, distribute and/or modify this document
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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
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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* g77: (g77).                  The GNU Fortran compiler.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU Fortran
(`g77') compiler.  It corresponds to the GCC-3.3.2 version of `g77'.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

Copyright (C) 1995,1996,1997,1998,1999,2000,2001,2002,2003 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 1.2 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.
   Contributed by James Craig Burley (<craig@jcb-sc.com>).  Inspired by
a first pass at translating `g77-0.5.16/f/DOC' that was contributed to
Craig by David Ronis (<ronis@onsager.chem.mcgill.ca>).


File: g77.info,  Node: Multiple Definitions of External Names,  Next: Limitation on Implicit Declarations,  Prev: Mangling of Names,  Up: Disappointments

Multiple Definitions of External Names
--------------------------------------

   `g77' doesn't allow a common block and an external procedure or
`BLOCK DATA' to have the same name.  Some systems allow this, but `g77'
does not, to be compatible with `f2c'.

   `g77' could special-case the way it handles `BLOCK DATA', since it
is not compatible with `f2c' in this particular area (necessarily,
since `g77' offers an important feature here), but it is likely that
such special-casing would be very annoying to people with programs that
use `EXTERNAL FOO', with no other mention of `FOO' in the same program
unit, to refer to external procedures, since the result would be that
`g77' would treat these references as requests to force-load BLOCK DATA
program units.

   In that case, if `g77' modified names of `BLOCK DATA' so they could
have the same names as `COMMON', users would find that their programs
wouldn't link because the `FOO' procedure didn't have its name
translated the same way.

   (Strictly speaking, `g77' could emit a
null-but-externally-satisfying definition of `FOO' with its name
transformed as if it had been a `BLOCK DATA', but that probably invites
more trouble than it's worth.)


File: g77.info,  Node: Limitation on Implicit Declarations,  Prev: Multiple Definitions of External Names,  Up: Disappointments

Limitation on Implicit Declarations
-----------------------------------

   `g77' disallows `IMPLICIT CHARACTER*(*)'.  This is not
standard-conforming.


File: g77.info,  Node: Non-bugs,  Next: Warnings and Errors,  Prev: Disappointments,  Up: Trouble

Certain Changes We Don't Want to Make
=====================================

   This section lists changes that people frequently request, but which
we do not make because we think GNU Fortran is better without them.

* Menu:

* Backslash in Constants::           Why `'\\'' is a constant that
                                       is one, not two, characters long.
* Initializing Before Specifying::   Why `DATA VAR/1/' can't precede
                                       `COMMON VAR'.
* Context-Sensitive Intrinsicness::  Why `CALL SQRT' won't work.
* Context-Sensitive Constants::      Why `9.435784839284958' is a
                                       single-precision constant,
                                       and might be interpreted as
                                       `9.435785' or similar.
* Equivalence Versus Equality::      Why `.TRUE. .EQ. .TRUE.' won't work.
* Order of Side Effects::            Why `J = IFUNC() - IFUNC()' might
                                       not behave as expected.


File: g77.info,  Node: Backslash in Constants,  Next: Initializing Before Specifying,  Up: Non-bugs

Backslash in Constants
----------------------

   In the opinion of many experienced Fortran users, `-fno-backslash'
should be the default, not `-fbackslash', as currently set by `g77'.

   First of all, you can always specify `-fno-backslash' to turn off
this processing.

   Despite not being within the spirit (though apparently within the
letter) of the ANSI FORTRAN 77 standard, `g77' defaults to
`-fbackslash' because that is what most UNIX `f77' commands default to,
and apparently lots of code depends on this feature.

   This is a particularly troubling issue.  The use of a C construct in
the midst of Fortran code is bad enough, worse when it makes existing
Fortran programs stop working (as happens when programs written for
non-UNIX systems are ported to UNIX systems with compilers that provide
the `-fbackslash' feature as the default--sometimes with no option to
turn it off).

   The author of GNU Fortran wished, for reasons of linguistic purity,
to make `-fno-backslash' the default for GNU Fortran and thus require
users of UNIX `f77' and `f2c' to specify `-fbackslash' to get the UNIX
behavior.

   However, the realization that `g77' is intended as a replacement for
_UNIX_ `f77', caused the author to choose to make `g77' as compatible
with `f77' as feasible, which meant making `-fbackslash' the default.

   The primary focus on compatibility is at the source-code level, and
the question became "What will users expect a replacement for `f77' to
do, by default?"  Although at least one UNIX `f77' does not provide
`-fbackslash' as a default, it appears that the majority of them do,
which suggests that the majority of code that is compiled by UNIX `f77'
compilers expects `-fbackslash' to be the default.

   It is probably the case that more code exists that would _not_ work
with `-fbackslash' in force than code that requires it be in force.

   However, most of _that_ code is not being compiled with `f77', and
when it is, new build procedures (shell scripts, makefiles, and so on)
must be set up anyway so that they work under UNIX.  That makes a much
more natural and safe opportunity for non-UNIX users to adapt their
build procedures for `g77''s default of `-fbackslash' than would exist
for the majority of UNIX `f77' users who would have to modify existing,
working build procedures to explicitly specify `-fbackslash' if that was
not the default.

   One suggestion has been to configure the default for `-fbackslash'
(and perhaps other options as well) based on the configuration of `g77'.

   This is technically quite straightforward, but will be avoided even
in cases where not configuring defaults to be dependent on a particular
configuration greatly inconveniences some users of legacy code.

