To: J3                                                     J3/21-142
From: Yasuharu Hayashi
Subject: NOTE on the coarray memory model
Date: 2021-June-02

1. Introduction

NOTE 4 in clause 11.7.2 says
    "In practice, apart from executions of atomic subroutines, the
     processor could make a copy of a nonvolatile coarray on an image
     (in cache or a register, for example) and, as an optimization,
     defer copying a changed value back to the permanent memory location
     while it is still being used. Since the variable is not volatile, it
     is safe to defer this transfer until the end of the segment and
     thereafter to reload from permanent memory any coarray that was not
     defined within the segment. It might not be safe to defer these
     actions beyond the end of the segment since another image might
     reference the variable then."

The intention of the latter half of the second sentence
    "thereafter to reload..."
related to cache optimizations for nonvolatile coarrays is unclear, because
 + Reload from permanent memory after the copy back should be safe
   irrespective of whether the nonvolatile coarray was defined or not,
 + The copy back and reload of the nonvolatile coarray that was not defined
   are not necessary in the first place, and
 + "thereafter" could be interpreted as "after the end of the segment",
   which seems to be in contradiction with the description in the last
   sentence
       "not be safe to defer these actions beyond the end of the segment".

Therefore this paper suggests modifying NOTE 4 in clause 11.7.2 in
ISO/IEC JTC1/SC22/WG5/N2184 as follows:

2. Edits to ISO/IEC JTC1/SC22/WG5/N2184

Change
   "Since the variable is not volatile, it is safe to defer this transfer
    until the end of the segment and thereafter to reload from permanent
    memory any coarray that was not defined within the segment."
to
   "Since the variable is not volatile, it is safe to defer this transfer
    or refer to the value of the local copy without reloading it from
    permanent memory until the end of the segment."

===END===