Unit test have fought well-earned respect among programmers long ago. Unit tests will help to check the correctness of the program after the port on a new platform. But there is one nuance which you should keep in mind.
Unit test may not allow you to check the new ranges of input values which become accessible on 64-bit systems. Unit tests are originally developed in such a way that they can be passed in a short time. And the function which usually works with an array with the size of tens of Mb, will probably process tens of Kb in unit tests. It is justified for this function in tests may be called many times with different sets of input values. But suppose you have a 64-bit variant of the program. And now the function we study is processing more than 4 Gb of data. Surely there appears a necessity to raise the input size of an array in the tests either up to sizes more than 4 Gb. The problem is that the time of passing the tests will increase greatly in such a case.
That’s why while modifying the sets of tests keep in mind the compromise between speed of passing unit tests and the fullness of the checks. Fortunately, there are other methods which can help you to make sure of the efficiency of your applications.