This is g77.info, produced by makeinfo version 4.2 from g77.texi.

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-3.1 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 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.

   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: Gotchas (Transforming),  Next: TBD (Transforming),  Prev: ste.c,  Up: Overview of Translation Process

Gotchas (Transforming)
----------------------

   This section is not about transforming "gotchas" into something else.
It is about the weirder aspects of transforming Fortran, however that's
defined, into a more modern, canonical form.

Multi-character Lexemes
.......................

   Each lexeme carries with it a pointer to where it appears in the
source.

   To provide the ability for diagnostics to point to column numbers,
in addition to line numbers and names, lexemes that represent more than
one (significant) character in the source code need, generally, to
provide pointers to where each _character_ appears in the source.

   This provides the ability to properly identify the precise location
of the problem in code like

     SUBROUTINE X
     END
     BLOCK DATA X
     END

   which, in fixed-form source, would result in single lexemes
consisting of the strings `SUBROUTINEX' and `BLOCKDATAX'.  (The problem
is that `X' is defined twice, so a pointer to the `X' in the second
definition, as well as a follow-up pointer to the corresponding pointer
in the first, would be preferable to pointing to the beginnings of the
statements.)

   This need also arises when parsing (and diagnosing) `FORMAT'
statements.

   Further, it arises when diagnosing `FMT=' specifiers that contain
constants (or partial constants, or even propagated constants!)  in I/O
statements, as in:

     PRINT '(I2, 3HAB)', J

   (A pointer to the beginning of the prematurely-terminated Hollerith
constant, and/or to the close parenthese, is preferable to a pointer to
the open-parenthese or the apostrophe that precedes it.)

   Multi-character lexemes, which would seem to naturally include at
least digit strings, alphanumeric strings, `CHARACTER' constants, and
Hollerith constants, therefore need to provide location information on
each character.  (Maybe Hollerith constants don't, but it's unnecessary
to except them.)

   The question then arises, what about _other_ multi-character lexemes,
such as `**' and `//', and Fortran 90's `(/', `/)', `::', and so on?

   Turns out there's a need to identify the location of the second
character of these two-character lexemes.  For example, in `I(/J) = K',
the slash needs to be diagnosed as the problem, not the open parenthese.
Similarly, it is preferable to diagnose the second slash in `I = J //
K' rather than the first, given the implicit typing rules, which would
result in the compiler disallowing the attempted concatenation of two
integers.  (Though, since that's more of a semantic issue, it's not
_that_ much preferable.)

   Even sequences that could be parsed as digit strings could use
location info, for example, to diagnose the `9' in the octal constant
`O'129''.  (This probably will be parsed as a character string, to be
consistent with the parsing of `Z'129A''.)

   To avoid the hassle of recording the location of the second
character, while also preserving the general rule that each significant
character is distinctly pointed to by the lexeme that contains it, it's
best to simply not have any fixed-size lexemes larger than one
character.

   This new design is expected to make checking for two `*' lexemes in
a row much easier than the old design, so this is not much of a
sacrifice.  It probably makes the lexer much easier to implement than
it makes the parser harder.

Space-padding Lexemes
.....................

   Certain lexemes need to be padded with virtual spaces when the end
of the line (or file) is encountered.

   This is necessary in fixed form, to handle lines that don't extend
to column 72, assuming that's the line length in effect.

Bizarre Free-form Hollerith Constants
.....................................

   Last I checked, the Fortran 90 standard actually required the
compiler to silently accept something like

     FORMAT ( 1 2   Htwelve chars )

   as a valid `FORMAT' statement specifying a twelve-character
Hollerith constant.

   The implication here is that, since the new lexer is a zero-feedback
one, it won't know that the special case of a `FORMAT' statement being
parsed requires apparently distinct lexemes `1' and `2' to be treated as
a single lexeme.

   (This is a horrible misfeature of the Fortran 90 language.  It's one
of many such misfeatures that almost make me want to not support them,
and forge ahead with designing a new "GNU Fortran" language that has
the features, but not the misfeatures, of Fortran 90, and provide
utility programs to do the conversion automatically.)

   So, the lexer must gather distinct chunks of decimal strings into a
single lexeme in contexts where a single decimal lexeme might start a
Hollerith constant.

   (Which probably means it might as well do that all the time for all
multi-character lexemes, even in free-form mode, leaving it to
subsequent phases to pull them apart as they see fit.)

   Compare the treatment of this to how

     CHARACTER * 4 5 HEY

   and

     CHARACTER * 12 HEY

   must be treated--the former must be diagnosed, due to the separation
between lexemes, the latter must be accepted as a proper declaration.

Hollerith Constants
...................

   Recognizing a Hollerith constant--specifically, that an `H' or `h'
after a digit string begins such a constant--requires some knowledge of
context.

