Distributed Array Sections

In Chapter 1 we introduced the HPJava syntax for forming sections of multiarrays. This syntax carries over with very few changes to distributed arrays.

We have seen that the subscripts in a distributed array element reference are generally either index symbols or (restrictedly) integer expressions. Options for subscripts in distributed array section expressions are wider. As for multiarrays, a section subscript is allowed be a triplet. The section may also have some scalar subscripts, similar to those appearing in element references.

This fragment includes two examples of distributed array section expressions:

$$\begin{minipage}[t]{\linewidth}\small\begin{verbatim}Proc... ... : N / 2 - 1, 0 : N - 1 : 2]]) ; }\end{verbatim}\end{minipage}$$

The first array section expression appears on the right hand side of the definition of b. It specifies b as an alias for the first row of a (Figure 4.1). In an array section expression, unlike in an array element reference, a scalar subscript is always allowed to be an integer expression^4.1. The second array section expression appears as an argument to the method foo. It represents a two-dimensional, $N/2$ by $N/2$, subset of the elements of a, visualized in Figure 4.2.

Figure 4.1: A one-dimensional section of a two-dimensional array (shaded area).

Figure 4.2: A two-dimensional section of a two-dimensional array (shaded area).

Distributed array sections allow us to implement a number of interesting applications. They are often passed as arguments to library functions like remap, implementing various interesting patterns of communication and arithmetic on subarrays.