PR fortran/23815: Add byte-swapping to gfortran

[Tobias Schlüter]

François-Xavier Coudert wrote:
>>Currently, there is only one method of supplying the information
>>of which files to byte-swap, via a CONVERT keyword on an OPEN
>>statement.
> 
> 
> Is that an already existent extension used by other compilers (and
> which)? I am *strongly* against new extensions: we can't complain that
> we have to support lots of stupid extensions that didn't age well
> *and* introduce new ones. Most compilers allow to do that with a
> compile-time (or link-time) switch; that's what i have read on most
> tutorials about this problem in scientific codes. We have such a
> possibility (I introduced it, see how the -ffpe-trap option is
> handled).

A compile-time or link-time switch would make it impossible to write a program
that converts between different endiannesses or reads files with different
endiannesses.  Without a language extension, this would only be possible with
per-unit environment variables.

- Tobi