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.
g77 and gcc, but
they apply only when compiling Fortran programs.
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)
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)
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.