   Many users appreciate the GNU compilers because they provide an
environment that is uniform across machines.  These users would be
inconvenienced if the compiler treated things like the format of the
source code differently on certain machines.

   Occasionally users write programs intended only for a particular
machine type.  On these occasions, the users would benefit if the GNU
Fortran compiler were to support by default the same dialect as the
other compilers on that machine.  But such applications are rare.  And
users writing a program to run on more than one type of machine cannot
possibly benefit from this kind of compatibility.  (This is consistent
with the design goals for `gcc'.  To change them for `g77', you must
first change them for `gcc'.  Do not ask the maintainers of `g77' to do
this for you, or to disassociate `g77' from the widely understood, if
not widely agreed-upon, goals for GNU compilers in general.)

   This is why GNU Fortran does and will treat backslashes in the same
fashion on all types of machines (by default).  *Note Direction of
Language Development::, for more information on this overall philosophy
guiding the development of the GNU Fortran language.

   Of course, users strongly concerned about portability should indicate
explicitly in their build procedures which options are expected by
their source code, or write source code that has as few such
expectations as possible.

   For example, avoid writing code that depends on backslash (`\')
being interpreted either way in particular, such as by starting a
program unit with:

     CHARACTER BACKSL
     PARAMETER (BACKSL = '\\')

Then, use concatenation of `BACKSL' anyplace a backslash is desired.
In this way, users can write programs which have the same meaning in
many Fortran dialects.

   (However, this technique does not work for Hollerith constants--which
is just as well, since the only generally portable uses for Hollerith
constants are in places where character constants can and should be
used instead, for readability.)


File: g77.info,  Node: Initializing Before Specifying,  Next: Context-Sensitive Intrinsicness,  Prev: Backslash in Constants,  Up: Non-bugs

Initializing Before Specifying
------------------------------

   `g77' does not allow `DATA VAR/1/' to appear in the source code
before `COMMON VAR', `DIMENSION VAR(10)', `INTEGER VAR', and so on.  In
general, `g77' requires initialization of a variable or array to be
specified _after_ all other specifications of attributes (type, size,
placement, and so on) of that variable or array are specified (though
_confirmation_ of data type is permitted).

   It is _possible_ `g77' will someday allow all of this, even though
it is not allowed by the FORTRAN 77 standard.

   Then again, maybe it is better to have `g77' always require
placement of `DATA' so that it can possibly immediately write constants
to the output file, thus saving time and space.

   That is, `DATA A/1000000*1/' should perhaps always be immediately
writable to canonical assembler, unless it's already known to be in a
`COMMON' area following as-yet-uninitialized stuff, and to do this it
cannot be followed by `COMMON A'.


File: g77.info,  Node: Context-Sensitive Intrinsicness,  Next: Context-Sensitive Constants,  Prev: Initializing Before Specifying,  Up: Non-bugs

Context-Sensitive Intrinsicness
-------------------------------

   `g77' treats procedure references to _possible_ intrinsic names as
always enabling their intrinsic nature, regardless of whether the
_form_ of the reference is valid for that intrinsic.

   For example, `CALL SQRT' is interpreted by `g77' as an invalid
reference to the `SQRT' intrinsic function, because the reference is a
subroutine invocation.

   First, `g77' recognizes the statement `CALL SQRT' as a reference to
a _procedure_ named `SQRT', not to a _variable_ with that name (as it
would for a statement such as `V = SQRT').

   Next, `g77' establishes that, in the program unit being compiled,
`SQRT' is an intrinsic--not a subroutine that happens to have the same
name as an intrinsic (as would be the case if, for example, `EXTERNAL
SQRT' was present).

   Finally, `g77' recognizes that the _form_ of the reference is
invalid for that particular intrinsic.  That is, it recognizes that it
is invalid for an intrinsic _function_, such as `SQRT', to be invoked as
a _subroutine_.

   At that point, `g77' issues a diagnostic.

   Some users claim that it is "obvious" that `CALL SQRT' references an
external subroutine of their own, not an intrinsic function.

   However, `g77' knows about intrinsic subroutines, not just
functions, and is able to support both having the same names, for
example.

   As a result of this, `g77' rejects calls to intrinsics that are not
subroutines, and function invocations of intrinsics that are not
functions, just as it (and most compilers) rejects invocations of
intrinsics with the wrong number (or types) of arguments.

   So, use the `EXTERNAL SQRT' statement in a program unit that calls a
user-written subroutine named `SQRT'.


File: g77.info,  Node: Context-Sensitive Constants,  Next: Equivalence Versus Equality,  Prev: Context-Sensitive Intrinsicness,  Up: Non-bugs

Context-Sensitive Constants
---------------------------

   `g77' does not use context to determine the types of constants or
named constants (`PARAMETER'), except for (non-standard) typeless
constants such as `'123'O'.

   For example, consider the following statement:

     PRINT *, 9.435784839284958 * 2D0

`g77' will interpret the (truncated) constant `9.435784839284958' as a
`REAL(KIND=1)', not `REAL(KIND=2)', constant, because the suffix `D0'
is not specified.

   As a result, the output of the above statement when compiled by
`g77' will appear to have "less precision" than when compiled by other
compilers.

   In these and other cases, some compilers detect the fact that a
single-precision constant is used in a double-precision context and
therefore interpret the single-precision constant as if it was
_explicitly_ specified as a double-precision constant.  (This has the
effect of appending _decimal_, not _binary_, zeros to the fractional
part of the number--producing different computational results.)

