Source : Free On-Line Dictionary of Computing

loop combination
     
        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
        {register}s) rather than requiring them to be stored in an
        {array}.
     
        The functional equivalent of horizontal and vertical loop
        combination are {tupling} and {fusion}.