To:       J3                                                08-143r3
From:     Jim Xia/Bill Long
Subject:  Comment: to disallow coindexed object as a data-ref in
          procedure-designator
Date: 2008 May 15
References: J3/08-007r2 (WG5/N1723)


Discussion:

The 2008 standard explicitly declares that a coindexed object can be
used as the <data-ref> in a <procedure-designator> or <variable> in a
<proc-component-ref>.  In particular, Note 12.16 on page 286 provides
the following interpretation for the case

   "If image I executes a procedure reference in which the variable
    of a proc-component-ref specifies a procedure pointer on image J,
    the procedure pointer association is fetched from image J but the
    invocation of the associated procedure occurs on image I."

This capability was included because it is irregular that one can read
data pointers on other images but not procedure pointers. This was
particularly argued for the case when a polymorphic object has
different dynamic types on different images---there is no way for an
image to call the right type-bound procedure for the objects on other
images.

While simple to implement on a homogeneous system if the same static
executable is running for each image, this capability is more
difficult to implement on a heterogeneous system, and execution of
such procedure references would likely result in poor performance.

It is also possible that users might be confused by the notation and
assume that the procedure is executing on the remote image.

The main use of this is when there is a PASSed dummy argument, or the
object is polymorphic. For either case the notation

 call x[j]%proc()

is a short hand for

 call target_of_proc_on_image_j (x[j])

which likely results in a copy-in/copy-out of the argument.  This is
not what users would want or expect and is likely to be inefficient in
most cases. Furthermore, this could be problematic if there are
pointer components of the type.

Overall, we feel the negatives outweigh the benefits, and this
capability should be removed.



EDITS to 08-007r2 (WG5 N1723):

[157:7.2.2.2] Near the end of "Pointer assignment/Syntax" add a new
constraint:

"C728a (R741) The <scalar-variable> shall not be a coindexed object."

[285:12.5.1p1] In "Procedure reference/Syntax" add a new constraint:

"C1230a (R1221) <data-ref> shall not be a coindexed object."

[286:12.5.1p3+] Delete Note 12.16.