Re: [Patch, Fortran] libgfortran's ISO_Fortran_binding.c: Use GCC11 version for backward-only code [PR108056]

[Harald Anlauf <anlauf@gmx.de>]

Hi Tobias,

thanks for your explanation!  Now things are clearer.

Am 14.12.22 um 08:57 schrieb Tobias Burnus:
>>> @@ -63 +66 @@ cfi_desc_to_gfc_desc (gfc_array_void *d,
>>> -  d->dtype.version = s->version;
>>> +  d->dtype.version = 0;
>>
>> I was wondering what the significance of "version" is.
>> In ISO_Fortran_binding.h we seem to always have
>>    #define CFI_VERSION 1
>> and it did not change with gcc-12.
>
> The version is 1 for CFI but it is 0 for GFC. However, as we do not
> check the GFC version anywhere and it is not publicly exposed, it does
> not really matter. Still, "d->dtype.version = 0;" matches what the
> compiler itself produces – and for consistency, setting it to 0 is
> better than setting it to 1 (via CFI's version field).
>
> Actually 'dtype.version' is not really set anywhere; at least
> gfc_get_dtype_rank_type(...) does not set it; zero initialization is
> most common but it could be also some random value. In libgfortran,
> GFC_DTYPE_CLEAR explicitly sets it to 0.

OK, that was not easy to find.

>>> @@ -107 +117 @@ gfc_desc_to_cfi_desc (CFI_cdesc_t **d_pt
>>> -  d->version = s->dtype.version;
>>> +  d->version = CFI_VERSION;
>>
>> This treatment of "version" was the equivalent to the above that
>> confused me.  Assuming we were to change CFI_VERSION in gcc-13+,
>> is this the right choice here regarding backward compatibility?
>
> I don't think we will change CFI version any time soon as we rather
> closely follow the Fortran standard and I do not see any changes which
> are required there.
>
> NOTE: As s->dtype.version is either 0 or some random value, setting
> version in the CFI / ISO C descriptor to 1, be it as literal or as macro
> constant, makes it the same as CFI_VERSION.

OK

> And: I don't think we will change CFI_VERSION or the structure of the
> CFI array descriptor any time soon; there does not seem to be any need
> for it, it matches the Fortran standard one well (and no plans seem to
> be planed on that side) and, finally, changing an array descriptor is
> painful!

Agreed.

> However, using '1;  /* CFI_VERSION in GCC 11 and at time of writing. */'
> would also work – but I would expect that we will go through all CFI
> users if we ever change the descriptor (and bump the version), possibly
> adding version-number dependent code.

Agreed.  Searching for a string that can be guessed is
easier that looking for a magic '1'.

>> So besides the "version" question ok from my side.
>
> I hope I could answer the latter.

Yes, and thanks for the patch!

Harald

> Tobias
>
> -----------------
> Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201,
> 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer:
> Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München;
> Registergericht München, HRB 106955
>