This chapter describes some aspects of the design and implementation
of the @code{g77} front end.
+To find about things that are ``To Be Determined'' or ``To Be Done'',
+search for the string TBD.
+If you want to help by working on one or more of these items,
+email me at @email{@value{email-burley}}.
+If you're planning to do more than just research issues and offer comments,
+see @uref{http://egcs.cygnus.com/contribute.html} for steps you might
+need to take first.
+
@menu
+* Overview of Translation Process::
* Philosophy of Code Generation::
* Two-pass Design::
* Challenges Posed::
* Transforming Expressions::
@end menu
+@node Overview of Translation Process
+@section Overview of Translation Process
+
+The order of phases translating source code to the form accepted
+by the GBE is:
+
+@enumerate
+@item
+Stripping punched-card sources (@file{g77stripcard.c})
+
+@item
+Lexing (@file{lex.c})
+
+@item
+Stand-alone statement identification (@file{sta.c})
+
+@item
+Parsing (@file{stb.c} and @file{expr.c})
+
+@item
+Constructing (@file{stc.c})
+
+@item
+Collecting (@file{std.c})
+
+@item
+Expanding (@file{ste.c})
+@end enumerate
+
+To get a rough idea of how a particularly twisted Fortran statement
+gets treated by the passes, consider:
+
+@smallexample
+ FORMAT(I2 4H)=(J/
+ & I3)
+@end smallexample
+
+The job of @file{lex.c} is to know enough about Fortran syntax rules
+to break the statement up into distinct lexemes without requiring
+any feedback from subsequent phases:
+
+@smallexample
+`FORMAT'
+`('
+`I24H'
+`)'
+`='
+`('
+`J'
+`/'
+`I3'
+`)'
+@end smallexample
+
+The job of @file{sta.c} is to figure out the kind of statement,
+or, at least, statement form, that sequence of lexemes represent.
+
+The sooner it can do this (in terms of using the smallest number of
+lexemes, starting with the first for each statement), the better,
+because that leaves diagnostics for problems beyond the recognition
+of the statement form to subsequent phases,
+which can usually better describe the nature of the problem.
+
+In this case, the @samp{=} at ``level zero''
+(not nested within parentheses)
+tells @file{sta.c} that this is an @emph{assignment-form},
+not @code{FORMAT}, statement.
+
+An assignment-form statement might be a statement-function
+definition or an executable assignment statement.
+
+To make that determination,
+@file{sta.c} looks at the first two lexemes.
+
+Since the second lexeme is @samp{(},
+the first must represent an array for this to be an assignment statement,
+else it's a statement function.
+
+Either way, @file{sta.c} hands off the statement to @file{stb.c}
+(either its statement-function parser or its assignment-statement parser).
+
+@file{stb.c} forms a
+statement-specific record containing the pertinent information.
+That information includes a source expression and,
+for an assignment statement, a destination expression.
+Expressions are parsed by @file{expr.c}.
+
+This record is passed to @file{stc.c},
+which copes with the implications of the statement
+within the context established by previous statements.
+
+For example, if it's the first statement in the file
+or after an @code{END} statement,
+@file{stc.c} recognizes that, first of all,
+a main program unit is now being lexed
+(and tells that to @file{std.c}
+before telling it about the current statement).
+
+@file{stc.c} attaches whatever information it can,
+usually derived from the context established by the preceding statements,
+and passes the information to @file{std.c}.
+
+@file{std.c} saves this information away,
+since the GBE cannot cope with information
+that might be incomplete at this stage.
+
+For example, @samp{I3} might later be determined
+to be an argument to an alternate @code{ENTRY} point.
+
+When @file{std.c} is told about the end of an external (top-level)
+program unit,
+it passes all the information it has saved away
+on statements in that program unit
+to @file{ste.c}.
+
+@file{ste.c} ``expands'' each statement, in sequence, by
+constructing the appropriate GBE information and calling
+the appropriate GBE routines.
+
+Details on the transformational phases follow.
+Keep in mind that Fortran numbering is used,
+so the first character on a line is column 1,
+decimal numbering is used, and so on.
