A program transformation where the bodies of two loops are
merged into one thus reducing the overhead of manipulating and
testing the control variable and branching. Further
optimisation of the merged code may then become possible.
In horizontal loop combination the bodies of the loops are
largely independent so only the loop overhead is saved.
Vertical loop combination applies where the results of the
first loop are used by the second. Combining the two allows
the intermediate results to be used immediately (in
registers) rather than requiring them to be stored in an
array.
The functional equivalent of horizontal and vertical loop
combination are tupling and fusion.
