Wire sorts: a language abstraction for safe hardware composition

Effective digital hardware design fundamentally requires decomposing a design into a set of interconnected modules, each a distinct unit of computation and state. However, naively connecting hardware modules leads to real-world pathological cases which are surprisingly far from obvious when looking at the interfaces alone and which are very difficult to debug after synthesis. We show for the first time that it is possible to soundly abstract even complex combinational dependencies of arbitrary hardware modules through the assignment of IO ports to one of four new sorts which we call: to-sync, to-port, from-sync, and from-port. This new taxonomy, and the reasoning it enables, facilitates modularity by escalating problematic aspects of module input/output interaction to the language-level interface specification. We formalize and prove the soundness of our new wire sorts, implement them in a practical hardware description language, and demonstrate they can be applied and even inferred automatically at scale. Through an examination of the Base-Jump STL, the OpenPiton manycore research platform, and a complete RISC-V implementation, we find that even on our biggest design containing 1.5 million primitive gates, analysis takes less than 31 seconds; that across 172 unique modules analyzed, the inferred sorts are widely distributed across our taxonomy; and that by using wire sorts, our tool is 2.6ś33.9x faster at finding loops than standard synthesis-time cycle detection.