Parameterized Hardware Design with Latency-Abstract Interfaces
Hardware designs must use latency-insensitive (LI) interfaces when timing is input-dependent. When timing is input-independent, designs should use latency-sensitive (LS) interfaces for maximum performance. However, designs commonly use LI interfaces to integrate with externally generated LS modules--from, e.g., IP generators, high-level synthesis, or domain specific languages. In every fully integrated design, such uses of LI represent pure overhead. The challenge is that generators can dramatically change timing interfaces of the modules to meet performance objectives, and LI interfaces act as a useful design abstraction and enable timing adaptation. We define latency-abstract (LA) interfaces, a new design abstraction, which provide the timing adaptability of LI interfaces at design-time and the efficient integration of LS interfaces. LA interfaces use output parameters, a novel compile-time mechanism for child modules to return values parent modules, to abstract and encapsulate timing behaviors at design time. During design elaboration, LA interfaces are compiled into efficient LS interfaces based on parameter values. While an attractive option, LA interfaces inherit the complexities of parameterized hardware design: the user must reason how parameters influence timing behaviors of modules and ensure that designs adapt to interface changes. To address this challenge and demonstrate the utility of LA interfaces, we design Lilac, a parameterized HDL that uses a type system track the influence of parameters on timing behaviors and formally guarantee that every parameterization of an LA design results in a circuit without structural hazards.