   Hollerith constants (such as `2HAB') can appear after:

   * `('

   * `,'

   * `='

   * `+', `-', `/'

   * `*', except as noted below

   Hollerith constants don't appear after:

   * `CHARACTER*', which can be treated generally as any `*' that is
     the second lexeme of a statement

Confusing Function Keyword
..........................

   While

     REAL FUNCTION FOO ()

   must be a `FUNCTION' statement and

     REAL FUNCTION FOO (5)

   must be a type-definition statement,

     REAL FUNCTION FOO (NAMES)

   where NAMES is a comma-separated list of names, can be one or the
other.

   The only way to disambiguate that statement (short of mandating
free-form source or a short maximum length for name for external
procedures) is based on the context of the statement.

   In particular, the statement is known to be within an
already-started program unit (but not at the outer level of the
`CONTAINS' block), it is a type-declaration statement.

   Otherwise, the statement is a `FUNCTION' statement, in that it
begins a function program unit (external, or, within `CONTAINS',
nested).

Weird READ
..........

   The statement

     READ (N)

   is equivalent to either

     READ (UNIT=(N))

   or

     READ (FMT=(N))

   depending on which would be valid in context.

   Specifically, if `N' is type `INTEGER', `READ (FMT=(N))' would not
be valid, because parentheses may not be used around `N', whereas they
may around it in `READ (UNIT=(N))'.

   Further, if `N' is type `CHARACTER', the opposite is true--`READ
(UNIT=(N))' is not valid, but `READ (FMT=(N))' is.

   Strictly speaking, if anything follows

     READ (N)

   in the statement, whether the first lexeme after the close
parenthese is a comma could be used to disambiguate the two cases,
without looking at the type of `N', because the comma is required for
the `READ (FMT=(N))' interpretation and disallowed for the `READ
(UNIT=(N))' interpretation.

   However, in practice, many Fortran compilers allow the comma for the
`READ (UNIT=(N))' interpretation anyway (in that they generally allow a
leading comma before an I/O list in an I/O statement), and much code
takes advantage of this allowance.

   (This is quite a reasonable allowance, since the juxtaposition of a
comma-separated list immediately after an I/O control-specification
list, which is also comma-separated, without an intervening comma,
looks sufficiently "wrong" to programmers that they can't resist the
itch to insert the comma.  `READ (I, J), K, L' simply looks cleaner than
`READ (I, J) K, L'.)

   So, type-based disambiguation is needed unless strict adherence to
the standard is always assumed, and we're not going to assume that.


File: g77.info,  Node: TBD (Transforming),  Prev: Gotchas (Transforming),  Up: Overview of Translation Process

TBD (Transforming)
------------------

   Continue researching gotchas, designing the transformational process,
and implementing it.

   Specific issues to resolve:

   * Just where should (if it was implemented) `USE' processing take
     place?

     This gets into the whole issue of how `g77' should handle the
     concept of modules.  I think GNAT already takes on this issue, but
     don't know more than that.  Jim Giles has written extensively on
     `comp.lang.fortran' about his opinions on module handling, as have
     others.  Jim's views should be taken into account.

     Actually, Richard M. Stallman (RMS) also has written up some
     guidelines for implementing such things, but I'm not sure where I
     read them.  Perhaps the old <gcc2@cygnus.com> list.

     If someone could dig references to these up and get them to me,
     that would be much appreciated!  Even though modules are not on
     the short-term list for implementation, it'd be helpful to know
     _now_ how to avoid making them harder to implement them _later_.

   * Should the `g77' command become just a script that invokes all the
     various preprocessing that might be needed, thus making it seem
     slower than necessary for legacy code that people are unwilling to
     convert, or should we provide a separate script for that, thus
     encouraging people to convert their code once and for all?

     At least, a separate script to behave as old `g77' did, perhaps
     named `g77old', might ease the transition, as might a
     corresponding one that converts source codes named `g77oldnew'.

     These scripts would take all the pertinent options `g77' used to
     take and run the appropriate filters, passing the results to `g77'
     or just making new sources out of them (in a subdirectory, leaving
     the user to do the dirty deed of moving or copying them over the
     old sources).

   * Do other Fortran compilers provide a prefix syntax to govern the
     treatment of backslashes in `CHARACTER' (or Hollerith) constants?

     Knowing what other compilers provide would help.

   * Is it okay to drop support for the `-fintrin-case-initcap',
     `-fmatch-case-initcap', `-fsymbol-case-initcap', and
     `-fcase-initcap' options?

     I've asked <info-gnu-fortran@gnu.org> for input on this.  Not
     having to support these makes it easier to write the new front end,
     and might also avoid complicated its design.

     The consensus to date (1999-11-17) has been to drop this support.
     Can't recall anybody saying they're using it, in fact.


File: g77.info,  Node: Philosophy of Code Generation,  Next: Two-pass Design,  Prev: Overview of Translation Process,  Up: Front End

Philosophy of Code Generation
=============================

   Don't poke the bear.

