The INDEPENDENT HPF Directive

The INDEPENDENT HPF Directive

Definition Back to Top
The INDEPENDENT directive gives the compiler the information that the statements in a particular section of code do not exhibit any sequentializing dependencies. Dangerous if used incorrectly!

It can precede a DO loop or FORALL, asserting that iterations may be executed independently.

The NEW clause in an INDEPENDENT directive may cause scalars or arrays to be "privatized" by the compiler.

Syntax of the INDEPENDENT directive: 

  INDEPENDENT [ , NEW variable(s) ]

General Examples Back to Top
Example 1:
!HPF$ INDEPENDENT
DO i = 1, 100
  a(i) = b(i)
  c(i) = d(i)
END DO
In example 1 it is easy to see that there are no data dependencies and that the loop may be safely parallelized, so it is safe to give the compiler the information that the loop is "INDEPENDENT". However, it is probably also unnecessary in such a simple loop.

Example 2: 

!HPF$ INDEPENDENT
DO i = 100
  a(b(i)) = c(i)
END DO
In example 2, the compiler may "play it safe" and not parallelize this loop if it cannot prove that there are no repeated values in b(i). If the programmer knows this information, it is useful to share that with the compiler via the directive.
Tariff Program Examples Back to Top
INDEPENDENT is used twice in the Tariff Program: 

!HPF$ INDEPENDENT,NEW(k,test,apath)
        do 2225 j = 1, ndemm
          do 2245, k = 1, nsupm
            test = (-supp(k) - thxlin(k,j)) * (1. + tar(k,j)) + demp(j)
            apath = max(0., apatho(k,j) + (rho * test))
	    apathn(k, j) = apath
            demand(j) = demand(j) + apath
2245      continue
2225    continue
Move the program display to this section 
!HPF$ INDEPENDENT,NEW(k,test,apath)
        do 12225 j = 1, ndemm
          do 12230 k = 1, nsupm
            test =(-supp(k) - thxlin(k, j))*(1. + tar(k, j)) + demp(j)
            apath = max( 0., apatho(k, j) + rho * test)
	    apathn(k, j) = apath
            demand(j) = demand(j) + apath
12230     continue
12225   continue
Move the program display to this section

In both loops, the NEW option must be used to indicate that the variables listed should be treated as if they were allocated anew for each iteration, or private within the loop body.

Exercise (long) Back to Top
The lab exercise for this section is shared with the Case Study Part 7 lab. As such it calls out prerequisites particular to the Case Study - you may proceed without reviewing them. The lab will start with a program that does not exactly match the program in this module.

This lab may be run using the Virtual Workshop Companion Web interface, which allows you to work through the lab exercise without opening a telnet session. If you would like to use the VW Companion, click on the preceding link, which will open up a separate browser window from which you will supply your userid and password. The rest of the interface will be launched from there.

Whether you prefer to log in or to use the VW Companion, you'll need to select the instructions for the lab, which will come up in this window.

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