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

[Steve Kargl]

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_EXT		(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

   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.

-- 
Steve
20170425 https://www.youtube.com/watch?v=VWUpyCsUKR4
20161221 https://www.youtube.com/watch?v=IbCHE-hONow