Re: Studying and Maintaining GNAT, Is There Any Interest in a New Group?

["Dan'l Miller" <optikos@verizon.net>]

On Saturday, August 25, 2018 at 4:48:07 PM UTC-5, Luke A. Guest wrote:
> <patrick@spellingbeewinnars.org> wrote:
> 
> > However I am discouraged with what you are saying about the bugs being on
> > the compiler side and not so much in the RTS.
> 
> It’s more the bugs are in the Ada front end, not the GCC backend, which is
> the codegen.

Although it might be quibbling over the definition of “front-end” and “back-end”, it is my belief that the vast majority of the bugs outside of the runtime is either:
1) front-end proper:
lack of fidelity (to the _LRM_ or to nuanced explanations in the _AARM_) in the Ada-language semantic-adornment of the AST
and
2) lack of fidelity in the GIGI tree-transducer middle layer (which might be truly one-of-a-kind unique to GNAT) effectively •outside• the Ada-language-source-code front-end proper and the GENERIC/GIMPLE/RTL back-end proper:
Between a) the Ada-language semantic-tree in the written-in-Ada front-end proper and b) the GCC GENERIC/GIMPLE/RTL back-end that is shared among all GCC languages, there exists the GIGI () tree-transducer that observes snippets of Ada-front-end's semantically-adorned AST to then instantiate the analogous C/C++-language semantics in that branch of the GENERIC/GIMPLE's back-end semantic tree.

There are 2 areas where GNAT can easily make a mistake:
1) misinterpret (or malenforce) the normative _LRM_ and/or the nuanced explanations in the _AARM_ in the Ada semantically-adorned AST written in Ada language
or
2) mistranscribe the ‘equivalent‘ C/C++ semantics in GENERIC/GIMPLE via the GIGI tree transducer, written in C language.  Just like beauty, ‘equivalent’ degenerates to:  equivalent is in the eye of the beholder.  (And the human being of high IQ has a bigger eye-of-the-beholder than GIGI's observation of mere subset of the Ada AST.  GIGI's eye-of-the-beholder naturally has tunnel vision.)

On Saturday, August 25, 2018 at 5:05:52 PM UTC-5, Luke A. Guest wrote:
> A new compiler needs a radical new design.

Luke, GIGI is the general vicinity for fulfilling your prophetic prediction of the future:  A radical new Ada-compiler design would eliminate the tree-transducer's transcription of snippets of Ada's semantically-adorned AST (in Ada-language source code) into GENERIC/GIMPLE C/C++ semantic tree (in C-language source code).  In short, a radical new Ada-compiler design would eliminate GIGI.  Luke & Shark8 take especial note:  eliminate the need for GIGI entirely, then one has a drastically entirely-different-than-GNAT design for a next-gen Ada compiler.  Luke, despite your ridicule of studying Ada's antiquity, •that• elimination of GIGI is precisely why studying William Wulf's DIANA from decades ago is intellectually stimulating & rewarding as getting the creative juices flowing in the mind when contemplating what a next-gen Ada compiler might have at its heart instead of the bug-prone complication that GNAT has at its heart:  AdaAST-GIGI-C/C++AST, 2 separate trees and a transducer-of-clever-snippets between them.  (I suspect sometimes the scope of that cleverness there in GIGI is insufficiently narrow/not-omniscient-enough, hence mistranscription bug.)

Although Figure 1.1 is the front-end/back-end diagram that most people hold in their head (and dear to their heart) from a compiler-construction textbook, I find Figure 1.2 to much more lucidly reveal GNAT's inner workings in a nutshell for repeated reference in the mind's eye:
https://www2.adacore.com/gap-static/GNAT_Book/html/node5.htm#SECTION03120000000000000000

It is not entirely clear (to me) what GIGI stands for, btw.  One of the “G”s could stand for GNAT; the other could stand for GENERIC or GIMPLE.  One of the “I”s could stand for instantiation (as in instantiating a snippet/extract of the Ada AST into GENERIC/GIMPLE's C-language tree).  One guess:  GNAT-Internal GENERIC/GIMPLE Instatiator.  Perhaps only the original designers of GIGI know for sure:  Robert Dewar, RWS, perhaps Edmond Schonberg.