Possible Front End optimization - remove entities USEd from module but not really used.

[Toon Moene]

Consider the following example:

$ cat loc-nomod.f
       module noot
       real :: x(10), y(5), z(16)
       end module noot
       program doit
       use noot
       common /aap/ a(2), b(10), c(16)
       real :: f(6), g(3), h(10)
       real, allocatable :: s(:), t(:), u(:)
       locmod(r) = mod(loc(r),32)
       allocate(s(3),t(10),u(13))
!     print *,locmod(x(1)),locmod(y(1)),locmod(z(1))
       print *,locmod(a(1)),locmod(b(1)),locmod(c(1))
       print *,locmod(f(1)),locmod(g(1)),locmod(h(1))
       print *,locmod(s(1)),locmod(t(1)),locmod(u(1))
       end

which I originally used to study default aligned for arrays of various 
origins (module, common, local variable, allocatable.

I commented out the arrays USEd from a module.  However, both with and 
without -ffrontend-optimize (using 4.7.2),

gfortran -Ofast -g -c -fdump-fortran-original loc-nomod.f

showed the following in the output:

   symtree: 'x'           || symbol: 'x'
     type spec : (REAL 4)
     attributes: (VARIABLE IMPLICIT-SAVE DIMENSION USE-ASSOC(noot))
     Array spec:(1 [0] AS_EXPLICIT 1 10 )
   symtree: 'y'           || symbol: 'y'
     type spec : (REAL 4)
     attributes: (VARIABLE IMPLICIT-SAVE DIMENSION USE-ASSOC(noot))
     Array spec:(1 [0] AS_EXPLICIT 1 5 )
   symtree: 'z'           || symbol: 'z'
     type spec : (REAL 4)
     attributes: (VARIABLE IMPLICIT-SAVE DIMENSION USE-ASSOC(noot))
     Array spec:(1 [0] AS_EXPLICIT 1 16 )

Surely, those could be removed, speeding up the compilation process 
(later on, middle end optimizations probably can't get rid of them), and 
possibly helping optimization passes by removing clutter and extra 
entities which count against the various *max*=<number> limits.

Such an optimization could also help to produce various useful warning 
messages, like:

<x> is in module <m>, but not referenced in subprogram <y>
<x> is in module <m>, used as <xx>, but not referenced in subprogram <y>

or, in case only a few entities are referenced from a USEd module:

Few entities from module <m> are used, suggest USE <m>, ONLY: <x>, <y>, <z>

with a (default) cutoff of e.g., 10 entities.

Would that be useful ?

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
At home: http://moene.org/~toon/; weather: http://moene.org/~hirlam/
Progress of GNU Fortran: http://gcc.gnu.org/wiki/GFortran#news