Latent modification instruction for substituting functionality of instructions during transactional execution

An instruction stream includes a transactional code region. The transactional code region includes a latent modification instruction (LMI), a next sequential instruction (NSI) following the LMI, and a set of target instructions following the NSI in program order. Each target instruction has an associated function, and the LMI at least partially specifies a substitute function for the associated function. A processor executes the LMI, the NSI, and at least one of the target instructions, employing the substitute function at least partially specified by the LMI. The LMI, the NSI, and the target instructions may be executed by the processor in sequential program order or out of order.