Veryl Crash With $clog2 In Proto Module Declaration Explained

July 13, 2025 by StackCamp Team 62 views

Veryl Crash When Using $clog2 in Proto Module Declaration

Introduction

This article addresses a critical issue encountered while using the Veryl hardware description language, specifically when employing the $clog2 function within a proto module declaration. The problem manifests as a crash in the Veryl compiler, hindering the compilation process and potentially impacting project development timelines. This article aims to provide a comprehensive understanding of the issue, its context, the root cause, and potential workarounds, thereby assisting Veryl users in navigating this challenge. Understanding the nuances of hardware description languages like Veryl is crucial for designing efficient and reliable digital systems. This issue highlights the importance of rigorous testing and debugging in the development of hardware compilers and tools. By delving into the specifics of this crash, we can gain insights into the inner workings of the Veryl compiler and contribute to its ongoing improvement.

Problem Description

The core issue arises when the $clog2 function, intended to calculate the base-2 logarithm of a value, is used within the declaration of a proto module. The Veryl compiler, version 0.16.1-nightly (0f86f5a 2025-07-07), encounters a panic during the build process, leading to a compilation failure. The provided code snippet demonstrates this issue:

proto module protoA #(
    const DEPTH : u64 = 1,
    const ADDR_WIDTH : u64 = $clog2(DEPTH+1)
)();

When this code is compiled using the command RUST_BACKTRACE=1 veryl build, the Veryl compiler crashes. The error message and stack trace, which will be detailed in the next section, reveal that the crash occurs during the analysis phase, specifically within the check_var_ref.rs module. This suggests that the compiler is unable to properly resolve or handle the $clog2 function call within the context of the proto module declaration. The use of $clog2 is common in hardware design to determine the number of bits required to address a memory of a certain depth, making this issue a significant impediment to using Veryl for certain types of hardware designs. The inability to use $clog2 directly within the proto module declaration forces developers to find alternative ways to calculate the address width, potentially leading to more complex or less readable code.

Error Message and Stack Trace

The error message generated by the Veryl compiler provides valuable clues about the nature and location of the crash. The key part of the message is:

thread 'main' panicked at crates/analyzer/src/handlers/check_var_ref.rs:63:51:
called `Option::unwrap()` on a `None` value

This indicates that the compiler encountered an unexpected None value where it was expecting a valid value. The unwrap() function in Rust is used to extract the value from an Option type, but it will panic if the Option is None. This suggests that a variable or expression that the compiler was trying to access or evaluate was not properly initialized or resolved.

The stack trace provides a more detailed view of the call sequence leading to the panic. It shows that the crash originates in the check_var_ref.rs module, specifically in the CheckVarRef::add_expression function. The stack trace further reveals that the $clog2 function call is processed through a series of parser and walker functions (VerylWalker::expression_identifier, VerylWalker::function_call, etc.) before reaching the point of failure. The presence of these functions in the stack trace indicates that the compiler is attempting to parse and analyze the expression containing $clog2 when the crash occurs. The stack trace is essential for developers working on the Veryl compiler itself, as it allows them to pinpoint the exact location in the code where the error occurs and to understand the sequence of events that led to the crash. For users of Veryl, the stack trace provides a valuable tool for reporting bugs and for communicating the specific circumstances under which the crash occurs.

Root Cause Analysis

The underlying cause of the crash appears to be the Veryl compiler's inability to properly handle the $clog2 function within the context of a proto module declaration. Specifically, the compiler seems to fail to resolve the function call or its result during the analysis phase when processing the module's generic parameters. The stack trace points to an issue within the check_var_ref.rs module, suggesting that the variable reference check mechanism is not correctly handling the $clog2 function. This might be due to the timing of the evaluation or the scope in which the function is being called. The compiler might be attempting to evaluate the $clog2 function before the DEPTH constant is fully resolved, or there might be an issue with how the compiler handles built-in functions like $clog2 within generic parameter declarations. The provided examples further illustrate this issue. The first example, which uses DEPTH+1 instead of $clog2(DEPTH+1), compiles successfully, suggesting that the problem is specific to the $clog2 function. The second example, which attempts to use a user-defined function test_pkg::add1, also fails, indicating a broader issue with referencing functions or expressions outside the immediate scope of the proto module declaration. This suggests that the compiler might have limitations on the types of expressions or functions that can be used within proto module generic parameters.

Workarounds and Solutions

While the issue is a bug in the Veryl compiler that needs to be addressed by the developers, there are several workarounds that users can employ to mitigate the problem in the short term. One approach is to calculate the ADDR_WIDTH outside of the proto module declaration and then pass it as a generic parameter. This can be achieved by defining a constant or using a function to calculate the value before instantiating the proto module. For instance:

const ADDR_WIDTH_CALC : u64 = $clog2(DEPTH + 1);

proto module protoA #(
    const DEPTH : u64 = 1,
    const ADDR_WIDTH : u64 = ADDR_WIDTH_CALC
)();

This approach moves the $clog2 calculation to a context where the compiler can properly handle it. Another workaround is to use a different method to calculate the logarithm, although this might be less efficient or readable. For example, a loop-based approach could be used to determine the number of bits required to represent the depth. However, this approach is generally not recommended as it can lead to more complex code and potentially introduce errors. The ideal solution is for the Veryl developers to fix the bug in the compiler. This will likely involve modifying the variable reference checking mechanism or the handling of built-in functions within generic parameter declarations. Users are encouraged to report the issue to the Veryl development team, providing the code snippet and error message to help them reproduce and fix the bug. In the meantime, the workarounds described above can help users continue to develop their hardware designs using Veryl.

Impact and Implications

The crash caused by using $clog2 in a proto module declaration has significant implications for Veryl users. It prevents the direct calculation of address widths based on depth, which is a common requirement in hardware design, especially when dealing with memories and buffers. This limitation can force developers to adopt workarounds, leading to less readable, more complex, and potentially less efficient code. The inability to use $clog2 directly in proto module declarations can also hinder the adoption of Veryl in projects where such calculations are frequent. The crash itself can be frustrating for users, as it interrupts the compilation process and requires them to debug the issue. The error message and stack trace, while helpful, might not be immediately clear to all users, especially those who are new to Veryl or hardware description languages in general. Furthermore, the bug highlights the importance of thorough testing in compiler development. Issues like this can slip through the cracks if the compiler is not tested with a wide range of code constructs and use cases. The impact of this bug extends beyond the immediate inconvenience it causes. It can also affect the overall perception of Veryl as a reliable hardware description language. If users encounter such issues frequently, they might be less likely to adopt Veryl for their projects. Therefore, addressing this bug and ensuring the stability of the compiler is crucial for the long-term success of Veryl.

Conclusion

The crash encountered when using $clog2 in a Veryl proto module declaration is a significant issue that can hinder hardware design workflows. The root cause appears to be related to the compiler's handling of built-in functions within generic parameter declarations. While workarounds exist, the ideal solution is a fix from the Veryl development team. Reporting the issue and providing detailed information, such as the code snippet and error message, can greatly assist in the bug-fixing process. This issue underscores the importance of robust testing and debugging in compiler development and the need for clear and informative error messages to aid users in troubleshooting. By understanding the problem, its context, and potential solutions, Veryl users can navigate this challenge and continue to leverage the language for their hardware design projects. The Veryl community can play a vital role in identifying and reporting such issues, contributing to the ongoing improvement and stability of the language. As Veryl continues to evolve, addressing these kinds of bugs and enhancing the overall user experience will be crucial for its widespread adoption and success in the hardware design community.