Integrated optical signal processors, in combination with conventional electrical signal processors, are envisioned to open a path to a new generation of signal processing hardware platform that allows for significant improvement in processing bandwidth, latency, and power efficiency. With its well-known features and potential, silicon photonics is considered as a favorable candidate for the device implementation, particularly with high circuit complexity, and hence has been the focus of the study. As an outlook from the previous discussions on such processors, we are considering new building blocks in the silicon photonics platform for further extending the processor capabilities and adding practical features, particularly the miniaturized devices that enable ultra-dense integration of complex circuits into such processor chips. As enlightening examples, we review here our recent contribution together with representative works from other groups of compact designs of silicon photonics devices that enrich functionalities of processor building blocks such as multiplexing, polarization handling, and optical I/Os. The results shown in this review reflect the significance and maturity of the state-of-the-art photonic fabrication technology and contribute to the implementation of high-capacity, general-purpose optical signal processing functionalities on the chip scale.