   The `g77' front end generates code via the `gcc' back end.

   The `gcc' back end (GBE) is a large, complex labyrinth of intricate
code written in a combination of the C language and specialized
languages internal to `gcc'.

   While the _code_ that implements the GBE is written in a combination
of languages, the GBE itself is, to the front end for a language like
Fortran, best viewed as a _compiler_ that compiles its own, unique,
language.

   The GBE's "source", then, is written in this language, which
consists primarily of a combination of calls to GBE functions and
"tree" nodes (which are, themselves, created by calling GBE functions).

   So, the `g77' generates code by, in effect, translating the Fortran
code it reads into a form "written" in the "language" of the `gcc' back
end.

   This language will heretofore be referred to as "GBEL", for GNU Back
End Language.

   GBEL is an evolving language, not fully specified in any published
form as of this writing.  It offers many facilities, but its "core"
facilities are those that corresponding most directly to those needed
to support `gcc' (compiling code written in GNU C).

   The `g77' Fortran Front End (FFE) is designed and implemented to
navigate the currents and eddies of ongoing GBEL and `gcc' development
while also delivering on the potential of an integrated FFE (as
compared to using a converter like `f2c' and feeding the output into
`gcc').

   Goals of the FFE's code-generation strategy include:

   * High likelihood of generation of correct code, or, failing that,
     producing a fatal diagnostic or crashing.

   * Generation of highly optimized code, as directed by the user via
     GBE-specific (versus `g77'-specific) constructs, such as
     command-line options.

   * Fast overall (FFE plus GBE) compilation.

   * Preservation of source-level debugging information.

   The strategies historically, and currently, used by the FFE to
achieve these goals include:

   * Use of GBEL constructs that most faithfully encapsulate the
     semantics of Fortran.

   * Avoidance of GBEL constructs that are so rarely used, or limited
     to use in specialized situations not related to Fortran, that
     their reliability and performance has not yet been established as
     sufficient for use by the FFE.

   * Flexible design, to readily accommodate changes to specific
     code-generation strategies, perhaps governed by command-line
     options.

   "Don't poke the bear" somewhat summarizes the above strategies.  The
GBE is the bear.  The FFE is designed and implemented to avoid poking it
in ways that are likely to just annoy it.  The FFE usually either
tackles it head-on, or avoids treating it in ways dissimilar to how the
`gcc' front end treats it.

   For example, the FFE uses the native array facility in the back end
instead of the lower-level pointer-arithmetic facility used by `gcc'
when compiling `f2c' output).  Theoretically, this presents more
opportunities for optimization, faster compile times, and the
production of more faithful debugging information.  These benefits were
not, however, immediately realized, mainly because `gcc' itself makes
little or no use of the native array facility.

   Complex arithmetic is a case study of the evolution of this strategy.
When originally implemented, the GBEL had just evolved its own native
complex-arithmetic facility, so the FFE took advantage of that.

   When porting `g77' to 64-bit systems, it was discovered that the GBE
didn't really implement its native complex-arithmetic facility properly.

   The short-term solution was to rewrite the FFE to instead use the
lower-level facilities that'd be used by `gcc'-compiled code (assuming
that code, itself, didn't use the native complex type provided, as an
extension, by `gcc'), since these were known to work, and, in any case,
if shown to not work, would likely be rapidly fixed (since they'd
likely not work for vanilla C code in similar circumstances).

   However, the rewrite accommodated the original, native approach as
well by offering a command-line option to select it over the emulated
approach.  This allowed users, and especially GBE maintainers, to try
out fixes to complex-arithmetic support in the GBE while `g77'
continued to default to compiling more code correctly, albeit producing
(typically) slower executables.

   As of April 1999, it appeared that the last few bugs in the GBE's
support of its native complex-arithmetic facility were worked out.  The
FFE was changed back to default to using that native facility, leaving
emulation as an option.

   Later during the release cycle (which was called EGCS 1.2, but soon
became GCC 2.95), bugs in the native facility were found.  Reactions
among various people included "the last thing we should do is change
the default back", "we must change the default back", and "let's figure
out whether we can narrow down the bugs to few enough cases to allow
the now-months-long-tested default to remain the same".  The latter
viewpoint won that particular time.  The bugs exposed other concerns
regarding ABI compliance when the ABI specified treatment of complex
data as different from treatment of what Fortran and GNU C consider the
equivalent aggregation (structure) of real (or float) pairs.

   Other Fortran constructs--arrays, character strings, complex
division, `COMMON' and `EQUIVALENCE' aggregates, and so on--involve
issues similar to those pertaining to complex arithmetic.

   So, it is possible that the history of how the FFE handled complex
arithmetic will be repeated, probably in modified form (and hopefully
over shorter timeframes), for some of these other facilities.