   The reason this misfeature is dangerous is that a slight, apparently
innocuous change to the source code can change the computational
results.  Consider:

     REAL ALMOST, CLOSE
     DOUBLE PRECISION FIVE
     PARAMETER (ALMOST = 5.000000000001)
     FIVE = 5
     CLOSE = 5.000000000001
     PRINT *, 5.000000000001 - FIVE
     PRINT *, ALMOST - FIVE
     PRINT *, CLOSE - FIVE
     END

Running the above program should result in the same value being printed
three times.  With `g77' as the compiler, it does.

   However, compiled by many other compilers, running the above program
would print two or three distinct values, because in two or three of
the statements, the constant `5.000000000001', which on most systems is
exactly equal to `5.' when interpreted as a single-precision constant,
is instead interpreted as a double-precision constant, preserving the
represented precision.  However, this "clever" promotion of type does
not extend to variables or, in some compilers, to named constants.

   Since programmers often are encouraged to replace manifest constants
or permanently-assigned variables with named constants (`PARAMETER' in
Fortran), and might need to replace some constants with variables
having the same values for pertinent portions of code, it is important
that compilers treat code so modified in the same way so that the
results of such programs are the same.  `g77' helps in this regard by
treating constants just the same as variables in terms of determining
their types in a context-independent way.

   Still, there is a lot of existing Fortran code that has been written
to depend on the way other compilers freely interpret constants' types
based on context, so anything `g77' can do to help flag cases of this
in such code could be very helpful.


File: g77.info,  Node: Equivalence Versus Equality,  Next: Order of Side Effects,  Prev: Context-Sensitive Constants,  Up: Non-bugs

Equivalence Versus Equality
---------------------------

   Use of `.EQ.' and `.NE.' on `LOGICAL' operands is not supported,
except via `-fugly-logint', which is not recommended except for legacy
code (where the behavior expected by the _code_ is assumed).

   Legacy code should be changed, as resources permit, to use `.EQV.'
and `.NEQV.' instead, as these are permitted by the various Fortran
standards.

   New code should never be written expecting `.EQ.' or `.NE.' to work
if either of its operands is `LOGICAL'.

   The problem with supporting this "feature" is that there is unlikely
to be consensus on how it works, as illustrated by the following sample
program:

     LOGICAL L,M,N
     DATA L,M,N /3*.FALSE./
     IF (L.AND.M.EQ.N) PRINT *,'L.AND.M.EQ.N'
     END

   The issue raised by the above sample program is: what is the
precedence of `.EQ.' (and `.NE.') when applied to `LOGICAL' operands?

   Some programmers will argue that it is the same as the precedence
for `.EQ.' when applied to numeric (such as `INTEGER') operands.  By
this interpretation, the subexpression `M.EQ.N' must be evaluated first
in the above program, resulting in a program that, when run, does not
execute the `PRINT' statement.

   Other programmers will argue that the precedence is the same as the
precedence for `.EQV.', which is restricted by the standards to
`LOGICAL' operands.  By this interpretation, the subexpression
`L.AND.M' must be evaluated first, resulting in a program that _does_
execute the `PRINT' statement.

   Assigning arbitrary semantic interpretations to syntactic expressions
that might legitimately have more than one "obvious" interpretation is
generally unwise.

   The creators of the various Fortran standards have done a good job
in this case, requiring a distinct set of operators (which have their
own distinct precedence) to compare `LOGICAL' operands.  This
requirement results in expression syntax with more certain precedence
(without requiring substantial context), making it easier for
programmers to read existing code.  `g77' will avoid muddying up
elements of the Fortran language that were well-designed in the first
place.

   (Ask C programmers about the precedence of expressions such as `(a)
& (b)' and `(a) - (b)'--they cannot even tell you, without knowing more
context, whether the `&' and `-' operators are infix (binary) or unary!)

   Most dangerous of all is the fact that, even assuming consensus on
its meaning, an expression like `L.AND.M.EQ.N', if it is the result of
a typographical error, doesn't _look_ like it has such a typo.  Even
experienced Fortran programmers would not likely notice that
`L.AND.M.EQV.N' was, in fact, intended.

   So, this is a prime example of a circumstance in which a quality
compiler diagnoses the code, instead of leaving it up to someone
debugging it to know to turn on special compiler options that might
diagnose it.


File: g77.info,  Node: Order of Side Effects,  Prev: Equivalence Versus Equality,  Up: Non-bugs

Order of Side Effects
---------------------

   `g77' does not necessarily produce code that, when run, performs
side effects (such as those performed by function invocations) in the
same order as in some other compiler--or even in the same order as
another version, port, or invocation (using different command-line
options) of `g77'.

   It is never safe to depend on the order of evaluation of side
effects.  For example, an expression like this may very well behave
differently from one compiler to another:

     J = IFUNC() - IFUNC()

There is no guarantee that `IFUNC' will be evaluated in any particular
order.  Either invocation might happen first.  If `IFUNC' returns 5 the
first time it is invoked, and returns 12 the second time, `J' might end
up with the value `7', or it might end up with `-7'.

   Generally, in Fortran, procedures with side-effects intended to be
visible to the caller are best designed as _subroutines_, not functions.
Examples of such side-effects include:

   * The generation of random numbers that are intended to influence
     return values.

   * Performing I/O (other than internal I/O to local variables).

   * Updating information in common blocks.

   An example of a side-effect that is not intended to be visible to
the caller is a function that maintains a cache of recently calculated
results, intended solely to speed repeated invocations of the function
with identical arguments.  Such a function can be safely used in
expressions, because if the compiler optimizes away one or more calls
to the function, operation of the program is unaffected (aside from
being speeded up).