+
+@menu
+* g77stripcard::
+* lex.c::
+* sta.c::
+* stb.c::
+* expr.c::
+* stc.c::
+* std.c::
+* ste.c::
+
+* Gotchas (Transforming)::
+* TBD (Transforming)::
+@end menu
+
+@node g77stripcard
+@subsection g77stripcard
+
+The @code{g77stripcard} program handles removing content beyond
+column 72 (adjustable via a command-line option),
+optionally warning about that content being something other
+than trailing whitespace or Fortran commentary.
+
+This program is needed because @code{lex.c} doesn't pay attention
+to maximum line lengths at all, to make it easier to maintain,
+as well as faster (for sources that don't depend on the maximum
+column length vis-a-vis trailing non-blank non-commentary content).
+
+Just how this program will be run---whether automatically for
+old source (perhaps as the default for @file{.f} files?)---is not
+yet determined.
+
+In the meantime, it might as well be implemented as a typical UNIX pipe.
+
+It should accept a @samp{-fline-length-@var{n}} option,
+with the default line length set to 72.
+
+When the text it strips off the end of a line is not blank
+(not spaces and tabs),
+it should insert an additional comment line
+(beginning with @samp{!},
+so it works for both fixed-form and free-form files)
+containing the text,
+following the stripped line.
+The inserted comment should have a prefix of some kind,
+TBD, that distinguishes the comment as representing stripped text.
+Users could use that to @code{sed} out such lines, if they wished---it
+seems silly to provide a command-line option to delete information
+when it can be so easily filtered out by another program.
+
+(This inserted comment should be designed to ``fit in'' well
+with whatever the Fortran community is using these days for
+preprocessor, translator, and other such products, like OpenMP.
+What that's all about, and how @code{g77} can elegantly fit its
+special comment conventions into it all, is TBD as well.
+We don't want to reinvent the wheel here, but if there turn out
+to be too many conflicting conventions, we might have to invent
+one that looks nothing like the others, but which offers their
+host products a better infrastructure in which to fit and coexist
+peacefully.)
+
+@node lex.c
+@subsection lex.c
+
+To help make the lexer simple, fast, and easy to maintain,
+while also having @code{g77} generally encourage Fortran programmers
+to write simple, maintainable, portable code by maximizing the
+performance of compiling that kind of code:
+
+@itemize @bullet
+@item
+There'll be just one lexer, for both fixed-form and free-form source.
+
+@item
+It'll care about the form only when handling the first 7 columns of
+text, stuff like spaces between strings of alphanumerics, and
+how lines are continued.
+
+Some other distinctions will be handled by subsequent phases,
+so at least one of them will have to know which form is involved.
+
+For example, @samp{I = 2 . 4} is acceptable in fixed form,
+and works in free form as well given the implementation @code{g77}
+presently uses.
+But the standard requires a diagnostic for it in free form,
+so the parser has to be able to recognize that
+the lexemes aren't contiguous
+(information the lexer @emph{does} have to provide)
+and that free-form source is being parsed,
+so it can provide the diagnostic.
+
+The @code{g77} lexer doesn't try to gather @samp{2 . 4} into a single lexeme.
+Otherwise, it'd have to know a whole lot more about how to parse Fortran,
+or subsequent phases (mainly parsing) would have two paths through
+lots of critical code---one to handle the lexeme @samp{2}, @samp{.},
+and @samp{4} in sequence, another to handle the lexeme @samp{2.4}.
+
+@item
+It won't worry about line lengths
+(beyond the first 7 columns for fixed-form source).
+
+That is, once it starts parsing the ``statement'' part of a line
+(column 7 for fixed-form, column 1 for free-form),
+it'll keep going until it finds a newline,
+rather than ignoring everything past a particular column
+(72 or 132).
+
+The implication here is that there shouldn't @emph{be}
+anything past that last column, other than whitespace or
+commentary, because users using typical editors
+(or viewing output as typically printed)
+won't necessarily know just where the last column is.
