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

[Jakub Jelinek]

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:

  /* See if we want to use defaults for missing exponents in real transfers.  */
  if (flag_dec)
    dt->default_exp = 1;
...
      if (dt->default_exp)
        mask |= IOPARM_dt_default_exp;
are the only spots that refer to default_exp in the FE.

If it is acceptable that users can't choose I want default widths but not
default exponent, or I want default exponent but not default width, then
yes, we can just check
dtp->common.flags & IOPARM_DT_DEFAULT_EXP
for whether default widths are allowed or not, rather than
dtp->common.flags & IOPARM_DT_DEFAULT_WIDTH
or
compile_options.allow_std & GFC_STD_DEC

> > Any way to deal with the FMT_L missing width inconsistency (libgfortran
> > under notify_std (..., GFC_STD_GNU, ...) using default length of 1 while
> > the other compilers all using 2?
> 
> 	  fmt->value = flag_dec ? 2 : 1;	/* Default width */ 

flag_dec would need to be one of the above.

	Jakub