File: g77.info,  Node: Two-pass Design,  Next: Challenges Posed,  Prev: Philosophy of Code Generation,  Up: Front End

Two-pass Design
===============

   The FFE does not tell the GBE anything about a program unit until
after the last statement in that unit has been parsed.  (A program unit
is a Fortran concept that corresponds, in the C world, mostly closely
to functions definitions in ISO C.  That is, a program unit in Fortran
is like a top-level function in C.  Nested functions, found among the
extensions offered by GNU C, correspond roughly to Fortran's statement
functions.)

   So, while parsing the code in a program unit, the FFE saves up all
the information on statements, expressions, names, and so on, until it
has seen the last statement.

   At that point, the FFE revisits the saved information (in what
amounts to a second "pass" over the program unit) to perform the actual
translation of the program unit into GBEL, ultimating in the generation
of assembly code for it.

   Some lookahead is performed during this second pass, so the FFE
could be viewed as a "two-plus-pass" design.

* Menu:

* Two-pass Code::
* Why Two Passes::


File: g77.info,  Node: Two-pass Code,  Next: Why Two Passes,  Up: Two-pass Design

Two-pass Code
-------------

   Most of the code that turns the first pass (parsing) into a second
pass for code generation is in `gcc/gcc/f/std.c'.

   It has external functions, called mainly by siblings in
`gcc/gcc/f/stc.c', that record the information on statements and
expressions in the order they are seen in the source code.  These
functions save that information.

   It also has an external function that revisits that information,
calling the siblings in `gcc/gcc/f/ste.c', which handles the actual
code generation (by generating GBEL code, that is, by calling GBE
routines to represent and specify expressions, statements, and so on).


File: g77.info,  Node: Why Two Passes,  Prev: Two-pass Code,  Up: Two-pass Design

Why Two Passes
--------------

   The need for two passes was not immediately evident during the
design and implementation of the code in the FFE that was to produce
GBEL.  Only after a few kludges, to handle things like
incorrectly-guessed `ASSIGN' label nature, had been implemented, did
enough evidence pile up to make it clear that `std.c' had to be
introduced to intercept, save, then revisit as part of a second pass,
the digested contents of a program unit.

   Other such missteps have occurred during the evolution of the FFE,
because of the different goals of the FFE and the GBE.

   Because the GBE's original, and still primary, goal was to directly
support the GNU C language, the GBEL, and the GBE itself, requires more
complexity on the part of most front ends than it requires of `gcc''s.

   For example, the GBEL offers an interface that permits the `gcc'
front end to implement most, or all, of the language features it
supports, without the front end having to make use of non-user-defined
variables.  (It's almost certainly the case that all of K&R C, and
probably ANSI C as well, is handled by the `gcc' front end without
declaring such variables.)

   The FFE, on the other hand, must resort to a variety of "tricks" to
achieve its goals.

   Consider the following C code:

     int
     foo (int a, int b)
     {
       int c = 0;
     
       if ((c = bar (c)) == 0)
         goto done;
     
       quux (c << 1);
     
     done:
       return c;
     }

   Note what kinds of objects are declared, or defined, before their
use, and before any actual code generation involving them would
normally take place:

   * Return type of function

   * Entry point(s) of function

   * Dummy arguments

   * Variables

   * Initial values for variables

   Whereas, the following items can, and do, suddenly appear "out of
the blue" in C:

   * Label references

   * Function references

   Not surprisingly, the GBE faithfully permits the latter set of items
to be "discovered" partway through GBEL "programs", just as they are
permitted to in C.

   Yet, the GBE has tended, at least in the past, to be reticent to
fully support similar "late" discovery of items in the former set.

   This makes Fortran a poor fit for the "safe" subset of GBEL.
Consider:

           FUNCTION X (A, ARRAY, ID1)
           CHARACTER*(*) A
           DOUBLE PRECISION X, Y, Z, TMP, EE, PI
           REAL ARRAY(ID1*ID2)
           COMMON ID2
           EXTERNAL FRED
     
           ASSIGN 100 TO J
           CALL FOO (I)
           IF (I .EQ. 0) PRINT *, A(0)
           GOTO 200
     
           ENTRY Y (Z)
           ASSIGN 101 TO J
     200   PRINT *, A(1)
           READ *, TMP
           GOTO J
     100   X = TMP * EE
           RETURN
     101   Y = TMP * PI
           CALL FRED
           DATA EE, PI /2.71D0, 3.14D0/
           END

   Here are some observations about the above code, which, while
somewhat contrived, conforms to the FORTRAN 77 and Fortran 90 standards:

   * The return type of function `X' is not known until the `DOUBLE
     PRECISION' line has been parsed.

   * Whether `A' is a function or a variable is not known until the
     `PRINT *, A(0)' statement has been parsed.

   * The bounds of the array of argument `ARRAY' depend on a
     computation involving the subsequent argument `ID1' and the
     blank-common member `ID2'.

