In the mire of timing constraint

I'm a little concerned about your flow of seeing an issue, adding a timing constraint(to the .sdc, as alteraaditya points out) and then re-compiling. For the most part, all the constraints like multicycles and false paths should be entered up front to reflect how your hardware behaves. It certainly happens where users miss a constraint and have to add it in the back-end, but usually not too much. Are you sure the constraints being added are correct?