File: g77.info,  Node: Warnings and Errors,  Prev: Non-bugs,  Up: Trouble

Warning Messages and Error Messages
===================================

   The GNU compiler can produce two kinds of diagnostics: errors and
warnings.  Each kind has a different purpose:

     _Errors_ report problems that make it impossible to compile your
     program.  GNU Fortran reports errors with the source file name,
     line number, and column within the line where the problem is
     apparent.

     _Warnings_ report other unusual conditions in your code that
     _might_ indicate a problem, although compilation can (and does)
     proceed.  Warning messages also report the source file name, line
     number, and column information, but include the text `warning:' to
     distinguish them from error messages.

   Warnings might indicate danger points where you should check to make
sure that your program really does what you intend; or the use of
obsolete features; or the use of nonstandard features of GNU Fortran.
Many warnings are issued only if you ask for them, with one of the `-W'
options (for instance, `-Wall' requests a variety of useful warnings).

   _Note:_ Currently, the text of the line and a pointer to the column
is printed in most `g77' diagnostics.

   *Note Options to Request or Suppress Warnings: Warning Options, for
more detail on these and related command-line options.


File: g77.info,  Node: Open Questions,  Next: Bugs,  Prev: Trouble,  Up: Top

Open Questions
**************

   Please consider offering useful answers to these questions!

   * `LOC()' and other intrinsics are probably somewhat misclassified.
     Is the a need for more precise classification of intrinsics, and
     if so, what are the appropriate groupings?  Is there a need to
     individually enable/disable/delete/hide intrinsics from the
     command line?


File: g77.info,  Node: Bugs,  Next: Service,  Prev: Open Questions,  Up: Top

Reporting Bugs
**************

   Your bug reports play an essential role in making GNU Fortran
reliable.

   When you encounter a problem, the first thing to do is to see if it
is already known.  *Note Trouble::.  If it isn't known, then you should
report the problem.

* Menu:

* Criteria: Bug Criteria.    Have you really found a bug?
* Reporting: Bug Reporting.  How to report a bug effectively.

   *Note Known Causes of Trouble with GNU Fortran: Trouble, for
information on problems we already know about.

   *Note How To Get Help with GNU Fortran: Service, for information on
where to ask for help.


File: g77.info,  Node: Bug Criteria,  Next: Bug Reporting,  Up: Bugs

Have You Found a Bug?
=====================

   If you are not sure whether you have found a bug, here are some
guidelines:

   * If the compiler gets a fatal signal, for any input whatever, that
     is a compiler bug.  Reliable compilers never crash--they just
     remain obsolete.

   * If the compiler produces invalid assembly code, for any input
     whatever, that is a compiler bug, unless the compiler reports
     errors (not just warnings) which would ordinarily prevent the
     assembler from being run.

   * If the compiler produces valid assembly code that does not
     correctly execute the input source code, that is a compiler bug.

     However, you must double-check to make sure, because you might
     have run into an incompatibility between GNU Fortran and
     traditional Fortran.  These incompatibilities might be considered
     bugs, but they are inescapable consequences of valuable features.

     Or you might have a program whose behavior is undefined, which
     happened by chance to give the desired results with another
     Fortran compiler.  It is best to check the relevant Fortran
     standard thoroughly if it is possible that the program indeed does
     something undefined.

     After you have localized the error to a single source line, it
     should be easy to check for these things.  If your program is
     correct and well defined, you have found a compiler bug.

     It might help if, in your submission, you identified the specific
     language in the relevant Fortran standard that specifies the
     desired behavior, if it isn't likely to be obvious and agreed-upon
     by all Fortran users.

   * If the compiler produces an error message for valid input, that is
     a compiler bug.

   * If the compiler does not produce an error message for invalid
     input, that is a compiler bug.  However, you should note that your
     idea of "invalid input" might be someone else's idea of "an
     extension" or "support for traditional practice".

   * If you are an experienced user of Fortran compilers, your
     suggestions for improvement of GNU Fortran are welcome in any case.

   Many, perhaps most, bug reports against `g77' turn out to be bugs in
the user's code.  While we find such bug reports educational, they
sometimes take a considerable amount of time to track down or at least
respond to--time we could be spending making `g77', not some user's
code, better.

   Some steps you can take to verify that the bug is not certainly in
the code you're compiling with `g77':

   * Compile your code using the `g77' options `-W -Wall -O'.  These
     options enable many useful warning; the `-O' option enables flow
     analysis that enables the uninitialized-variable warning.

     If you investigate the warnings and find evidence of possible bugs
     in your code, fix them first and retry `g77'.

   * Compile your code using the `g77' options `-finit-local-zero',
     `-fno-automatic', `-ffloat-store', and various combinations
     thereof.

     If your code works with any of these combinations, that is not
     proof that the bug isn't in `g77'--a `g77' bug exposed by your
     code might simply be avoided, or have a different, more subtle
     effect, when different options are used--but it can be a strong
     indicator that your code is making unwarranted assumptions about
     the Fortran dialect and/or underlying machine it is being compiled
     and run on.

     *Note Overly Convenient Command-Line Options: Overly Convenient
     Options, for information on the `-fno-automatic' and
     `-finit-local-zero' options and how to convert their use into
     selective changes in your own code.

   * Validate your code with `ftnchek' or a similar code-checking tool.
     `ftnchek' can be found at `ftp://ftp.netlib.org/fortran' or
     `ftp://ftp.dsm.fordham.edu'.