+
+Code that has ``garbage'' beyond the last column
+(almost certainly only fixed-form code with a punched-card legacy,
+such as code using columns 73-80 for ``sequence numbers'')
+will have to be run through @code{g77stripcard} first.
+
+Also, keeping track of the maximum column position while also watching out
+for the end of a line @emph{and} while reading from a file
+just makes things slower.
+Since a file must be read, and watching for the end of the line
+is necessary (unless the typical input file was preprocessed to
+include the necessary number of trailing spaces),
+dropping the tracking of the maximum column position
+is the only way to reduce the complexity of the pertinent code
+while maintaining high performance.
+
+@item
+ASCII encoding is assumed for the input file.
+
+Code written in other character sets will have to be converted first.
+
+@item
+Tabs (ASCII code 9)
+will be converted to spaces via the straightforward
+approach.
+
+Specifically, a tab is converted to between one and eight spaces
+as necessary to reach column @var{n},
+where dividing @samp{(@var{n} - 1)} by eight
+results in a remainder of zero.
+
+@item
+Linefeeds (ASCII code 10)
+mark the ends of lines.
+
+@item
+A carriage return (ASCII code 13)
+is accept if it immediately precedes a linefeed,
+in which case it is ignored.
+
+Otherwise, it is rejected (with a diagnostic).
+
+@item
+Any other characters other than the above
+that are not part of the GNU Fortran Character Set
+(@pxref{Character Set})
+are rejected with a diagnostic.
+
+This includes backspaces, form feeds, and the like.
+
+(It might make sense to allow a form feed in column 1
+as long as that's the only character on a line.
+It certainly wouldn't seem to cost much in terms of performance.)
+
+@item
+The end of the input stream (EOF)
+ends the current line.
+
+@item
+The distinction between uppercase and lowercase letters
+will be preserved.
+
+It will be up to subsequent phases to decide to fold case.
+
+Current plans are to permit any casing for Fortran (reserved) keywords
+while preserving casing for user-defined names.
+(This might not be made the default for @file{.f} files, though.)
+
+Preserving case seems necessary to provide more direct access
+to facilities outside of @code{g77}, such as to C or Pascal code.
+
+Names of intrinsics will probably be matchable in any case,
+However, there probably won't be any option to require
+a particular mixed-case appearance of intrinsics
+(as there was for @code{g77} prior to version 0.6),
+because that's painful to maintain,
+and probably nobody uses it.
+
+(How @samp{external SiN; r = sin(x)} would be handled is TBD.
+I think old @code{g77} might already handle that pretty elegantly,
+but whether we can cope with allowing the same fragment to reference
+a @emph{different} procedure, even with the same interface,
+via @samp{s = SiN(r)}, needs to be determined.
+If it can't, we need to make sure that when code introduces
+a user-defined name, any intrinsic matching that name
+using a case-insensitive comparison
+is ``turned off''.)
+
+@item
+Backslashes in @code{CHARACTER} and Hollerith constants
+are not allowed.
+
+This avoids the confusion introduced by some Fortran compiler vendors
+providing C-like interpretation of backslashes,
+while others provide straight-through interpretation.
+
+Some kind of lexical construct (TBD) will be provided to allow
+flagging of a @code{CHARACTER}
+(but probably not a Hollerith)
+constant that permits backslashes.
+It'll necessarily be a prefix, such as:
+
+@smallexample
+PRINT *, C'This line has a backspace \b here.'
+PRINT *, F'This line has a straight backslash \ here.'
+@end smallexample
+
+Further, command-line options might be provided to specify that
+one prefix or the other is to be assumed as the default
+for @code{CHARACTER} constants.
+
+However, it seems more helpful for @code{g77} to provide a program
+that converts prefix all constants
+(or just those containing backslashes)
+with the desired designation,
+so printouts of code can be read
+without knowing the compile-time options used when compiling it.
+
+If such a program is provided
+(let's name it @code{g77slash} for now),
+then a command-line option to @code{g77} should not be provided.
+(Though, given that it'll be easy to implement, it might be hard
+to resist user requests for it ``to compile faster than if we
+have to invoke another filter''.)
