Hi Harald,

On 18.08.21 23:01, Harald Anlauf wrote:
>> Von: "Tobias Burnus"<tobias@codesourcery.com>
>>> Note, however, that gfc_simplify_len still won't handle neither
>>> deferred strings nor their substrings.
>> Obviously, nonsubstrings cannot be simplified but I do not
>> see why  len(str(1:2))  cannot or should not be simplified.
> well, here's an example that Intel rejects:
> ...
>       character(:), allocatable :: str
>    end type u
>    type(u) :: q
> ...
>    integer, parameter :: k3 = len (q% str (3:4)) ! Rejected by Intel
>
> pr100950-ww.f90(7): error #6814: When using this inquiry function, the length of this object cannot be evaluated to a constant.   [LEN]

I think the question is really how to interpret "10.1.12 Constant expression"

"(4) a specification inquiry where each designator or argument is
    ...
  (b) a variable whose properties inquired about are not
     (i) assumed,
     (ii) deferred, or
     (iii) defined by an expression that is not a constant expression,"

And as the substring bounds are constant expressions,
one can argue that (4)(b) is fulfilled as (i)–(iii) do not apply.

I am inclined to say that the Intel compiler has a bug by not
accepting it – but as written before, I regard sub-string length
(esp. with const expr) inquiries as an odd corner case which
is unlikely to occur in real-world code.

>> However, there is no reason why the user cannot do [...]
> Maybe you can enlighten me here.  [...]
I can't as I did not understand your question. However ...
>> But, IMHO, the latter remark does_not_  imply that we
>> shall/must/have to accept code like:
>>
>> if (allocated(str)) then
>>     block
>>        integer, parameter :: n = len(str(:5))
>>     end block
>> endif
> So shall we not simplify here (and thus reject it)?
> This is important!  Or silently simplify and accept it?

I tried to draw the line between simplification – to generate better code –
and 'constant expression' handling (accept where permitted, diagnose
non-standard-conforming code). — However, nearly but not quite always:
if it can be simplified to a constant the standard also regards it as
constant expression.

I think in for the purpose of the examples in this email thread,
we do not need to distinguish the two. — And can always simplify
deferred-length substrings where the substring bounds are const
expressions (or the lower-bound is absent and, hence, 1).

>> With the caveat from above that len(<substring>) is rather special,
>> there is no real reason why:  str_array(:)(4:5)  cannot be handled.
>> (→ len = 2).
> Good point.  This is fixed in the revised patch and tested for.

Still does not work – or rather: ...%t(:)(3:4) [i.e. substring with array section]
and ...%str(3:4) [i.e. substring of deferred-length scalar] both do work
but if one combines the two (→ ...%str2(:)(3:4), i.e. substring of deferred-length
array section), it does not:

Array ‘r’ at (1) is a variable, which does not reduce to a constant expression

for:

--- a/gcc/testsuite/gfortran.dg/pr100950.f90
+++ b/gcc/testsuite/gfortran.dg/pr100950.f90
@@ -15,2 +15,3 @@ program p
       character(len=:), allocatable :: str
+     character(len=:), allocatable :: str2(:)
    end type t_
@@ -24,2 +25,4 @@ program p
    integer,      parameter :: l6 = len (r(1)%str (3:4))
+  integer,      parameter :: l7 = len (r(1)%str2(1)(3:4))
+  integer,      parameter :: l8 = len (r(1)%str2(:)(3:4))


which feels odd.

>>> The updated patch regtests fine.  OK?
>> Looks good to me except for the caveats.
> Regtested again.
[...]
> Well, there's already
>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101735

I have added the example to the PR.

>> For deferred length, I have no strong opinion; [...]
> Actually, this is now an important point.  If we really want
> to allow to handle substrings of deferred length strings
> in constant expressions, the new patch would be fine,
I think handling len=: substrings is fine.

In principle, LGTM – except I wonder what we do about the
len(r(1)%str(1)(3:4));
I think we really do handle most code available and I would like to
close this
topic – but still it feels a bit odd to leave this bit out.

I was also wondering whether we should check that the
compile-time simplification works – i.e. use -fdump-tree-original for this;
I attached a patch for this.

Thanks,

Tobias

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