High-level synthesis

High-level synthesis (HLS), sometimes referred to as C synthesis, electronic system-level (ESL) synthesis, algorithmic synthesis, or behavioral synthesis, is an automated design process that interprets an algorithmic description of a desired behavior and creates digital hardware that implements that behavior. Synthesis begins with a high-level specification of the problem, where behavior is generally decoupled from e.g. clock-level timing. Early HLS explored a variety of input specification languages., although recent research and commercial applications generally accept synthesizable subsets of ANSI C/C++/SystemC/Matlab. The code is analyzed, architecturally constrained, and scheduled to create a register-transfer level (RTL) hardware description language (HDL), which is then in turn commonly synthesized to the gate level by the use of a logic synthesis tool. The goal of HLS is to let hardware designers efficiently build and verify hardware, by giving them better control over optimization of their design architecture, and through the nature of allowing the designer to describe the design at a higher level of abstraction while the tool does the RTL implementation. Verification of the RTL is an important part of the process.

Hardware design can be created at a variety of levels of abstraction. The commonly used levels of abstraction are gate level, register-transfer level (RTL), and algorithmic level.

While logic synthesis uses an RTL description of the design, high-level synthesis works at a higher level of abstraction, starting with an algorithmic description in a high-level language such as SystemC and Ansi C/C++. The designer typically develops the module functionality and the interconnect protocol. The high-level synthesis tools handle the micro-architecture and transform untimed or partially timed functional code into fully timed RTL implementations, automatically creating cycle-by-cycle detail for hardware implementation. The (RTL) implementations are then used directly in a conventional logic synthesis flow to create a gate-level implementation.

Early academic work extracted scheduling, allocation, and binding as the basic steps for high-level-synthesis. Scheduling partitions the algorithm in control steps that are used to define the states in the FSM. Each control step contains one small section of the algorithm that can be performed in a single clock cycle in the hardware. Allocation and binding maps the instructions and variables to the hardware components, multiplexers, registers and wires of the data path.

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