     Here are some sample `Makefile' rules using `ftnchek' "project"
     files to do cross-file checking and `sfmakedepend' (from
     `ftp://ahab.rutgers.edu/pub/perl/sfmakedepend') to maintain
     dependencies automatically.  These assume the use of GNU `make'.

          # Dummy suffix for ftnchek targets:
          .SUFFIXES: .chek
          .PHONY: chekall
          
          # How to compile .f files (for implicit rule):
          FC = g77
          # Assume `include' directory:
          FFLAGS = -Iinclude -g -O -Wall
          
          # Flags for ftnchek:
          CHEK1 = -array=0 -include=includes -noarray
          CHEK2 = -nonovice -usage=1 -notruncation
          CHEKFLAGS = $(CHEK1) $(CHEK2)
          
          # Run ftnchek with all the .prj files except the one corresponding
          # to the target's root:
          %.chek : %.f ; \
            ftnchek $(filter-out $*.prj,$(PRJS)) $(CHEKFLAGS) \
              -noextern -library $<
          
          # Derive a project file from a source file:
          %.prj : %.f ; \
            ftnchek $(CHEKFLAGS) -noextern -project -library $<
          
          # The list of objects is assumed to be in variable OBJS.
          # Sources corresponding to the objects:
          SRCS = $(OBJS:%.o=%.f)
          # ftnchek project files:
          PRJS = $(OBJS:%.o=%.prj)
          
          # Build the program
          prog: $(OBJS) ; \
            $(FC) -o $ $(OBJS)
          
          chekall: $(PRJS) ; \
            ftnchek $(CHEKFLAGS) $(PRJS)
          
          prjs: $(PRJS)
          
          # For Emacs M-x find-tag:
          TAGS: $(SRCS) ; \
            etags $(SRCS)
          
          # Rebuild dependencies:
          depend: ; \
            sfmakedepend -I $(PLTLIBDIR) -I includes -a prj $(SRCS1)

   * Try your code out using other Fortran compilers, such as `f2c'.
     If it does not work on at least one other compiler (assuming the
     compiler supports the features the code needs), that is a strong
     indicator of a bug in the code.

     However, even if your code works on many compilers _except_ `g77',
     that does _not_ mean the bug is in `g77'.  It might mean the bug
     is in your code, and that `g77' simply exposes it more readily
     than other compilers.


File: g77.info,  Node: Bug Reporting,  Prev: Bug Criteria,  Up: Bugs

How to Report Bugs
==================

   Bugs should be reported to our bug database.  Please refer to
`http://gcc.gnu.org/bugs.html' for up-to-date instructions how to
submit bug reports.  Copies of this file in HTML (`bugs.html') and
plain text (`BUGS') are also part of GCC releases.


File: g77.info,  Node: Service,  Next: Adding Options,  Prev: Bugs,  Up: Top

How To Get Help with GNU Fortran
********************************

   If you need help installing, using or changing GNU Fortran, there
are two ways to find it:

   * Look in the service directory for someone who might help you for a
     fee.  The service directory is found in the file named `SERVICE'
     in the GNU CC distribution.

   * Send a message to <gcc-help@gcc.gnu.org>.


File: g77.info,  Node: Adding Options,  Next: Projects,  Prev: Service,  Up: Top

Adding Options
**************

   To add a new command-line option to `g77', first decide what kind of
option you wish to add.  Search the `g77' and `gcc' documentation for
one or more options that is most closely like the one you want to add
(in terms of what kind of effect it has, and so on) to help clarify its
nature.

   * _Fortran options_ are options that apply only when compiling
     Fortran programs.  They are accepted by `g77' and `gcc', but they
     apply only when compiling Fortran programs.

   * _Compiler options_ are options that apply when compiling most any
     kind of program.

   _Fortran options_ are listed in the file `gcc/gcc/f/lang-options.h',
which is used during the build of `gcc' to build a list of all options
that are accepted by at least one language's compiler.  This list goes
into the `documented_lang_options' array in `gcc/toplev.c', which uses
this array to determine whether a particular option should be offered
to the linked-in front end for processing by calling
`lang_option_decode', which, for `g77', is in `gcc/gcc/f/com.c' and just
calls `ffe_decode_option'.

   If the linked-in front end "rejects" a particular option passed to
it, `toplev.c' just ignores the option, because _some_ language's
compiler is willing to accept it.

   This allows commands like `gcc -fno-asm foo.c bar.f' to work, even
though Fortran compilation does not currently support the `-fno-asm'
option; even though the `f771' version of `lang_decode_option' rejects
`-fno-asm', `toplev.c' doesn't produce a diagnostic because some other
language (C) does accept it.

   This also means that commands like `g77 -fno-asm foo.f' yield no
diagnostics, despite the fact that no phase of the command was able to
recognize and process `-fno-asm'--perhaps a warning about this would be
helpful if it were possible.

   Code that processes Fortran options is found in `gcc/gcc/f/top.c',
function `ffe_decode_option'.  This code needs to check positive and
negative forms of each option.

   The defaults for Fortran options are set in their global
definitions, also found in `gcc/gcc/f/top.c'.  Many of these defaults
are actually macros defined in `gcc/gcc/f/target.h', since they might be
machine-specific.  However, since, in practice, GNU compilers should
behave the same way on all configurations (especially when it comes to
language constructs), the practice of setting defaults in `target.h' is
likely to be deprecated and, ultimately, stopped in future versions of
`g77'.

