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(g77-295.info)Philosophy of Code Generation

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Philosophy of Code Generation
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   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.

   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.