cDEC$ IVDEP

  The IVDEP directive assists the compiler's dependence analysis.  It
  can only be applied to iterative DO loops.  This directive can also
  be specified as INIT_DEP_FWD (INITialize DEPendences ForWarD).

  The IVDEP directive takes the following form:

  cDEC$ IVDEP

    c        Is one of the following: C (or c), !, or *.

  The IVDEP directive is an assertion to the compiler's optimizer
  about the order of memory references inside a DO loop.

  The IVDEP directive tells the compiler to begin dependence analysis
  by assuming all dependences occur in the same forward direction as
  their appearance in the normal scalar execution order.  This
  contrasts with normal compiler behavior, which is for the
  dependence analysis to make no initial assumptions about the
  direction of a dependence.

  The IVDEP directive must precede the DO statement for each DO loop
  it affects.  No source code lines, other than the following, can be
  placed between the IVDEP directive statement and the DO statement:

   o  An UNROLL directive

   o  Placeholder lines

   o  Comment lines

   o  Blank lines

  The IVDEP directive is applied to a DO loop in which the user knows
  that dependences are in lexical order.  For example, if two memory
  references in the loop touch the same memory location and one of
  them modifies the memory location, then the first reference to
  touch the location has to be the one that appears earlier lexically
  in the program source code.  This assumes that the right-hand side
  of an assignment statement is "earlier" than the left-hand side.

  The IVDEP directive informs the compiler that the program would
  behave correctly if the statements were executed in certain orders
  other than the sequential execution order, such as executing the
  first statement or block to completion for all iterations, then the
  next statement or block for all iterations, and so forth.  The
  optimizer can use this information, along with whatever else it can
  prove about the dependences, to choose other execution orders.

  Examples:

  In the following example, the IVDEP directive provides more
  information about the dependences within the loop, which may enable
  loop transformations to occur:

  !DEC$ IVDEP
        DO I=1, N
           A(INDARR(I)) = A(INDARR(I)) + B(I)
        END DO

  In this case, the scalar execution order follows:

  1.  Retrieve INDARR(I).

  2.  Use the result from step 1 to retrieve A(INDARR(I)).

  3.  Retrieve B(I).

  4.  Add the results from steps 2 and 3.

  5.  Store the results from step 4 into the location indicated by
      A(INDARR(I)) from step 1.

  IVDEP directs the compiler to initially assume that when steps 1
  and 5 access a common memory location, step 1 always accesses the
  location first because step 1 occurs earlier in the execution
  sequence.  This approach lets the compiler reorder instructions, as
  long as it chooses an instruction schedule that maintains the
  relative order of the array references.