   Accessor macros for Fortran options, used by code in the `g77' FFE,
are defined in `gcc/gcc/f/top.h'.

   _Compiler options_ are listed in `gcc/toplev.c' in the array
`f_options'.  An option not listed in `lang_options' is looked up in
`f_options' and handled from there.

   The defaults for compiler options are set in the global definitions
for the corresponding variables, some of which are in `gcc/toplev.c'.

   You can set different defaults for _Fortran-oriented_ or
_Fortran-reticent_ compiler options by changing the source code of
`g77' and rebuilding.  How to do this depends on the version of `g77':

`G77 0.5.24 (EGCS 1.1)'
`G77 0.5.25 (EGCS 1.2 - which became GCC 2.95)'
     Change the `lang_init_options' routine in `gcc/gcc/f/com.c'.

     (Note that these versions of `g77' perform internal consistency
     checking automatically when the `-fversion' option is specified.)

`G77 0.5.23'
`G77 0.5.24 (EGCS 1.0)'
     Change the way `f771' handles the `-fset-g77-defaults' option,
     which is always provided as the first option when called by `g77'
     or `gcc'.

     This code is in `ffe_decode_options' in `gcc/gcc/f/top.c'.  Have
     it change just the variables that you want to default to a
     different setting for Fortran compiles compared to compiles of
     other languages.

     The `-fset-g77-defaults' option is passed to `f771' automatically
     because of the specification information kept in
     `gcc/gcc/f/lang-specs.h'.  This file tells the `gcc' command how
     to recognize, in this case, Fortran source files (those to be
     preprocessed, and those that are not), and further, how to invoke
     the appropriate programs (including `f771') to process those
     source files.

     It is in `gcc/gcc/f/lang-specs.h' that `-fset-g77-defaults',
     `-fversion', and other options are passed, as appropriate, even
     when the user has not explicitly specified them.  Other "internal"
     options such as `-quiet' also are passed via this mechanism.


File: g77.info,  Node: Projects,  Next: Front End,  Prev: Adding Options,  Up: Top

Projects
********

   If you want to contribute to `g77' by doing research, design,
specification, documentation, coding, or testing, the following
information should give you some ideas.  More relevant information
might be available from `ftp://alpha.gnu.org/gnu/g77/projects/'.

* Menu:

* Efficiency::               Make `g77' itself compile code faster.
* Better Optimization::      Teach `g77' to generate faster code.
* Simplify Porting::         Make `g77' easier to configure, build,
                             and install.
* More Extensions::          Features many users won't know to ask for.
* Machine Model::            `g77' should better leverage `gcc'.
* Internals Documentation::  Make maintenance easier.
* Internals Improvements::   Make internals more robust.
* Better Diagnostics::       Make using `g77' on new code easier.


File: g77.info,  Node: Efficiency,  Next: Better Optimization,  Up: Projects

Improve Efficiency
==================

   Don't bother doing any performance analysis until most of the
following items are taken care of, because there's no question they
represent serious space/time problems, although some of them show up
only given certain kinds of (popular) input.

   * Improve `malloc' package and its uses to specify more info about
     memory pools and, where feasible, use obstacks to implement them.

   * Skip over uninitialized portions of aggregate areas (arrays,
     `COMMON' areas, `EQUIVALENCE' areas) so zeros need not be output.
     This would reduce memory usage for large initialized aggregate
     areas, even ones with only one initialized element.

     As of version 0.5.18, a portion of this item has already been
     accomplished.

   * Prescan the statement (in `sta.c') so that the nature of the
     statement is determined as much as possible by looking entirely at
     its form, and not looking at any context (previous statements,
     including types of symbols).  This would allow ripping out of the
     statement-confirmation, symbol retraction/confirmation, and
     diagnostic inhibition mechanisms.  Plus, it would result in
     much-improved diagnostics.  For example, `CALL
     some-intrinsic(...)', where the intrinsic is not a subroutine
     intrinsic, would result actual error instead of the
     unimplemented-statement catch-all.

   * Throughout `g77', don't pass line/column pairs where a simple
     `ffewhere' type, which points to the error as much as is desired
     by the configuration, will do, and don't pass `ffelexToken' types
     where a simple `ffewhere' type will do.  Then, allow new default
     configuration of `ffewhere' such that the source line text is not
     preserved, and leave it to things like Emacs' next-error function
     to point to them (now that `next-error' supports column, or,
     perhaps, character-offset, numbers).  The change in calling
     sequences should improve performance somewhat, as should not
     having to save source lines.  (Whether this whole item will
     improve performance is questionable, but it should improve
     maintainability.)

   * Handle `DATA (A(I),I=1,1000000)/1000000*2/' more efficiently,
     especially as regards the assembly output.  Some of this might
     require improving the back end, but lots of improvement in
     space/time required in `g77' itself can be fairly easily obtained
     without touching the back end.  Maybe type-conversion, where
     necessary, can be speeded up as well in cases like the one shown
     (converting the `2' into `2.').

   * If analysis shows it to be worthwhile, optimize `lex.c'.

   * Consider redesigning `lex.c' to not need any feedback during
     tokenization, by keeping track of enough parse state on its own.


File: g77.info,  Node: Better Optimization,  Next: Simplify Porting,  Prev: Efficiency,  Up: Projects

Better Optimization
===================

   Much of this work should be put off until after `g77' has all the
features necessary for its widespread acceptance as a useful F77
compiler.  However, perhaps this work can be done in parallel during
the feature-adding work.