   * Whether `Y' and `Z' are local variables, additional function entry
     points, or dummy arguments to additional entry points is not known
     until the `ENTRY' statement is parsed.

   * Similarly, whether `TMP' is a local variable is not known until
     the `READ *, TMP' statement is parsed.

   * The initial values for `EE' and `PI' are not known until after the
     `DATA' statement is parsed.

   * Whether `FRED' is a function returning type `REAL' or a subroutine
     (which can be thought of as returning type `void' _or_, to support
     alternate returns in a simple way, type `int') is not known until
     the `CALL FRED' statement is parsed.

   * Whether `100' is a `FORMAT' label or the label of an executable
     statement is not known until the `X =' statement is parsed.
     (These two types of labels get _very_ different treatment,
     especially when `ASSIGN''ed.)

   * That `J' is a local variable is not known until the first `ASSIGN'
     statement is parsed.  (This happens _after_ executable code has
     been seen.)

   Very few of these "discoveries" can be accommodated by the GBE as it
has evolved over the years.  The GBEL doesn't support several of them,
and those it might appear to support don't always work properly,
especially in combination with other GBEL and GBE features, as
implemented in the GBE.

   (Had the GBE and its GBEL originally evolved to support `g77', the
shoe would be on the other foot, so to speak--most, if not all, of the
above would be directly supported by the GBEL, and a few C constructs
would probably not, as they are in reality, be supported.  Both this
mythical, and today's real, GBE caters to its GBEL by, sometimes,
scrambling around, cleaning up after itself--after discovering that
assumptions it made earlier during code generation are incorrect.
That's not a great design, since it indicates significant code paths
that might be rarely tested but used in some key production
environments.)

   So, the FFE handles these discrepancies--between the order in which
it discovers facts about the code it is compiling, and the order in
which the GBEL and GBE support such discoveries--by performing what
amounts to two passes over each program unit.

   (A few ambiguities can remain at that point, such as whether, given
`EXTERNAL BAZ' and no other reference to `BAZ' in the program unit, it
is a subroutine, a function, or a block-data--which, in C-speak,
governs its declared return type.  Fortunately, these distinctions are
easily finessed for the procedure, library, and object-file interfaces
supported by `g77'.)


File: g77.info,  Node: Challenges Posed,  Next: Transforming Statements,  Prev: Two-pass Design,  Up: Front End

Challenges Posed
================

   Consider the following Fortran code, which uses various extensions
(including some to Fortran 90):

     SUBROUTINE X(A)
     CHARACTER*(*) A
     COMPLEX CFUNC
     INTEGER*2 CLOCKS(200)
     INTEGER IFUNC
     
     CALL SYSTEM_CLOCK (CLOCKS (IFUNC (CFUNC ('('//A//')'))))

   The above poses the following challenges to any Fortran compiler
that uses run-time interfaces, and a run-time library, roughly similar
to those used by `g77':

   * Assuming the library routine that supports `SYSTEM_CLOCK' expects
     to set an `INTEGER*4' variable via its `COUNT' argument, the
     compiler must make available to it a temporary variable of that
     type.

   * Further, after the `SYSTEM_CLOCK' library routine returns, the
     compiler must ensure that the temporary variable it wrote is
     copied into the appropriate element of the `CLOCKS' array.  (This
     assumes the compiler doesn't just reject the code, which it should
     if it is compiling under some kind of a "strict" option.)

   * To determine the correct index into the `CLOCKS' array, (putting
     aside the fact that the index, in this particular case, need not
     be computed until after the `SYSTEM_CLOCK' library routine
     returns), the compiler must ensure that the `IFUNC' function is
     called.

     That requires evaluating its argument, which requires, for `g77'
     (assuming `-ff2c' is in force), reserving a temporary variable of
     type `COMPLEX' for use as a repository for the return value being
     computed by `CFUNC'.

   * Before invoking `CFUNC', is argument must be evaluated, which
     requires allocating, at run time, a temporary large enough to hold
     the result of the concatenation, as well as actually performing
     the concatenation.

   * The large temporary needed during invocation of `CFUNC' should,
     ideally, be deallocated (or, at least, left to the GBE to dispose
     of, as it sees fit) as soon as `CFUNC' returns, which means before
     `IFUNC' is called (as it might need a lot of dynamically allocated
     memory).

   `g77' currently doesn't support all of the above, but, so that it
might someday, it has evolved to handle at least some of the above
requirements.

   Meeting the above requirements is made more challenging by
conforming to the requirements of the GBEL/GBE combination.


File: g77.info,  Node: Transforming Statements,  Next: Transforming Expressions,  Prev: Challenges Posed,  Up: Front End

Transforming Statements
=======================

   Most Fortran statements are given their own block, and, for
temporary variables they might need, their own scope.  (A block is what
distinguishes `{ foo (); }' from just `foo ();' in C.  A scope is
included with every such block, providing a distinct name space for
local variables.)

