RFH: Impose code-movement restrictions and value assumption (for ASYNCHRONOUS/Coarrays)

[Tobias Burnus]

Hello all,

I seek a tree attribute which tells that a "pointer" (in the 
C/middle-end sense) does not alias with any other variable in the 
translation unit (i.e. like "restrict"), but on the other hand, it 
should prevent code movements and value assumptions across (impure) 
function calls - as it is done for non-restrict pointers.

The primary usage are Fortran's coarrays. Those variables exists on all 
processes ("images") and can be accessed remotely using one-side 
communication semantics. As coarrays are also used in hot loops, I would 
like avoid using a non-restricted pointer. A similar issue exists for 
variables with the ASYNCHRONOUS attribute.


Middle-end question: How to handle this best with regards to the middle end?

C/C++ question: As one can also with C/C++ use asynchronous I/O, 
asynchronous communication via libraries as MPI, or single-sided 
communication via POSIX threads - or with C++0x's std:thread: How do you 
handle it? Just by avoiding "restrict"? Or do you have a solution, which 
can also be applied for Fortran? I'm sure that a "restrict + hope & 
pray" solution won't work reliably and thus is not used ;-)

Fortran question: Do my requirements make sense? That is: No code 
movements for any variable which is a coarray or has the asynchronous 
attribute in the scoping unit. Plus, no assumption of the value after 
any call to any impure function? Can something be relaxed or has 
anything to be tightened?


ASYNCHRONOUS is defined in the Fortran standard (e.g 2008, Section 
5.3.4) and extended to explicitly allow for asynchronous user functions 
in Technical Report 29113. The latter functionality will be used in the 
Message Passing Interface (MPI) specification 3.0. Like VOLATILE, the 
asynchronous attribute might be restricted to a block (in C: { ... }). 
Coarrays are defined in the Fortran 2008 standard. (For semantics of 
interest, see especially Section 8.5 and, in particular, Subsection 8.5.2.)

The Fortran 2008 standard is available at 
ftp://ftp.nag.co.uk/sc22wg5/N1801-N1850/N1830.pdf and the PDTR 29113 at 
ftp://ftp.nag.co.uk/sc22wg5/N1851-N1900/N1866.pdf


Example 1: Asynchronous I/O; in this example using build-in functions, 
but asynchronous MPI communication would be another example
   integer, ASYNCHRONOUS :: a
   ...
   READ(unit_number,ID=idvar, asynchronous='yes') a
     ...
   WAIT(ID=idvar)
   ... = a

Here, "= a" may not be moved before WAIT.

Example 2: Coarray with sync. The SYNC is not directly called, but via a 
wrapper function to increase the fun factor.
     subroutine sub(coarray)
       integer :: coarray[*]
       coarray = 5
       call SYNC_calling_proc()
       ! coarray is modified remotely
       call SYNC_calling_proc()
       if (coarray /= 5) ...
     end subroutine sub
Here, the "if" may not be removed as the image could have been changed 
remotely.

Example 3: Allow other optimizations
     subroutine sub(coarray1, coarray2)
       integer :: coarray1[*], coarray2[*]
       coarray1 = 5
       coarray2 = 7
       if (coarray1 /= 5)
Here, the "if" can be removed as "coarray1" cannot alias with any other 
variable in "sub" as it is not TARGET - and, in particular, it cannot 
alias with "coarray2" as neither of them is a pointer.

Tobias