   * Do the equivalent of the trick of putting `extern inline' in front
     of every function definition in `libg2c' and #include'ing the
     resulting file in `f2c'+`gcc'--that is, inline all
     run-time-library functions that are at all worth inlining.  (Some
     of this has already been done, such as for integral
     exponentiation.)

   * When doing `CHAR_VAR = CHAR_FUNC(...)', and it's clear that types
     line up and `CHAR_VAR' is addressable or not a `VAR_DECL', make
     `CHAR_VAR', not a temporary, be the receiver for `CHAR_FUNC'.
     (This is now done for `COMPLEX' variables.)

   * Design and implement Fortran-specific optimizations that don't
     really belong in the back end, or where the front end needs to
     give the back end more info than it currently does.

   * Design and implement a new run-time library interface, with the
     code going into `libgcc' so no special linking is required to link
     Fortran programs using standard language features.  This library
     would speed up lots of things, from I/O (using precompiled formats,
     doing just one, or, at most, very few, calls for arrays or array
     sections, and so on) to general computing (array/section
     implementations of various intrinsics, implementation of commonly
     performed loops that aren't likely to be optimally compiled
     otherwise, etc.).

     Among the important things the library would do are:

        * Be a one-stop-shop-type library, hence shareable and usable
          by all, in that what are now library-build-time options in
          `libg2c' would be moved at least to the `g77' compile phase,
          if not to finer grains (such as choosing how list-directed
          I/O formatting is done by default at `OPEN' time, for
          preconnected units via options or even statements in the main
          program unit, maybe even on a per-I/O basis with appropriate
          pragma-like devices).

   * Probably requiring the new library design, change interface to
     normally have `COMPLEX' functions return their values in the way
     `gcc' would if they were declared `__complex__ float', rather than
     using the mechanism currently used by `CHARACTER' functions
     (whereby the functions are compiled as returning void and their
     first arg is a pointer to where to store the result).  (Don't
     append underscores to external names for `COMPLEX' functions in
     some cases once `g77' uses `gcc' rather than `f2c' calling
     conventions.)

   * Do something useful with `doiter' references where possible.  For
     example, `CALL FOO(I)' cannot modify `I' if within a `DO' loop
     that uses `I' as the iteration variable, and the back end might
     find that info useful in determining whether it needs to read `I'
     back into a register after the call.  (It normally has to do that,
     unless it knows `FOO' never modifies its passed-by-reference
     argument, which is rarely the case for Fortran-77 code.)


File: g77.info,  Node: Simplify Porting,  Next: More Extensions,  Prev: Better Optimization,  Up: Projects

Simplify Porting
================

   Making `g77' easier to configure, port, build, and install, either
as a single-system compiler or as a cross-compiler, would be very
useful.

   * A new library (replacing `libg2c') should improve portability as
     well as produce more optimal code.  Further, `g77' and the new
     library should conspire to simplify naming of externals, such as
     by removing unnecessarily added underscores, and to
     reduce/eliminate the possibility of naming conflicts, while making
     debugger more straightforward.

     Also, it should make multi-language applications more feasible,
     such as by providing Fortran intrinsics that get Fortran unit
     numbers given C `FILE *' descriptors.

   * Possibly related to a new library, `g77' should produce the
     equivalent of a `gcc' `main(argc, argv)' function when it compiles
     a main program unit, instead of compiling something that must be
     called by a library implementation of `main()'.

     This would do many useful things such as provide more flexibility
     in terms of setting up exception handling, not requiring
     programmers to start their debugging sessions with `breakpoint
     MAIN__' followed by `run', and so on.

   * The GBE needs to understand the difference between alignment
     requirements and desires.  For example, on Intel x86 machines,
     `g77' currently imposes overly strict alignment requirements, due
     to the back end, but it would be useful for Fortran and C
     programmers to be able to override these _recommendations_ as long
     as they don't violate the actual processor _requirements_.


File: g77.info,  Node: More Extensions,  Next: Machine Model,  Prev: Simplify Porting,  Up: Projects

More Extensions
===============

   These extensions are not the sort of things users ask for "by name",
but they might improve the usability of `g77', and Fortran in general,
in the long run.  Some of these items really pertain to improving `g77'
internals so that some popular extensions can be more easily supported.

   * Look through all the documentation on the GNU Fortran language,
     dialects, compiler, missing features, bugs, and so on.  Many
     mentions of incomplete or missing features are sprinkled
     throughout.  It is not worth repeating them here.

   * Consider adding a `NUMERIC' type to designate typeless numeric
     constants, named and unnamed.  The idea is to provide a
     forward-looking, effective replacement for things like the
     old-style `PARAMETER' statement when people really need
     typelessness in a maintainable, portable, clearly documented way.
     Maybe `TYPELESS' would include `CHARACTER', `POINTER', and
     whatever else might come along.  (This is not really a call for
     polymorphism per se, just an ability to express limited, syntactic
     polymorphism.)

   * Support `OPEN(...,KEY=(...),...)'.

   * Support arbitrary file unit numbers, instead of limiting them to 0
     through `MXUNIT-1'.  (This is a `libg2c' issue.)

   * `OPEN(NOSPANBLOCKS,...)' is treated as
     `OPEN(UNIT=NOSPANBLOCKS,...)', so a later `UNIT=' in the first
     example is invalid.  Make sure this is what users of this feature
     would expect.

   * Currently `g77' disallows `READ(1'10)' since it is an obnoxious
     syntax, but supporting it might be pretty easy if needed.  More
     details are needed, such as whether general expressions separated
     by an apostrophe are supported, or maybe the record number can be
     a general expression, and so on.

