To: J3                                                     J3/22-195
From: Bill Long
Subject: F202Y feature : BITS
Date: 2022-October-17


Introduction:

At a late WG5 meeting for F2008, we downsized the proposal to reduce
the number of "large" features from three to one. Kept was the SPMD
parallel programming using coarrys, and dropped were the BITS feature
and the Intelligent Macros feature.  The macros feature has been
supplanted by the Generic Programming feature for F202Y, and some of
the F2023 language syntax features.  The BITS feature was described in
the last F2008 draft before the reduction (07-007r2.pdf) in the
Introduction as "A BITS data type for non-numeric programming and
enhanced handling of binary, octal, and hexadecimal constants.".

The BITS type was originally proposed as a performance and portability
feature that got around the problem of the leftmost bit in an INTEGER
being unspecified.  As a feature for F202Y, we can leverage (and not
have to repeat) all of the discussion and work done for the previous
standard draft and can reuse a lot of the old edits.

Discussion:

The BITS data type is an intrinsic type with one kind-type parameter
that indicates the number of bits in a data element of the
type. Constants for the type are the existing
boz-literal-constants. Conversion of boz-constants to type INTEGER and
REAL were already specified in F2003 and that same scheme can be used
for conversion of the values of BITS variables to INTEGER and REAL
data items.  The binary operators for BITS objects are the existing
.and., .or., and .xor., with .not. as the unary operator. The
descriptions of several intrinsic functions, such as IAND, IOR, IEOR,
and NOT can be simplified if rewritten for BITS data.  The previous
draft allowed association of INTEGER arguments with BiTS dummy
arguments of the same BIT_SIZE().

BITS data elements can be used to represent compressed genomic data,
initially represented by a string of A, C, T, and G characters, with
the letters (case-independent) efficiently mapped to {00, 01, 10, 11}
permitting 32 characters to be packed into a single BITS(64) data
elements.  Similarly, signal data that is provided as a sequence of
bits can be more easily handled.

This paper proposes for F202Y the semantics and syntax for BITS from
J3/07-007r2.pdf, as modified where necessary to account for changes
made in F2018 and F2023.  The edits are not specified in this paper
but can be largely copied from the old 007 draft, with modifications
where needed to accommodate changes in later standards.




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