To: J3                                                     J3/18-257r1
From: Dan Nagle
Subject: log<n> and friends
Date: 2018-October-16



I Introduction

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

log8 - an integer constant whose value is the kind value
       of an 8-bit logical

log16 - an integer constant whose value is the kind value
       of a 16-bit logical

log32 - an integer constant whose value is the kind value
        of a 32-bit logical

log64 - an integer constant whose value is the kind value
        of an 64-bit logical

real16 - an integer constant whose value is the kind value
         of a 16-bit real


II Use-cases

At times, one wants the smallest, or the fastest, logical.
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 useless 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 log[8-64] and real16 to iso_fortran_env.



IV (Rough) Requirements

iso_fortran_env is expanded to include:

log8, log16, log32, log64

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.