23. 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/.

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

23.1 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.

23.2 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.)

23.3 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.

23.4 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 OPENed,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.

23.5 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.

23.6 Internals Documentation

Better info on how g77 works and how to port it is needed. Much of this should be done only after the redesign planned for 0.6 is complete.

See section 24. Front End, which contains some information on g77 internals.

23.7 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 ffewheres 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.

23.8 Better Diagnostics

These are things users might not ask about, or that need to be looked into, before worrying about. Also here are items that involve reducing unnecessary diagnostic clutter.

• When FUNCTION and ENTRY point types disagree (CHARACTER lengths, type classes, and so on), ANY-ize the offending ENTRY point and any new dummies it specifies.

• Speed up and improve error handling for data when repeat-count is specified. For example, don't output 20 unnecessary messages after the first necessary one for:

  INTEGER X(20) CONTINUE DATA (X(I), J= 1, 20) /20*5/ END

  (The CONTINUE statement ensures the DATA statement is processed in the context of executable, not specification, statements.)