[patch, Fortran] PR 58146, enable array slice compile-time bounds checking

[Thomas Koenig]

Hello world,

the attached patch enables more sophisticated bounds-checking on
array slices by using gfc_dep_difference to calculate extents.
The information may also be useful in other places of the
front end, I don't really know.

There is one wrinkle (alluded to in the comments) which makes
this harder.  When somebody changes the value of a variable
used in detemining the size of an array, such as

subroutine foo(a,n)
  real, dimension(n) :: a

  n = n -2

  print *,ubound(a(n-1:))

we cannot compare n-1 against n and think that their difference is
one :-(

This is why I restricted myself to expressions where all
indices are specified, e.g. in a(n+1:n+4) or none are specified,
as in a(:).

In order for this to work on 64-bit systems, it was necessary
to look through widening integer conversions, so I added that
functionality to discard_nops.  Using this function in
gfc_dep_compare_expr made this function shorter and cleaner.

Regression-tested.  OK for trunk?

	Thomas

2013-08-14  Thomas Koenig  <tkoenig@gcc.gnu.org>

        PR fortran/58146
        * array.c (gfc_ref_dimen_size):  If possible, use
        gfc_dep_difference to calculate array refrence
        sizes.  Fall back to integer code otherwise.
        * dependency.c (discard_nops).  Move up.
        Also discarde widening integer conversions.
        (gfc_dep_compare_expr):  Use discard_nops.

2013-08-14  Thomas Koenig  <tkoenig@gcc.gnu.org>

        PR fortran/58146
        * gfortran.dg/bounds_check_18.f90:  New test.
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