# OpenMP nested loop collapse

Version's name: OpenMP nested loop collapse ; a version of the OMP Collapse program.
Implemented best practices: Collapsing OpenMP parallel loops ·

The following pseudo-code shows the proposed best-practice, namely, to use the COLLAPSE clause to fuse the outer loops

!\$OMP PARALLEL DO COLLAPSE(2) DEFAULT(NONE) SHARED(...)
DO K = 1, Nk
DO J = 1, Nj
DO  I = 1, Ni
!! work to do
END DO
END DO
END DO


The COLLAPSE(2) clause allows the parallelization of the perfectly nested first two loops where the compiler forms a single loop with size Nk*Nj and then parallelizes it. In this way the work from the outer two loops is better balanced among the threads.

There are specific conditions to be fulfilled to use the COLLAPSE clause (see, for more details on the conditions, this link for an exhaustive list of rules). The most important ones are:

• The loops must form a rectangular iteration space;
• The loops must be perfectly nested;
• The associated do-loops must be structured blocks. Their execution must not be terminated by an EXIT statement.
The following experiments have been registered: