[patch, fortran, 4.9] Dependency and string length calculation improvements

[Thomas Koenig]

Hello world,

below is a patch which improves dependency checking for array
assignments and calculation of string lengths.

The new function gfc_dep_difference is used for both.  This allows
us to detect, for example, that the difference between the lower
indices of

     a(n:m:2) = a(n+1:m+1:2)

is one, thus making an array temporary unnecessary.

I have tried to cover every code path through gfc_dep_difference in
string_length_2.f90. dependency_40.f90 is just there to test
the basic functionality.

OK for trunk?

	Thomas

2013-03-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/45159
         * gfortran.h (gfc_dep_difference):  Add prototype.
         * dependency.c (discard_nops):  New function.
         (gfc_dep_difference):  New function.
         (check_section_vs_section):  Use gfc_dep_difference
         to calculate the difference of starting indices.
         * trans-expr.c (gfc_conv_substring):  Use
         gfc_dep_difference to calculate the length of
         substrings where possible.

2013-03-16  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/45159
         * gfortran.dg/string_length_2.f90:  New test.
         * gfortran.dg/dependency_40.f90:  New test.
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