Here's now my third attempt to fix this PR, taking into account
the comments by Tobias and Bernhard.

> > On 10.06.21 20:52, Harald Anlauf via Fortran wrote:
> > > +static bool
> > > +substring_has_constant_len (gfc_expr *e)
> > > +{
> > > +  ptrdiff_t istart, iend;
> > > +  size_t length;
> > > +  bool equal_length = false;
> > > +
> > > +  if (e->ts.type != BT_CHARACTER
> > > +      || !e->ref
> > > +      || e->ref->type != REF_SUBSTRING
> > 
> > Is there a reason why you do not handle:
> > 
> > type t
> >    character(len=5) :: str1
> >    character(len=:), allocatable :: str2
> > end type
> > type(t) :: x
> > 
> > allocate(x%str2, source="abd")
> > if (len (x%str)) /= 1) ...
> > if (len (x%str2(1:2) /= 2) ...
> > etc.
> > 
> > Namely: Search the last_ref = expr->ref->next->next ...?
> > and then check that lastref?

The mentioned search is now implemented.

Note, however, that gfc_simplify_len still won't handle neither
deferred strings nor their substrings.

I think there is nothing to simplify at compile time here.  Otherwise
there would be a conflict/inconsistency with type parameter inquiry,
see F2018:9.4.5(2):

"A deferred type parameter of a pointer that is not associated or
of an unallocated allocatable variable shall not be inquired about."

> >    * * *
> > 
> > Slightly unrelated: I think the following does not violate
> > F2018's R916 / C923 – but is rejected, namely:
> >    R916  type-param-inquiry  is  designator % type-param-name
> > the latter is 'len' or 'kind' for intrinsic types. And:
> >    R901  designator is ...
> >                     or substring
> > But
> > 
> > character(len=5) :: str
> > print *, str(1:3)%len
> > end
> > 
> > fails with
> > 
> >      2 | print *, str(1:3)%len
> >        |                  1
> > Error: Syntax error in PRINT statement at (1)
> > 
> > 
> > Assuming you don't want to handle it, can you open a new PR?
> > Thanks!

I tried to look into this, but there appear to be several unrelated
issues requiring a separate treatment.  I therefore opened:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101735

> > > +  istart = gfc_mpz_get_hwi (e->ref->u.ss.start->value.integer);
> > > +  iend = gfc_mpz_get_hwi (e->ref->u.ss.end->value.integer);
> > > +  length = gfc_mpz_get_hwi (e->ref->u.ss.length->length->value.integer);
> > > +
> > > +  if (istart <= iend)
> > > +    {
> > > +      if (istart < 1)
> > > +     {
> > > +       gfc_error ("Substring start index (%ld) at %L below 1",
> > > +                  (long) istart, &e->ref->u.ss.start->where);
> > 
> > As mentioned by Bernhard, you could use HOST_WIDE_INT_PRINT_DEC.
> > 
> > (It probably only matters on Windows which uses long == int = 32bit for
> > strings longer than INT_MAX.)

Done.

The updated patch regtests fine.  OK?

Thanks,
Harald


Fortran - simplify length of substring with constant bounds

gcc/fortran/ChangeLog:

	PR fortran/100950
	* simplify.c (substring_has_constant_len): New.
	(gfc_simplify_len): Handle case of substrings with constant
	bounds.

gcc/testsuite/ChangeLog:

	PR fortran/100950
	* gfortran.dg/pr100950.f90: New test.