   Label definitions for the statement precede this block, so `10 PRINT
*, I' is handled more like `fl10: { ... }' than `{ fl10: ... }' (where
`fl10' is just a notation meaning "Fortran Label 10" for the purposes
of this document).

* Menu:

* Statements Needing Temporaries::
* Transforming DO WHILE::
* Transforming Iterative DO::
* Transforming Block IF::
* Transforming SELECT CASE::


File: g77.info,  Node: Statements Needing Temporaries,  Next: Transforming DO WHILE,  Up: Transforming Statements

Statements Needing Temporaries
------------------------------

   Any temporaries needed during, but not beyond, execution of a
Fortran statement, are made local to the scope of that statement's
block.

   This allows the GBE to share storage for these temporaries among the
various statements without the FFE having to manage that itself.

   (The GBE could, of course, decide to optimize management of these
temporaries.  For example, it could, theoretically, schedule some of
the computations involving these temporaries to occur in parallel.
More practically, it might leave the storage for some temporaries
"live" beyond their scopes, to reduce the number of manipulations of
the stack pointer at run time.)

   Temporaries needed across distinct statement boundaries usually are
associated with Fortran blocks (such as `DO'/`END DO').  (Also, there
might be temporaries not associated with blocks at all--these would be
in the scope of the entire program unit.)

   Each Fortran block _should_ get its own block/scope in the GBE.
This is best, because it allows temporaries to be more naturally
handled.  However, it might pose problems when handling labels (in
particular, when they're the targets of `GOTO's outside the Fortran
block), and generally just hassling with replicating parts of the `gcc'
front end (because the FFE needs to support an arbitrary number of
nested back-end blocks if each Fortran block gets one).

   So, there might still be a need for top-level temporaries, whose
"owning" scope is that of the containing procedure.

   Also, there seems to be problems declaring new variables after
generating code (within a block) in the back end, leading to, e.g.,
`label not defined before binding contour' or similar messages, when
compiling with `-fstack-check' or when compiling for certain targets.

   Because of that, and because sometimes these temporaries are not
discovered until in the middle of of generating code for an expression
statement (as in the case of the optimization for `X**I'), it seems
best to always pre-scan all the expressions that'll be expanded for a
block before generating any of the code for that block.

   This pre-scan then handles discovering and declaring, to the back
end, the temporaries needed for that block.

   It's also important to treat distinct items in an I/O list as
distinct statements deserving their own blocks.  That's because there's
a requirement that each I/O item be fully processed before the next one,
which matters in cases like `READ (*,*), I, A(I)'--the element of `A'
read in the second item _must_ be determined from the value of `I' read
in the first item.


File: g77.info,  Node: Transforming DO WHILE,  Next: Transforming Iterative DO,  Prev: Statements Needing Temporaries,  Up: Transforming Statements

Transforming DO WHILE
---------------------

   `DO WHILE(expr)' _must_ be implemented so that temporaries needed to
evaluate `expr' are generated just for the test, each time.

   Consider how `DO WHILE (A//B .NE. 'END'); ...; END DO' is
transformed:

     for (;;)
       {
         int temp0;
     
         {
           char temp1[large];
     
           libg77_catenate (temp1, a, b);
           temp0 = libg77_ne (temp1, 'END');
         }
     
         if (! temp0)
           break;
     
         ...
       }

   In this case, it seems like a time/space tradeoff between allocating
and deallocating `temp1' for each iteration and allocating it just once
for the entire loop.

   However, if `temp1' is allocated just once for the entire loop, it
could be the wrong size for subsequent iterations of that loop in cases
like `DO WHILE (A(I:J)//B .NE. 'END')', because the body of the loop
might modify `I' or `J'.

   So, the above implementation is used, though a more optimal one can
be used in specific circumstances.


File: g77.info,  Node: Transforming Iterative DO,  Next: Transforming Block IF,  Prev: Transforming DO WHILE,  Up: Transforming Statements

Transforming Iterative DO
-------------------------

   An iterative `DO' loop (one that specifies an iteration variable) is
required by the Fortran standards to be implemented as though an
iteration count is computed before entering the loop body, and that
iteration count used to determine the number of times the loop body is
to be performed (assuming the loop isn't cut short via `GOTO' or
`EXIT').

   The FFE handles this by allocating a temporary variable to contain
the computed number of iterations.  Since this variable must be in a
scope that includes the entire loop, a GBEL block is created for that
loop, and the variable declared as belonging to the scope of that block.


File: g77.info,  Node: Transforming Block IF,  Next: Transforming SELECT CASE,  Prev: Transforming Iterative DO,  Up: Transforming Statements

Transforming Block IF
---------------------

   Consider:

     SUBROUTINE X(A,B,C)
     CHARACTER*(*) A, B, C
     LOGICAL LFUNC
     
     IF (LFUNC (A//B)) THEN
       CALL SUBR1
     ELSE IF (LFUNC (A//C)) THEN
       CALL SUBR2
     ELSE
       CALL SUBR3
     END

   The arguments to the two calls to `LFUNC' require dynamic allocation
(at run time), but are not required during execution of the `CALL'
statements.

