A tool is provided for modifying the code of a multi-threaded computer program undergoing testing. The program executes in an environment that has a governing memory model. It is assumed that there is a global heap and a thread-local heap, which are synchronized from time to time. The modifications are of two types: (1) code instructions are reordered while remaining in compliance with the memory model; and (2) thread-local variables are added to functions, together with inserted heap synchronizing instructions. The modified programs are then used by a test generator to prepare test cases for execution. The modifications have the effect of changing the interleavings that occur among different threads, and increase the likelihood of exposing flaws that may become evident under different memory models.