PWR034: avoid strided array access to improve performance

This recommendation is part of the open catalog of best practice rules for performance that is automatically detected and reported by Codee.

Issue

Strided array access may impact performance.

Relevance

Accessing an array using a non-unit stride is less efficient than accessing consecutive positions because the latter improves the  locality of reference. Note that C/C++ column-wise matrix access is an example non-unit stride, where the stride is the column width.

Actions

Consider using techniques like loop fusion, loop interchange, loop tiling or changing the data layout to avoid non-sequential access in hot loops.

Code example

The following code accesses both variables a and b with a stride LEN.

        for (int i = 0; i < LEN; ++i) {
            for (int j = 1; j < LEN; j++) {
                a[j * LEN + i] = a[j * LEN + i - 1] + b[j * LEN + i];
            }
        }

By doing loop interchange of loops over a and b, we get rid of the strided accesses with regard to the innermost loop.

 for (int j = 1; j < LEN; ++j) {
            for (int i = 0; i < LEN; i++) {
                a[j * LEN + i] = a[j * LEN + i - 1] + b[j * LEN + i];
            }
        }