   So, the scopes of those temporaries must be within blocks inside the
block corresponding to the Fortran `IF' block.

   This cannot be represented "naturally" in vanilla C, nor in GBEL.
The `if', `elseif', `else', and `endif' constructs provided by both
languages must, for a given `if' block, share the same C/GBE block.

   Therefore, any temporaries needed during evaluation of `expr' while
executing `ELSE IF(expr)' must either have been predeclared at the top
of the corresponding `IF' block, or declared within a new block for
that `ELSE IF'--a block that, since it cannot contain the `else' or
`else if' itself (due to the above requirement), actually implements
the rest of the `IF' block's `ELSE IF' and `ELSE' statements within an
inner block.

   The FFE takes the latter approach.


File: g77.info,  Node: Transforming SELECT CASE,  Prev: Transforming Block IF,  Up: Transforming Statements

Transforming SELECT CASE
------------------------

   `SELECT CASE' poses a few interesting problems for code generation,
if efficiency and frugal stack management are important.

   Consider `SELECT CASE (I('PREFIX'//A))', where `A' is
`CHARACTER*(*)'.  In a case like this--basically, in any case where
largish temporaries are needed to evaluate the expression--those
temporaries should not be "live" during execution of any of the `CASE'
blocks.

   So, evaluation of the expression is best done within its own block,
which in turn is within the `SELECT CASE' block itself (which contains
the code for the CASE blocks as well, though each within their own
block).

   Otherwise, we'd have the rough equivalent of this pseudo-code:

     {
       char temp[large];
     
       libg77_catenate (temp, 'prefix', a);
     
       switch (i (temp))
         {
         case 0:
           ...
         }
     }

   And that would leave temp[large] in scope during the CASE blocks
(although a clever back end *could* see that it isn't referenced in
them, and thus free that temp before executing the blocks).

   So this approach is used instead:

     {
       int temp0;
     
       {
         char temp1[large];
     
         libg77_catenate (temp1, 'prefix', a);
         temp0 = i (temp1);
       }
     
       switch (temp0)
         {
         case 0:
           ...
         }
     }

   Note how `temp1' goes out of scope before starting the switch, thus
making it easy for a back end to free it.

   The problem _that_ solution has, however, is with `SELECT
CASE('prefix'//A)' (which is currently not supported).

   Unless the GBEL is extended to support arbitrarily long character
strings in its `case' facility, the FFE has to implement `SELECT CASE'
on `CHARACTER' (probably excepting `CHARACTER*1') using a cascade of
`if', `elseif', `else', and `endif' constructs in GBEL.

   To prevent the (potentially large) temporary, needed to hold the
selected expression itself (`'prefix'//A'), from being in scope during
execution of the `CASE' blocks, two approaches are available:

   * Pre-evaluate all the `CASE' tests, producing an integer ordinal
     that is used, a la `temp0' in the earlier example, as if `SELECT
     CASE(temp0)' had been written.

     Each corresponding `CASE' is replaced with `CASE(I)', where I is
     the ordinal for that case, determined while, or before, generating
     the cascade of `if'-related constructs to cope with `CHARACTER'
     selection.

   * Make `temp0' above just large enough to hold the longest `CASE'
     string that'll actually be compared against the expression (in
     this case, `'prefix'//A').

     Since that length must be constant (because `CASE' expressions are
     all constant), it won't be so large, and, further, `temp1' need
     not be dynamically allocated, since normal `CHARACTER' assignment
     can be used into the fixed-length `temp0'.

   Both of these solutions require `SELECT CASE' implementation to be
changed so all the corresponding `CASE' statements are seen during the
actual code generation for `SELECT CASE'.


File: g77.info,  Node: Transforming Expressions,  Next: Internal Naming Conventions,  Prev: Transforming Statements,  Up: Front End

Transforming Expressions
========================

   The interactions between statements, expressions, and subexpressions
at program run time can be viewed as:

     ACTION(EXPR)

   Here, ACTION is the series of steps performed to effect the
statement, and EXPR is the expression whose value is used by ACTION.

   Expanding the above shows a typical order of events at run time:

     Evaluate EXPR
     Perform ACTION, using result of evaluation of EXPR
     Clean up after evaluating EXPR

   So, if evaluating EXPR requires allocating memory, that memory can
be freed before performing ACTION only if it is not needed to hold the
result of evaluating EXPR.  Otherwise, it must be freed no sooner than
after ACTION has been performed.

   The above are recursive definitions, in the sense that they apply to
subexpressions of EXPR.

   That is, evaluating EXPR involves evaluating all of its
subexpressions, performing the ACTION that computes the result value of
EXPR, then cleaning up after evaluating those subexpressions.

