Loop fission and fusion

In computer science, loop fission (or loop distribution) is a compiler optimization in which a loop is broken into multiple loops over the same index range with each taking only a part of the original loop's body. The goal is to break down a large loop body into smaller ones to achieve better utilization of locality of reference. This optimization is most efficient in multi-core processors that can split a task into multiple tasks for each processor.
Conversely, loop fusion (or loop jamming) is a compiler optimization and loop transformation which replaces multiple loops with a single one. It is possible when two loops iterate over the same range and do not reference each other's data. Loop fusion does not always improve run-time speed. On some architectures, two loops may actually perform better than one loop because, for example, there is increased data locality within each loop.
Fission.
Example in C.
int i, a[100], b[100];
for (i = 0; i < 100; i++) {
 a[i] = 1;
 b[i] = 2;
is equivalent to:
int i, a[100], b[100];
for (i = 0; i < 100; i++) {
 a[i] = 1;
for (i = 0; i < 100; i++) {
 b[i] = 2;