backwards incompatibility

[Steve Kargl]

On Mon, Mar 12, 2018 at 03:40:48PM +0000, Dave Love wrote:
> It's quite a pain for supporting research systems that the gfortran
> module format changes incompatibly with every (?) new version.  For
> instance, you can't mix development packages built with the RHEL system
> compiler with the more recent "devtoolset" compiler needed for hardware
> support.  As far as I know, the proprietary compilers don't have the
> same issue.  I can't find anything written about that, but I haven't
> tried to understand the implementation.  Is there something written?
> Otherwise, is it feasible to convert older module files to newer
> formats, given that the ABI doesn't change, as I've seen in release
> notes (and would hope)?  If so, where would one look in the code to see
> about doing so?
> 
> Version 8 makes the situation worse by breaking run time compatibility,
> not just build-time, with a different libgfortran shared library
> version.  That already caused considerable disruption with Fedora
> packaging after the switch to GCC 8.  With ELF, can't you use versioned
> symbols to avoid a new soname, like libgcc and libgomp do?
> 
> Regardless of any annoyances, thanks for keeping a competitive free
> compiler going!

Hi Dave,

While I do not speak for the other contributors, I can
assure you that we take breaking the library ABI quite
serious.  We maintain a list of changes that we know will
cause problems.  If a change occurs that breaks the ABI,
we try to merge as many changes from the list as possible
to minimize on-going breakage.  See

https://gcc.gnu.org/wiki/LibgfortranAbiCleanup

for those that have yet to be merged/fixed.  Unfortunately,
the addition of user defined derived type IO, which was a
much requested feature, required a change to the library
internals for the upcoming 8.0.

Maintaining the internals of *.mod files is even more difficult
as changes are sometimes required for bug fixes as well as when
new features are implemented (e.g., parameterized derived type).

-- 
Steve