   The recursive nature of this evaluation is implemented via
recursive-descent transformation of the top-level statements, their
expressions, _their_ subexpressions, and so on.

   However, that recursive-descent transformation is, due to the nature
of the GBEL, focused primarily on generating a _single_ stream of code
to be executed at run time.

   Yet, from the above, it's clear that multiple streams of code must
effectively be simultaneously generated during the recursive-descent
analysis of statements.

   The primary stream implements the primary ACTION items, while at
least two other streams implement the evaluation and clean-up items.

   Requirements imposed by expressions include:

   * Whether the caller needs to have a temporary ready to hold the
     value of the expression.

   * Other stuff???


File: g77.info,  Node: Internal Naming Conventions,  Prev: Transforming Expressions,  Up: Front End

Internal Naming Conventions
===========================

   Names exported by FFE modules have the following
(regular-expression) forms.  Note that all names beginning `ffeMOD' or
`FFEMOD', where MOD is lowercase or uppercase alphanumerics,
respectively, are exported by the module `ffeMOD', with the source code
doing the exporting in `MOD.h'.  (Usually, the source code for the
implementation is in `MOD.c'.)

   Identifiers that don't fit the following forms are not considered
exported, even if they are according to the C language.  (For example,
they might be made available to other modules solely for use within
expansions of exported macros, not for use within any source code in
those other modules.)

`ffeMOD'
     The single typedef exported by the module.

`FFEUMOD_[A-Z][A-Z0-9_]*'
     (Where UMOD is the uppercase for of MOD.)

     A `#define' or `enum' constant of the type `ffeMOD'.

`ffeMOD[A-Z][A-Z][a-z0-9]*'
     A typedef exported by the module.

     The portion of the identifier after `ffeMOD' is referred to as
     `ctype', a capitalized (mixed-case) form of `type'.

`FFEUMOD_TYPE[A-Z][A-Z0-9_]*[A-Z0-9]?'
     (Where UMOD is the uppercase for of MOD.)

     A `#define' or `enum' constant of the type `ffeMODTYPE', where
     TYPE is the lowercase form of CTYPE in an exported typedef.

`ffeMOD_VALUE'
     A function that does or returns something, as described by VALUE
     (see below).

`ffeMOD_VALUE_INPUT'
     A function that does or returns something based primarily on the
     thing described by INPUT (see below).

   Below are names used for VALUE and INPUT, along with their
definitions.

`col'
     A column number within a line (first column is number 1).

`file'
     An encapsulation of a file's name.

`find'
     Looks up an instance of some type that matches specified criteria,
     and returns that, even if it has to create a new instance or crash
     trying to find it (as appropriate).

`initialize'
     Initializes, usually a module.  No type.

`int'
     A generic integer of type `int'.

`is'
     A generic integer that contains a true (non-zero) or false (zero)
     value.

`len'
     A generic integer that contains the length of something.

`line'
     A line number within a source file, or a global line number.

`lookup'
     Looks up an instance of some type that matches specified criteria,
     and returns that, or returns nil.

`name'
     A `text' that points to a name of something.

`new'
     Makes a new instance of the indicated type.  Might return an
     existing one if appropriate--if so, similar to `find' without
     crashing.

`pt'
     Pointer to a particular character (line, column pairs) in the
     input file (source code being compiled).

`run'
     Performs some herculean task.  No type.

`terminate'
     Terminates, usually a module.  No type.

`text'
     A `char *' that points to generic text.


File: g77.info,  Node: Diagnostics,  Next: Index,  Prev: Front End,  Up: Top

Diagnostics
***********

   Some diagnostics produced by `g77' require sufficient explanation
that the explanations are given below, and the diagnostics themselves
identify the appropriate explanation.

   Identification uses the GNU Info format--specifically, the `info'
command that displays the explanation is given within square brackets
in the diagnostic.  For example:

     foo.f:5: Invalid statement [info -f g77 M FOOEY]

   More details about the above diagnostic is found in the `g77' Info
documentation, menu item `M', submenu item `FOOEY', which is displayed
by typing the UNIX command `info -f g77 M FOOEY'.

   Other Info readers, such as EMACS, may be just as easily used to
display the pertinent node.  In the above example, `g77' is the Info
document name, `M' is the top-level menu item to select, and, in that
node (named `Diagnostics', the name of this chapter, which is the very
text you're reading now), `FOOEY' is the menu item to select.

* Menu:

* CMPAMBIG::    Ambiguous use of intrinsic.
* EXPIMP::      Intrinsic used explicitly and implicitly.
* INTGLOB::     Intrinsic also used as name of global.
* LEX::         Various lexer messages
* GLOBALS::     Disagreements about globals.
* LINKFAIL::    When linking `f771' fails.
* Y2KBAD::      Use of non-Y2K-compliant intrinsic.

