To:      J3                                        J3/14-184r2
From:    David Muxworthy & Steve Lionel
Subject: UK proposal UK-22 RANDOM_NUMBER intrinsic
Date: 2014 June 25


Status: For Consideration
Basic Functionality: Independent random number generators

Rationale:

In N2013, Van Snyder says:
"It is processor dependent whether common or independent random number
generators are used, and there's no way to detect which is the
situation.  Therefore taking action using one assumption or the other
(e.g., calling or not calling RANDOM_SEED on each image with a
hopefully-different value, maybe dependent on the image number) might
be the wrong thing to do.  It is preferable to provide a named
constant in ISO_FORTRAN_ENV to indicate whether the processor uses a
common random number generator on all images, or intrinsic procedures
so to inquire or specify.  References to the intrinsic subroutine that
specifies whether common or independent random number generators are
used should be image control statements.  13.5p4 should be adjusted so
that the segment ordering requirement applies only if a common random
number generator is used."

Further discussion indicated that it was undesireable for
RANDOM_NUMBER to be different/unpredictable by default. The solution
chosen is to modify the behavior of RANDOM_SEED without arguments,
currently processor-dependent, to assign an unpredictable value
to the seed that is different from that of other calls to
RANDOM_SEED without arguments in the same program. This implies that
calls in different images would result in different seeds. Some
implementations already have this behavior for RANDOM_SEED.

Specification:

Specify that calling RANDOM_SEED without arguments asigns a new,
unpredictable seed value that is different from those of
other calls to RANDOM_SEED in the same program.

Syntax:
No syntax change.

Edits to J3/14-007r1(= N2014):

[xviii] Introduction

Add to the bullet "Intrinsic procedures and modules" the sentence
"Calling the intrinsic procedure RANDOM_SEED without arguments assigns
an unpredictable seed value."

[382:6] 13.7.136 RANDOM_SEED - replace:

"If no argument is present, the processor assigns a processor-dependent
value to the seed."

with:

"If no argument is present, the processor assigns a processor-dependent,
unpredictable value to the seed different from those assigned by
other calls to RANDOM_SEED without arguments in the same program."