High Level Synthesis

1. Definition: What is High Level Synthesis?

High Level Synthesis (HLS) is a critical process in the realm of Digital Circuit Design that transforms high-level programming languages, such as C, C++, or SystemC, into Register Transfer Level (RTL) representations. This transformation is pivotal in the design of Very Large Scale Integration (VLSI) systems, where the complexity and scale of designs necessitate efficient methodologies to expedite the design process while maintaining performance and power efficiency. HLS bridges the gap between algorithmic design and hardware implementation, enabling designers to focus on functionality rather than low-level hardware details.

The importance of HLS arises from its ability to automate the generation of hardware descriptions, which significantly reduces design time and enhances productivity. Traditional design methodologies often require extensive manual coding in hardware description languages (HDLs) like VHDL or Verilog, which can be time-consuming and error-prone. HLS allows designers to specify their algorithms at a higher level of abstraction, facilitating rapid prototyping and iterative design processes. This capability is particularly beneficial in industries where time-to-market is critical, such as telecommunications, consumer electronics, and automotive systems.

Technically, HLS encompasses various features, including scheduling, allocation, and binding. Scheduling determines the order of operations, allocation assigns resources (like functional units), and binding maps high-level constructs to hardware components. These features collectively contribute to optimizing the design for performance metrics such as timing, area, and power consumption. By employing HLS, designers can explore a broader design space and make informed trade-offs early in the design cycle, ultimately leading to more efficient and effective hardware implementations.

2. Components and Operating Principles

High Level Synthesis comprises several interrelated components and stages, each playing a crucial role in converting high-level specifications into a functional hardware design. The primary components of HLS include the front-end analysis, scheduling, resource allocation, and code generation.

2.1 Front-End Analysis

The front-end analysis is the initial phase of HLS, where the high-level code is parsed and analyzed. This stage involves syntax checking, semantic analysis, and the generation of an intermediate representation (IR) of the code. The IR serves as a bridge between the high-level language and the RTL description, encapsulating the program’s behavior and structure while abstracting away low-level details. This representation allows for further optimization and transformation during the synthesis process.

2.2 Scheduling

Once the front-end analysis is complete, the scheduling phase begins. Scheduling is the process of determining the order and timing of operations in the design. It involves creating a schedule that meets the timing constraints of the target hardware while optimizing for resource utilization. The scheduler must consider the dependencies between operations, ensuring that data hazards are resolved and that the design adheres to the specified clock frequency. Advanced scheduling techniques, such as list scheduling and modulo scheduling, can be employed to enhance performance and efficiency.

2.3 Resource Allocation

Resource allocation is the next critical component of HLS, where the synthesized operations are mapped to specific hardware resources. This stage involves assigning functional units (such as adders, multipliers, and memory) to operations based on the scheduling results. The allocation process must balance the trade-offs between area, power, and performance, often requiring sophisticated algorithms to optimize resource usage effectively. The objective is to minimize the overall chip area while ensuring that performance requirements are met.

2.4 Binding

The final stage of HLS is binding, where the high-level constructs are mapped to specific hardware components in the RTL description. This process includes generating the necessary control signals and ensuring that the data paths are correctly established. Binding is essential for producing a functional RTL output that can be further processed by traditional synthesis tools to create a gate-level netlist.

The interactions between these components are crucial for the successful implementation of HLS. Each stage feeds into the next, with decisions made in earlier phases influencing the outcomes in later stages. The iterative nature of HLS allows for refinements and optimizations, enabling designers to explore various design alternatives and achieve the desired performance metrics.

High Level Synthesis is often compared to other methodologies and technologies in the realm of hardware design, particularly traditional RTL design and high-level programming approaches.

3.1 Comparison with Traditional RTL Design

Traditional RTL design involves writing detailed hardware descriptions using HDLs like VHDL or Verilog. Unlike HLS, which abstracts away many low-level details, traditional methods require designers to specify every aspect of the hardware explicitly. This approach can lead to longer design cycles and increased susceptibility to errors, as designers must manage intricate timing and resource constraints manually. In contrast, HLS streamlines this process by allowing designers to focus on algorithmic behavior, resulting in faster iterations and reduced design time.

3.2 Comparison with High-Level Programming

Another related technology is high-level programming, where software engineers write algorithms without consideration for the underlying hardware. While high-level programming languages provide excellent abstractions for software development, they lack the capability to generate optimized hardware implementations. HLS serves as a bridge, enabling software-like descriptions to be transformed into efficient hardware designs. This capability is particularly advantageous in the context of application-specific integrated circuits (ASICs) and field-programmable gate arrays (FPGAs), where performance and resource utilization are paramount.

3.3 Advantages and Disadvantages

The primary advantages of HLS include reduced design time, improved productivity, and the ability to explore design alternatives rapidly. However, there are also disadvantages, such as potential loss of fine-grained control over the hardware implementation and the need for designers to have a solid understanding of both software and hardware principles. Additionally, the quality of the generated RTL can vary significantly depending on the HLS tool used and the complexity of the original high-level code.

Real-world examples of HLS applications can be found in various industries, including telecommunications, where HLS is used to design complex signal processing algorithms, and automotive systems, where it aids in the development of safety-critical applications. Companies like Synopsys, Cadence, and Mentor Graphics offer HLS tools that facilitate these processes, demonstrating the growing importance of HLS in modern hardware design workflows.

4. References

Synopsys Inc.
Cadence Design Systems
Mentor Graphics (Siemens EDA)
IEEE Computer Society
ACM Special Interest Group on Design Automation (SIGDA)

5. One-line Summary

High Level Synthesis is a transformative process that converts high-level programming languages into hardware descriptions, streamlining the design of complex VLSI systems while optimizing for performance and resource utilization.