To: J3                                                     J3/24-187
From: Malcolm Cohen
Subject: MAX_RANK proposal (part of auto-generic subprograms)
Date: 2024-October-30
References: N2217, 23-223r2, 23-244r1, 24-139r2, 24-168, 24-181.


1. Introduction

This paper presents specifications and syntax for MAX_RANK, viz a means of
discovering the maximum rank supported by a processor.

This is part of the auto-generic subprogram feature for Fortran 202y,
but is also more generally useful.


2. Constant or function?

Most of the papers so far have envisaged a named constant in the intrinsic
module ISO_FORTRAN_ENV, but 24-168 points out that the standard limit on
rank is actually a limit on rank+corank, and therefore a function might be
more appropriate.

If the processor limit is, like the standard, on rank+corank, the maximum
rank for a coarray would simply be (the constant) MAX_RANK minus the
intended corank.  However, there is no requirement that a processor's
maximum rank be different for a coarray: for example, a processor could
permit rank 15 and corank 15 for the same variable. If MAX_RANK were a
function, it could take that into account.


3. Specifications and syntax for MAX_RANK as a named constant

(MRS-100) That the intrinsic module ISO_FORTRAN_ENV shall contain a named
          scalar constant MAX_RANK, of type integer with default kind.

(MRS-101) The value of MAX_RANK the maximum rank of array that is supported
          by the processor.


4. Specifications and syntax for MAX_RANK as a function

(MRS-200) That the intrinsic module ISO_FORTRAN_ENV shall contain the
          simple scalar default integer function MAX_RANK. This function
          has one optional scalar argument CORANK, of type integer. The
          value of CORANK shall be non-negative.

(MRS-201) If the CORANK argument is absent, the result of MAX_RANK is the
          maximum rank of array that is supported by the processor.

(MRS-202) If the CORANK argument is present with a value less or equal to
          the maximum corank supported by the processor for a scalar, the
          result of MAX_RANK(CORANK) is the maximum rank supported for a
          coarray with corank CORANK.
          If the CORANK argument is present with a greater value, the
          result of MAX_RANK is equal to -HUGE(0).

Example: For a processor whose limits are exactly the minimum required by
         the standard,
            MAX_RANK() = 15
            MAX_RANK(1) = 14
            MAX_RANK(15) = 0
            MAX_RANK(16) = -2147483647 if default integer has 32 bits.
         For a processor whose maximum rank is 24, and whose maximum rank
         does not depend on the corank,
            MAX_RANK() = 24
            MAX_RANK(1) = 24
            MAX_RANK(24) = 24
            MAX_RANK(25) = -2147483647

Note: This function version of MAX_RANK thus also enables discovery of the
      maximum supported corank, as well as the relationship between maximum
      rank and corank.

(MRS-203) A reference to the function MAX_RANK may appear in a constant
          expression provided CORANK does not appear or is itself a
          constant expression.

Note: We already allow "transformational" functions from IEEE_ARITHMETIC
      and IEEE_EXCEPTIONS in constant expressions, so this would involve
      adding ISO_FORTRAN_ENV to the list in item (11) of 10.1.12 (p173).

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