   * Support `STRUCTURE', `UNION', `MAP', and `RECORD' fully.
     Currently there is no support at all for `%FILL' in `STRUCTURE'
     and related syntax, whereas the rest of the stuff has at least
     some parsing support.  This requires either major changes to
     `libg2c' or its replacement.

   * F90 and `g77' probably disagree about label scoping relative to
     `INTERFACE' and `END INTERFACE', and their contained procedure
     interface bodies (blocks?).

   * `ENTRY' doesn't support F90 `RESULT()' yet, since that was added
     after S8.112.

   * Empty-statement handling (10 ;;CONTINUE;;) probably isn't
     consistent with the final form of the standard (it was vague at
     S8.112).

   * It seems to be an "open" question whether a file, immediately
     after being `OPEN'ed,is positioned at the beginning, the end, or
     wherever--it might be nice to offer an option of opening to
     "undefined" status, requiring an explicit absolute-positioning
     operation to be performed before any other (besides `CLOSE') to
     assist in making applications port to systems (some IBM?) that
     `OPEN' to the end of a file or some such thing.


File: g77.info,  Node: Machine Model,  Next: Internals Documentation,  Prev: More Extensions,  Up: Projects

Machine Model
=============

   This items pertain to generalizing `g77''s view of the machine model
to more fully accept whatever the GBE provides it via its configuration.

   * Switch to using `REAL_VALUE_TYPE' to represent floating-point
     constants exclusively so the target float format need not be
     required.  This means changing the way `g77' handles
     initialization of aggregate areas having more than one type, such
     as `REAL' and `INTEGER', because currently it initializes them as
     if they were arrays of `char' and uses the bit patterns of the
     constants of the various types in them to determine what to stuff
     in elements of the arrays.

   * Rely more and more on back-end info and capabilities, especially
     in the area of constants (where having the `g77' front-end's IL
     just store the appropriate tree nodes containing constants might
     be best).

   * Suite of C and Fortran programs that a user/administrator can run
     on a machine to help determine the configuration for `g77' before
     building and help determine if the compiler works (especially with
     whatever libraries are installed) after building.


File: g77.info,  Node: Internals Documentation,  Next: Internals Improvements,  Prev: Machine Model,  Up: Projects

Internals Documentation
=======================

   Better info on how `g77' works and how to port it is needed.

   *Note Front End::, which contains some information on `g77'
internals.


File: g77.info,  Node: Internals Improvements,  Next: Better Diagnostics,  Prev: Internals Documentation,  Up: Projects

Internals Improvements
======================

   Some more items that would make `g77' more reliable and easier to
maintain:

   * Generally make expression handling focus more on critical syntax
     stuff, leaving semantics to callers.  For example, anything a
     caller can check, semantically, let it do so, rather than having
     `expr.c' do it.  (Exceptions might include things like diagnosing
     `FOO(I--K:)=BAR' where `FOO' is a `PARAMETER'--if it seems
     important to preserve the left-to-right-in-source order of
     production of diagnostics.)

   * Come up with better naming conventions for `-D' to establish
     requirements to achieve desired implementation dialect via
     `proj.h'.

   * Clean up used tokens and `ffewhere's in `ffeglobal_terminate_1'.

   * Replace `sta.c' `outpooldisp' mechanism with `malloc_pool_use'.

   * Check for `opANY' in more places in `com.c', `std.c', and `ste.c',
     and get rid of the `opCONVERT(opANY)' kludge (after determining if
     there is indeed no real need for it).

   * Utility to read and check `bad.def' messages and their references
     in the code, to make sure calls are consistent with message
     templates.

   * Search and fix `&ffe...' and similar so that `ffe...ptr...' macros
     are available instead (a good argument for wishing this could have
     written all this stuff in C++, perhaps).  On the other hand, it's
     questionable whether this sort of improvement is really necessary,
     given the availability of tools such as Emacs and Perl, which make
     finding any address-taking of structure members easy enough?

   * Some modules truly export the member names of their structures
     (and the structures themselves), maybe fix this, and fix other
     modules that just appear to as well (by appending `_', though it'd
     be ugly and probably not worth the time).

   * Implement C macros `RETURNS(value)' and `SETS(something,value)' in
     `proj.h' and use them throughout `g77' source code (especially in
     the definitions of access macros in `.h' files) so they can be
     tailored to catch code writing into a `RETURNS()' or reading from
     a `SETS()'.

   * Decorate throughout with `const' and other such stuff.

   * All F90 notational derivations in the source code are still based
     on the S8.112 version of the draft standard.  Probably should
     update to the official standard, or put documentation of the rules
     as used in the code...uh...in the code.

   * Some `ffebld_new' calls (those outside of `ffeexpr.c' or inside
     but invoked via paths not involving `ffeexpr_lhs' or
     `ffeexpr_rhs') might be creating things in improper pools, leading
     to such things staying around too long or (doubtful, but possible
     and dangerous) not long enough.

   * Some `ffebld_list_new' (or whatever) calls might not be matched by
     `ffebld_list_bottom' (or whatever) calls, which might someday
     matter.  (It definitely is not a problem just yet.)

   * Probably not doing clean things when we fail to `EQUIVALENCE'
     something due to alignment/mismatch or other problems--they end up
     without `ffestorag' objects, so maybe the backend (and other parts
     of the front end) can notice that and handle like an `opANY' (do
     what it wants, just don't complain or crash).  Most of this seems
     to have been addressed by now, but a code review wouldn't hurt.

