To: J3                                                     J3/18-257r2
From: Dan Nagle
Subject: log<n> and friends
Date: 2018-October-17



I Introduction

At 215, J3 agreed, and at 216 WG5 concurred, to add the following
to the iso_fortran_env intrinsic module.

logical8 - an integer constant whose value is the kind value
       of a logical whose storage size is 8 bits

logical16 - an integer constant whose value is the kind value
       of a logical whose storage size is 16 bits

logical32 - an integer constant whose value is the kind value
        of a logical whose storage size is 32 bits

logical64 - an integer constant whose value is the kind value
        of alogical whose storage size is 64 bits

real16 - an integer constant whose value is the kind value
         of a real whose storage size is 16 bits


II Use-cases

Presently, there is no way to get these kind values, other
than trying to decode the logical_kinds array.  However,
this array is unsorted, and so it is difficult for the purpose.

Newer hardware now supports a 16-bit real (for use in AI work).
This is missing from the current real32, real64, real128 set.


III What I have in mind

Add logical[8-64] and real16 to iso_fortran_env.



IV (Rough) Requirements

iso_fortran_env is expanded to include:

logical8, logical16, logical32, logical64

The values of these default integer scalar constants shall be those
of the kind type parameters that specify an LOGICAL type
whose storage size expressed in bits is 8, 16, 32, and 64, respectively.
If, for any of these constants, the processor supports more than one kind
of that size, it is processor dependent which kind value is provided.
If the processor supports no kind of a particular size, that constant
shall be equal to -2 if the processor supports a kind with larger size
and -1 otherwise.

real16

The value of this default integer scalar named constant shall be
that of the kind type parameters that specify a REAL type whose
storage size expressed in bits is 16.  If, for this constant,
the processor supports more than one kind of that size, it is
processor dependent which kind value is provided.  If the processor
supports no kind of a particular size, that constant shall be equal
to -2 if the processor supports kinds of a larger size and -1 otherwise.