[fortran,patch] Don't write common more than once in a module file (PR 30285)

[Jerry DeLisle]

FX Coudert wrote:
> PR 30285 is about deeply nested module files containing commons, which 
> take a huge amount of memory when they're read back (and loads of disk 
> space, but that's not so much of a problem). This commonly happens with 
> MPICH and openmpi, and leads some people to run out of memory when 
> compiling mpi-enabled programs.
> 
> The problem is that we write each common name more than once (a number 
> of times that grows exponentially with the nesting). I tried to change 
> the way we load modules, to avoid creating these duplicate symbols, but 
> the diagnostics code for commons with binding labels requires this 
> creation of multiple symbols, so I went for something that is a bit of a 
> hack: we get rid of duplicate common names (specificaly: duplicate pairs 
> of name and binding label) when writing the module files, by creating a 
> list of them as we go through and checking them against that list.
> 
> I agree it's not a perfect solution, but I couldn't make it work in any 
> other way (see the PR for a tentative patch), and I think it's worth 
> committing now, as it's a serious problem for our users. Regtested on 
> x86_64-linux, OK to commit?
> 
> FX
> 
> 
> 
> :ADDPATCH fortran:

Reviwed and with Heralds positive report.  OK to commit.

Jerry