Desire to allocate bit in DT_PARM bitmask for DEC FORMAT compatibility purposes

[Jerry DeLisle]

On 03/25/2018 11:48 AM, Jeff Law wrote:
> On 03/24/2018 12:20 PM, Jerry DeLisle wrote:
>> On 03/24/2018 10:11 AM, Steve Kargl wrote:
>>> On Sat, Mar 24, 2018 at 05:20:06PM +0100, Jakub Jelinek wrote:
>>>> On Sat, Mar 24, 2018 at 09:04:04AM -0700, Steve Kargl wrote:
>>>>>> Or shall we just check (warn_std & GFC_STD_LEGACY) != 0, if true,
>>>>>> set fmt->error if width etc. is missing, otherwise use the -1 and
>>>>>> later
>>>>>> resolve it to the default?
>>>>>
>>>>> No.  DEC extensions should be activated by the use of one of the
>>>>> -fdec-* options.  For this extension, I think it can be grouped
>>>>> under the general purpose -fdec option.  In the Fortran FE, -fdec
>>>>> then becomes "if (flag_dec)".  I don't recall if Fritz set up
>>>>> passing flag_dec into libgfortran.
>>>>
>>>> I don't see it passed directly.
>>>> With -fdec compile_options.warn_std is 0, rather than the default
>>>> GFC_STD_LEGACY | GFC_STD_F95_DEL, but _gfortran_set_options
>>>> argument is exactly the same for -fdec and for -std=legacy, so there
>>>> is no
>>>> way to differentiate the two.
>>>>
>>>> So, the only way to handle this DEC extension without adding new bits
>>>> (to either compile_options (i.e. GFC_STD_*), or the io flags as shown
>>>> by the
>>>> patch Jeff posted) would be to abuse the IOPARM_dt_default_exp bit
>>>> for it,
>>>> i.e. tie together the extension omitting exponent and the default width.
>>>> Seems the IOPARM_dt_default_exp is set whenever -fdec and not otherwise:
>>>>
>>>
>>> Well, if we add
>>>
>>> #define GFC_STD_DEC_EXTÂ Â Â Â Â Â Â  (1<<13)
>>>
>>> to fortran/libgfortran.h, we still have 18 bits left.
>>>
>>> Then change options.c(set_dec_flags) to include this in the
>>> list of "standards" passed to libgfortran.  The tests would
>>> then look like
>>>
>>>  Â Â Â  fmt->value = (gfc_option.allow_std & GFC_STD_DEC_EXT) ? 2 : 1;
>>>
>>> I don't know if this is any better than using Jeff's idea of reserving
>>> bit #28 in flag (and adding a flag2).  Other than 18 bits at 5 to 8
>>> years per revision to the standards, means we'll be long retired
>>> before someone has to deal with running out of bits.
>>>
>>
>> I like this approach because we can then use it for anything else that
>> crops up down the road for the runtime.
> In this scenario we communicate between the compiler and runtime via the
> ALLOW_STD field.  The technical difference is the change applies
> globally rather than on per-call basis (DT_PARMs IIUC is computed and
> passed to the runtime for each format call).  I think that's likely to
> be OK.
> 
> The downside of this approach is it puts these extensions under the -std
> namespace rather than in -fdec.  I think Steve K. has expressed a desire
> to keep this stuff under -fdec.  I'm not sure how to reconcile the
> difference of opinions here.

Definitely under -fdec. I assumed one could set a new option bit with 
that and have it available globally. If not the DT_PARM bit is fine too. 
Either way, keep under -fdec.

Jerry