g77.info: Better Optimization

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.)