X3J3/96-082

Date:    April 29, 1996
To:      X3J3
From:    Kurt W. Hirchert
Subject: JOR Item No. 116

  <FORTREQ>
  <NUMBER> 116
  <TITLE> "Simultaneous" assignment
  <KEYWORDS>
  <STATUS> Registered
  <TARGET>
  <SUBGROUP>
  <VERSION> 1

  <REQUIREMENT>
  Provide some mechanism for more general "simultaneous" assignment, i.e.
  evaluating all "right hand sides" before modifying the "left hand sides".

  <JUSTIFICATION>
  It is a common problem to have a set of variables that must be updated
  based on their collective current state.  Introducing explict temporary
  variables to achieve this obscures the logic and may produce less efficient
  code than what a compiler can do with architecture-specific temporary
  generation.

  <SUGGESTED IMPLEMENTATION>
  I can think of several very different notation approaches.  One would be
  to allow derived type "pseudo-variables" on the left hand side of an
  assignment (making use of the simultaneity within a derived type assignment):
        SWAP(X,Y)=SWAP(Y,X)
  Another would be to try to extend the simultaneity of the FORALL statement:
        FORALL(I=1:2)
        SELECT CASE(I)
        CASE(1); N=M
        CASE(2); M=N+M
        END SELECT
        END FORALL
  Yet another would be to create a new construct for this:
        SIMUL
          X=X*COS_THETA-Y*SIN_THETA
          Y=X*SIN_THETA+Y*COS_THETA
        END SIMUL

  <ESTIMATED IMPACT>
  The impact on the document is likely to be localized.  Impact on existing
  processors appears potentially significant.

  <SUBMITTED BY> Kurt W. Hirchert, hirchert@ncsa.uiuc.edu

  <HISTORY>
  <EVENT> 29 April 96: submitted
  </HISTORY>
  </FORTREQ>
--
Kurt W. Hirchert     hirchert@ncsa.uiuc.edu
National Center for Supercomputing Applications