To: J3                                                     J3/19-140
From: Bill Long
Subject: Reply for paper 19-109
Date: 2019-February-11


I Introduction
--------------

Paper 19-109 proposes a scheme for synchronizing execution of a code
section that is normally executed asynchronously, by employing syntax
to affect the concept of full and empty bits for variables. An
execution thread that attempts to reference a variable that is "empty"
would stall until a different thread defined the variable, changing
its state to "full".  Similarly for trying to define a variable that
is "full".

The main use case is to provide the ability to have cross-iteration
dependencies in a DO CONCURRENT construct, the iterations of which
normally execute asynchronously.


II. Discussion
--------------

The attempt to circumvent the most basic and useful concept of DO
CONCURRENT (independence of iteration execution) seems dubious, and
likely to confuse programmers.

Schemes to implement full and empty memory bits in hardware have been
employed in the past, mainly as a mechanism for hiding memory access
latency. These systems have been remarkably unsuccessful. It is very
likely that a software-only implementation would have serious
performance problems.

The amount of changes to the standard and implementations seem to
significantly exceed any benefit that might accrue from this feature.


III. Recommendation
-------------------

Subgroup recommends no action for the proposed feature in 19-109.