Understanding HLS and its Tools

Oct 13, 2025 by thesweetdesigns.com 58 views

Introduction

High-Level Synthesis (HLS) is a powerful tool for designing digital circuits using high-level programming languages like C++. However, when working with HLS, developers often encounter issues with non-specified input, which can be frustrating and time-consuming to resolve. In this article, we will delve into the reasons behind HLS C++ code generating non-specified input and provide practical solutions to overcome these challenges.

Understanding HLS and its Tools

HLS is a technology that allows developers to design digital circuits using high-level programming languages like C++. The process involves translating the C++ code into a netlist, which is then synthesized into a digital circuit. Xilinx, a leading provider of FPGA and SoC solutions, offers a range of HLS tools, including Vivado HLS and Vitis.

Vivado HLS vs. Vitis: What's the Difference?

Vivado HLS and Vitis are two different HLS tools offered by Xilinx. Vivado HLS is an older tool that was widely used in the past, while Vitis is the latest HLS tool, which was introduced in 2020. Vitis is a more modern and powerful tool that offers improved performance, better support for C++17, and enhanced debugging capabilities.

Why HLS C++ Code Generates Non-Specified Input

There are several reasons why HLS C++ code may generate non-specified input. Some of the common reasons include:

Lack of pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. Without pragmas, the tool may not be able to generate the desired output.
Incorrect data types: Using incorrect data types can lead to non-specified input. For example, using a 32-bit integer instead of a 16-bit integer can result in non-specified input.
Inadequate loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input.
Insufficient memory allocation: Insufficient memory allocation can lead to non-specified input. This is because the HLS tool may not be able to allocate enough memory to store the data.

Practical Solutions to Overcome Non-Specified Input Issues

To overcome non-specified input issues, developers can use the following practical solutions:

Use pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. By using pragmas, developers can specify the behavior of the tool and avoid non-specified input.
Use correct data types: Using correct data types is essential to avoid non-specified input. Developers should ensure that they use the correct data types for their variables.
Optimize loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input. Developers should optimize loop unrolling to avoid non-specified input.
Allocate sufficient memory: Insufficient memory allocation can lead to non-specified input. Developers should allocate sufficient memory to store the data.

Best Practices for HLS C++ Code Development

To avoid non-specified input issues, developers should follow the best practices outlined below:

Use modern HLS tools: Developers should use modern HLS tools like Vitis, which offer improved performance and better support for C++17.
Use pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. Developers should use pragmas to specify the behavior of the tool.
Use correct data types: Using correct data types is essential to avoid non-specified input. Developers should ensure that they use the correct data types for their variables.
Optimize loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input. Developers should optimize loop unrolling to avoid non-specified input.
Allocate sufficient memory: Insufficient memory allocation can lead to non-specified input. Developers should allocate sufficient memory to store the data.

Conclusion

In conclusion, non-specified input is a common issue that developers encounter when working with HLS C++ code. However, by understanding the reasons behind this issue and following the best practices outlined in this article, developers can overcome non-specified input issues and create high-quality HLS C++ code.

Common Issues and Solutions

Issue 1: Lack of Pragmas

Symptoms: Non-specified input, incorrect behavior of the HLS tool.
Solution: Use pragmas to specify the behavior of the HLS tool.

Issue 2: Incorrect Data Types

Symptoms: Non-specified input, incorrect behavior of the HLS tool.
Solution: Use correct data types for variables.

Issue 3: Inadequate Loop Unrolling

Symptoms: Non-specified input, incorrect behavior of the HLS tool.
Solution: Optimize loop unrolling to avoid non-specified input.

Issue 4: Insufficient Memory Allocation

Symptoms: Non-specified input, incorrect behavior of the HLS tool.
Solution: Allocate sufficient memory to store the data.

Frequently Asked Questions

Q: What is HLS C++ code?

A: HLS C++ code is a high-level programming language used for designing digital circuits using High-Level Synthesis (HLS) tools.

