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.[1][2] 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.[3][2] 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.

Fusion

Example in C

int i, a[100], b[100];
for (i = 0; i < 100; i++)
    a[i] = 1;                     
for (i = 0; i < 100; i++)
    b[i] = 2;

is equivalent to:

int i, a[100], b[100];
for (i = 0; i < 100; i++)
{
    a[i] = 1; 
    b[i] = 2;
}

See also

• Expression templates
• Loop transformation
• Nuclear computation

References

1. Y.N. Srikant; Priti Shankar (3 October 2018). The Compiler Design Handbook: Optimizations and Machine Code Generation, Second Edition. CRC Press. ISBN 978-1-4200-4383-9.
2. Kennedy, Ken & Allen, Randy. (2001). Optimizing Compilers for Modern Architectures: A Dependence-based Approach. Morgan Kaufmann. ISBN 1-55860-286-0.
3. Steven Muchnick; Muchnick and Associates (15 August 1997). Advanced Compiler Design Implementation. Morgan Kaufmann. ISBN 978-1-55860-320-2. loop fusion.

• Loop fission