Re: Gnat on OpenVMS

[Robert Dewar <dewar@gnat.com>]

In article <7i74fa$9e2$1@front6.grolier.fr>,
  "Thierry Lelegard" <lelegard@club-internet.fr> wrote:

> Honestly, I hardly understand why it was so difficult to
> integrate with the VMS debugger. The format of the debug
> tables
> is public (appendix of the linker manual).

Nobody said that it was "so difficult". I think Digital's
decision to require GDB as the base was not necessarily based
only bevcause it was difficult, although it would have been
relatively expensive, since it would have required even more
VMS-specific work in GNAT (which is a major maintenance issue
as well as a development issue).

Instead, my analysis is that Digital saw it as the best long
term choice, since they knew that with GDB, unlike the VMS
debugger, significant long term development and improvement
efforts would continue.

> All GNAT-specific
> features could have been implemented by GNAT using a DBGEXT
> entry in the GNAT RTL. The syntax of data is similar in Ada
> 83 and 95 and is already supported by the VMS debugger.

There are many features that would NOT have been well supported,
e.g. modular types, child units, tagged types (as you note),
Ada 95 attributes.

In addition, there are places where GNAT uses a different
approach from DEC Ada 83, and so the fact that the debugger
supports the Ada 83 approach would not help GNAT (e.g. proper
support of fat pointers for unconstrained types).

We did actually provide some minimal compatibility with
the VMS debugger to get the VMS traceback working correctly
(so line numbers and subprogram entries can be understood),
but the types do not attempt to use DEC Ada 83.

Robert Dewar
Ada Core Technologies




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