Q: Why does HLS C++ code generate non-specified input?

A: HLS C++ code generates non-specified input due to various reasons, including lack of pragmas, incorrect data types, inadequate loop unrolling, and insufficient memory allocation.

Q: How can I overcome non-specified input issues?

A: To overcome non-specified input issues, developers can use pragmas, correct data types, optimize loop unrolling, and allocate sufficient memory.

Q: What are the best practices for HLS C++ code development?

A: The best practices for HLS C++ code development include using modern HLS tools, using pragmas, using correct data types, optimizing loop unrolling, and allocating sufficient memory.

Conclusion

Q: What is HLS C++ code?

A: HLS C++ code is a high-level programming language used for designing digital circuits using High-Level Synthesis (HLS) tools. It is a C++-based language that allows developers to describe digital circuits using high-level abstractions, which are then synthesized into a netlist that can be implemented on a digital circuit.

Q: Why does HLS C++ code generate non-specified input?

A: HLS C++ code generates non-specified input due to various reasons, including:

Lack of pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. Without pragmas, the tool may not be able to generate the desired output.
Incorrect data types: Using incorrect data types can lead to non-specified input. For example, using a 32-bit integer instead of a 16-bit integer can result in non-specified input.
Inadequate loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input.
Insufficient memory allocation: Insufficient memory allocation can lead to non-specified input. This is because the HLS tool may not be able to allocate enough memory to store the data.

Q: How can I overcome non-specified input issues?

A: To overcome non-specified input issues, developers can use the following techniques:

Use pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. By using pragmas, developers can specify the behavior of the tool and avoid non-specified input.
Use correct data types: Using correct data types is essential to avoid non-specified input. Developers should ensure that they use the correct data types for their variables.
Optimize loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input. Developers should optimize loop unrolling to avoid non-specified input.
Allocate sufficient memory: Insufficient memory allocation can lead to non-specified input. Developers should allocate sufficient memory to store the data.

Q: What are the best practices for HLS C++ code development?

A: The best practices for HLS C++ code development include:

Use modern HLS tools: Developers should use modern HLS tools like Vitis, which offer improved performance and better support for C++17.
Use pragmas: Pragmas are directives that are used to specify the behavior of the HLS tool. Developers should use pragmas to specify the behavior of the tool.
Use correct data types: Using correct data types is essential to avoid non-specified input. Developers should ensure that they use the correct data types for their variables.
Optimize loop unrolling: Loop unrolling is a technique used to improve the performance of loops. However, if not done correctly, it can lead to non-specified input. Developers should optimize loop unrolling to avoid non-specified input.
Allocate sufficient memory: Insufficient memory allocation can lead to non-specified input. Developers should allocate sufficient memory to store the data.

Q: What are some common issues that can lead to non-specified input?

A: Some common issues that can lead to non-specified input include:

Lack of pragmas
Incorrect data types
Inadequate loop unrolling
Insufficient memory allocation

Q: How can I debug my HLS C++ code?

A: To debug your HLS C++ code, you can use the following techniques:

Use print statements: Print statements can be used to print the values of variables and debug the code.
Use debuggers: Debuggers can be used to step through the code and identify the source of the issue.
Use log files: Log files can be used to track the execution of the code and identify the source of the issue.

Q: What are some best practices for writing HLS C++ code?

A: Some best practices for writing HLS C++ code include:

Use clear and concise variable names
Use comments to explain the code
Use functions to organize the code
Use loops to reduce code duplication
Use pragmas to specify the behavior of the HLS tool

Q: How can I optimize my HLS C++ code for performance?

A: To optimize your HLS C++ code for performance, you can use the following techniques:

Use loop unrolling: Loop unrolling can be used to improve the performance of loops.
Use parallelization: Parallelization can be used to improve the performance of parallelizable code.
Use pipelining: Pipelining can be used to improve the performance of pipelizable code.
Use pragmas: Pragmas can be used to specify the behavior of the HLS tool and optimize the code for performance.