Some scalar dependencies are much harder to resolve and often require changes to the source code. A scalar data dependency could look like the following:
while (a != b) {
if (a > b) a -= b;
else b -= a;
}The next iteration of this loop cannot start until the current iteration
has calculated the updated the values of a and b, as shown in the following figure.
Figure 1. Scalar Dependency
If the result of the previous loop iteration must be available before the current iteration can begin, loop pipelining is not possible. If Vitis HLS cannot pipeline with the specified initiation interval, it increases the initiation internal. If it cannot pipeline at all, as shown by the above example, it halts pipelining and proceeds to output a non-pipelined design.