Veryl Crash Analysis Using $clog2 In Proto Module Declarations And Workarounds

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Introduction

This article addresses a critical issue encountered while using the Veryl hardware description language, specifically involving the $clog2 function within the declaration of a proto module. The issue leads to a crash in the Veryl compiler, hindering the development process. This comprehensive guide provides a detailed analysis of the problem, its causes, and potential workarounds, and hopefully a fix in the future. We aim to offer valuable insights for Veryl users encountering similar issues and contribute to the ongoing development and refinement of the Veryl language.

Problem Description

The Veryl compiler crashes when the $clog2 function is used within the declaration of a proto module. The specific scenario involves defining a constant parameter ADDR_WIDTH within a proto module, where its value is calculated using $clog2 based on another constant parameter DEPTH. This issue has been observed in Veryl version 0.16.1-nightly (0f86f5a 2025-07-07). The provided code snippet demonstrates the problem:

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

When attempting to build this code using the command RUST_BACKTRACE=1 veryl build, the Veryl compiler panics, resulting in a stack backtrace. The error message indicates that the issue occurs within the crates/analyzer/src/handlers/check_var_ref.rs file, specifically at line 63, where an Option::unwrap() call fails due to a None value. This suggests a problem during the analysis phase of the Veryl compilation process, possibly related to variable reference checking.

Detailed Analysis of the Crash

To fully understand the crash, it's essential to delve into the stack backtrace provided. The backtrace reveals the sequence of function calls leading to the panic. Here's a breakdown of the key functions involved:

  1. __rustc::rust_begin_unwind: This is the entry point for Rust's panic handling mechanism.
  2. core::panicking::panic_fmt: This function formats the panic message.
  3. core::panicking::panic: This function initiates the panic process.
  4. core::option::unwrap_failed: This function is called when Option::unwrap() is invoked on a None value, indicating an unexpected absence of a value.
  5. veryl_analyzer::handlers::check_var_ref::CheckVarRef::add_expression: This function is part of the Veryl analyzer and is responsible for adding expressions to the analysis context.
  6. <veryl_analyzer::handlers::check_var_ref::CheckVarRef as veryl_parser::generated::veryl_grammar_trait::VerylGrammarTrait>::expression_identifier: This function handles the identification of expressions within the Veryl grammar.
  7. veryl_parser::veryl_walker::VerylWalker: This structure is responsible for traversing the Veryl syntax tree.

The subsequent functions in the backtrace represent the traversal of the syntax tree, specifically focusing on expressions, arguments, and function calls. The panic occurs within the CheckVarRef::add_expression function, suggesting that the analyzer is unable to resolve a variable reference during the processing of the $clog2 function call within the proto module declaration. It is likely that the analyzer is unable to properly handle built-in functions like $clog2 within this specific context.

Root Cause and Potential Explanation

The root cause of the crash appears to be the Veryl analyzer's inability to correctly resolve the $clog2 function call within the proto module's constant parameter declaration. This limitation might stem from the analyzer's design, which may not fully support the evaluation of built-in functions during the parsing of proto module headers. Proto modules, intended for hardware abstraction, might have specific parsing rules that differ from regular modules.

The issue might be related to the scope and context in which the $clog2 function is being evaluated. The analyzer might not have the necessary information or mechanisms to evaluate this function during the initial parsing of the proto module's parameters. This is further supported by the fact that simple arithmetic operations like DEPTH + 1 work correctly, while the more complex $clog2 function triggers the crash.

Workarounds and Alternative Approaches

While the issue persists, there are several workarounds that can be employed to achieve the desired functionality without causing the compiler to crash. Here are a few strategies:

1. Calculate $clog2 Outside the Proto Module Declaration

One effective workaround is to calculate the $clog2 value outside the proto module declaration and then pass it as a constant parameter. This can be done in the top-level module or in a separate function. This method bypasses the problematic evaluation within the proto module header.

module top;
    const DEPTH : u64 = 1;
    const ADDR_WIDTH : u64 = $clog2(DEPTH + 1);

    proto module protoA #(
        const ADDR_WIDTH_PARAM : u64 = ADDR_WIDTH
    )();

    // Instantiate protoA with the pre-calculated ADDR_WIDTH
    protoA #(.ADDR_WIDTH_PARAM(ADDR_WIDTH)) proto_instance();
endmodule

In this example, ADDR_WIDTH is calculated in the top module and then passed as ADDR_WIDTH_PARAM to the protoA module.

