Why this message print??
WARNING: 0:28: Calls to any function that may require a gradient calculation inside a conditional block may return undefined results
this warning is given when you are using functions inside a conditional block which require gradient information.
Gradient information is based on a 2x2 pixel block processing:
if any of those pixels in a 2x2 block are not taking the same branch as the other pixels the resulting data,
which is used for gradient calculation, might be undefined and thus the resulting gradient invalid.