In this paper, we propose an efficient iterative optimization based approach for architectural synthesis of dark silicon heterogeneous chip multi-processors (CMPs). The goal is to determine the optimal number of cores of each type to provision the CMP with, such that the area and power budgets are met and the application performance is maximized. We consider general-purpose multi-threaded applications with a varying degree of parallelism (DOP) that can be set at run-time, and propose an accurate analytical model to predict the execution time of such applications on heterogeneous CMPs. Our experimental results illustrate that the synthesized heterogeneous dark silicon CMPs provide between 19% to 60% performance improvements over conventional homogeneous designs for variable and fixed DOP scenarios, respectively.