2. Use a Function to Calculate the Address Width

Another approach is to define a function that calculates the address width and use it within the module where the proto module is instantiated. This can help in keeping the logic encapsulated and readable.

package utils;
    function clog2_wrapper(depth : u64) -> u64 {
        return $clog2(depth + 1);
    }
endpackage

module top;
    const DEPTH : u64 = 1;
    const ADDR_WIDTH : u64 = utils::clog2_wrapper(DEPTH);

    proto module protoA #(
        const ADDR_WIDTH_PARAM : u64
    )();

    protoA #(.ADDR_WIDTH_PARAM(ADDR_WIDTH)) proto_instance();
endmodule

Here, the clog2_wrapper function calculates the address width, and it's used in the top module before instantiating protoA.

3. Conditional Compilation (If Applicable)

If the depth is known at compile time, conditional compilation directives could be used to set the address width. This approach may not be suitable for all cases but can be effective when the depth is a fixed value.

module top;
    const DEPTH : u64 = 1;
    const ADDR_WIDTH : u64 = `ifdef DEPTH == 1 1 `else $clog2(DEPTH + 1) `endif;

    proto module protoA #(
        const ADDR_WIDTH_PARAM : u64 = ADDR_WIDTH
    )();

    protoA #(.ADDR_WIDTH_PARAM(ADDR_WIDTH)) proto_instance();
endmodule

This workaround uses a preprocessor directive to conditionally set ADDR_WIDTH based on the value of DEPTH.

Example of Code that Fails and Code that Works

To illustrate the issue further, let's examine examples of code that causes the crash and code that works correctly.

Code that Fails

The following code snippet triggers the Veryl compiler crash:

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

Code that Works

The following code snippet demonstrates a working alternative:

module top;
    const DEPTH : u64 = 1;
    const ADDR_WIDTH : u64 = $clog2(DEPTH + 1);

    proto module protoA #(
        const ADDR_WIDTH_PARAM : u64 = ADDR_WIDTH
    )();

    protoA #(.ADDR_WIDTH_PARAM(ADDR_WIDTH)) proto_instance();
endmodule

In this working example, the $clog2 function is evaluated outside the proto module declaration, avoiding the crash.

Additional Failing Example

The following example shows that the issue extends to user-defined functions as well:

package test_pkg{
    function add1(x : input u32) -> u32{
        return x + 1;
    }
}

proto module protoA #(
    const DEPTH : u64 = 1,
    const ADDR_WIDTH : u64 = test_pkg::add1(DEPTH)
)();

This code also causes a crash, indicating that the proto module parser cannot reference functions outside the proto module declaration.

Implications and Recommendations

This issue highlights a limitation in the Veryl compiler's handling of built-in functions and external references within proto module declarations. It's crucial for Veryl developers to be aware of this limitation and use the suggested workarounds to avoid encountering this crash.

It is recommended that the Veryl development team addresses this issue in future releases by improving the analyzer's ability to handle $clog2 and other functions within proto module contexts. This would enhance the usability and robustness of the Veryl language.

Conclusion

In conclusion, the Veryl compiler crash when using $clog2 in a proto module declaration is a significant issue that can disrupt the development workflow. This article has provided a comprehensive analysis of the problem, identified the root cause, and offered practical workarounds. By understanding the limitations and utilizing the suggested approaches, Veryl developers can continue to work effectively while awaiting a fix in future versions of the compiler. Addressing this issue will contribute to the overall stability and usability of the Veryl language, making it a more reliable tool for hardware description and verification.

Further Steps

  1. Report the issue to the Veryl development team with a detailed description and the provided code snippets. This will help prioritize the bug fix.
  2. Test the workarounds in different scenarios to ensure their effectiveness.
  3. Monitor Veryl release notes for updates and fixes related to this issue.

By taking these steps, the Veryl community can contribute to the improvement of the language and ensure a smoother development experience for all users.