+
+This program would take a command-line option to specify the
+default interpretation of slashes,
+affecting which prefix it uses for constants.
+
+@code{g77slash} probably should automatically convert Hollerith
+constants that contain slashes
+to the appropriate @code{CHARACTER} constants.
+Then @code{g77} wouldn't have to define a prefix syntax for Hollerith
+constants specifying whether they want C-style or straight-through
+backslashes.
+@end itemize
+
+The above implements nearly exactly what is specified by
+@ref{Character Set},
+and
+@ref{Lines},
+except it also provides automatic conversion of tabs
+and ignoring of newline-related carriage returns.
+
+It also effects the ``pure visual'' model,
+by which is meant that a user viewing his code
+in a typical text editor
+(assuming it's not preprocessed via @code{g77stripcard} or similar)
+doesn't need any special knowledge
+of whether spaces on the screen are really tabs,
+whether lines end immediately after the last visible non-space character
+or after a number of spaces and tabs that follow it,
+or whether the last line in the file is ended by a newline.
+
+Most editors don't make these distinctions,
+the ANSI FORTRAN 77 standard doesn't require them to,
+and it permits a standard-conforming compiler
+to define a method for transforming source code to
+``standard form'' however it wants.
+
+So, GNU Fortran defines it such that users have the best chance
+of having the code be interpreted the way it looks on the screen
+of the typical editor.
+
+(Fancy editors should @emph{never} be required to correctly read code
+written in classic two-dimensional-plaintext form.
+By correct reading I mean ability to read it, book-like, without
+mistaking text ignored by the compiler for program code and vice versa,
+and without having to count beyond the first several columns.
+The vague meaning of ASCII TAB, among other things, complicates
+this somewhat, but as long as ``everyone'', including the editor,
+other tools, and printer, agrees about the every-eighth-column convention,
+the GNU Fortran ``pure visual'' model meets these requirements.
+Any language or user-visible source form
+requiring special tagging of tabs,
+the ends of lines after spaces/tabs,
+and so on, is broken by this definition.
+Fortunately, Fortran @emph{itself} is not broken,
+even if most vendor-supplied defaults for their Fortran compilers @emph{are}
+in this regard.)
+
+Further, this model provides a clean interface
+to whatever preprocessors or code-generators are used
+to produce input to this phase of @code{g77}.
+Mainly, they need not worry about long lines.
+
+@node sta.c
+@subsection sta.c
+
+@node stb.c
+@subsection stb.c
+
+@node expr.c
+@subsection expr.c
+
+@node stc.c
+@subsection stc.c
+
+@node std.c
+@subsection std.c
+
+@node ste.c
+@subsection ste.c
+
+@node Gotchas (Transforming)
+@subsection 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.
+
+@node TBD (Transforming)
+@subsection TBD (Transforming)
+
+Continue researching gotchas, designing the transformational process,
+and implementing it.
+
+Specific issues to resolve:
+
+@itemize @bullet
+@item
+Just where should @code{INCLUDE} processing take place?
+
+Clearly before (or part of) statement identification (@file{sta.c}),
+since determining whether @samp{I(J)=K} is a statement-function
+definition or an assignment statement requires knowing the context,
+which in turn requires having processed @code{INCLUDE} files.
+
+@item
+Just where should (if it was implemented) @code{USE} processing take place?
+
+This gets into the whole issue of how @code{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 @code{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 @email{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 @emph{now} how to avoid making them harder to
+implement them @emph{later}.
+
+@item
+Should the @code{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 @code{g77} did,
+perhaps named @code{g77old}, might ease the transition,
+as might a corresponding one that converts source codes
+named @code{g77oldnew}.
+
+These scripts would take all the pertinent options @code{g77} used
+to take and run the appropriate filters,
+passing the results to @code{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).
+
+@item
+Do other Fortran compilers provide a prefix syntax
+to govern the treatment of backslashes in @code{CHARACTER}
+(or Hollerith) constants?
+
+Knowing what other compilers provide would help.
+@end itemize
+
@node Philosophy of Code Generation
@section Philosophy of Code Generation
@item
Other stuff???
-
-
@end itemize
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