Independent DO Loops - Conditions

There are a number of conditions imposed on INDEPENDENT which ensure that it really does not matter which order the iterations are performed in.

If an INDEPENDENT loop

• assigns to same element twice parallel integrity would be lost,
• contains EXIT, STOP or PAUSE then the iterations must progress sequentially so as to ensure that the loop is terminated on the correct iteration,
• contains jumps out of loop then, again, the iterations must progress sequentially so as to ensure that the loop is terminated on the correct iteration,
• performs external I/O then the iterations must progress sequentially so as to ensure that the file is read from or written to in the correct order,
• prefixes statements other than DO or FORALL then it has no meaning and will cause an error.

(Hint: To decide whether a Fortran 90 or FORTRAN 77 loop is INDEPENDENT reverse the upper and lower bounds and negate the stride. If the loop still produces exactly the same effect then it must be INDEPENDENT.)

• Independent Example 1
• Independent Example 2