Verilator 5.038: Arithmetic Right Shift Bug!

Hey guys! Let's dive into a fascinating issue discovered in Verilator version 5.038 concerning the arithmetic right shift operator. If you're using Verilator for your Verilog projects, especially those involving signed arithmetic, this is something you'll definitely want to be aware of.

The Problem: Incorrect Arithmetic Right Shift

The core of the issue lies in how Verilator handles the arithmetic right shift (>>>) operator. In Verilog, the arithmetic right shift should preserve the sign of the number by filling the vacated bits with the most significant bit (MSB). This is crucial for maintaining the correct value when dealing with signed numbers. However, in Verilator 5.038, it appears that a bug causes the operator to perform a logical right shift instead, which fills the vacated bits with zeros. This can lead to incorrect results in your simulations and potentially affect the functionality of your designs.

To illustrate this, consider the provided Verilog code snippet:

// 
// 1111_1110_1011_1110_1101_0001_0010_0011 >>> 20
// 1111_1111_1111_1111_1111_1111_1111_1110_1011
// 

`timescale 1ns/1ps
`define stop   $stop
`define checkd(gotv, expv) \
  do if ((gotv) !== (expv)) begin \
       $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__, `__LINE__, (gotv), (expv)); \
       $write("%%Error: %s:%0d:  got=%32b exp=%32b\n", `__FILE__, `__LINE__, (gotv), (expv)); \
       `stop; \
     end while(0);

module top (out35);
    output wire [32:0] out35;
    wire   signed [32:0] wire_4;
    assign wire_4 = 32'b11111110_10111110_11010001_00100011;
    assign out35  = (wire_4 >>> 20);

    initial begin
        #10;
        `checkd(out35, 32'b11111111_11111111_11111111_11101011);
        $write("*-* All Finished *-*\n");
        $finish;
    end
endmodule

In this code, we have a 32-bit signed wire (wire_4) assigned a negative value. We then perform an arithmetic right shift by 20 bits and assign the result to out35. The expected result, as indicated in the comments, is 32'b11111111_11111111_11111111_11101011, where the vacated bits are filled with ones to preserve the sign. However, Verilator 5.038 produces the incorrect result 00000000000000000000111111101011, indicating a logical right shift.

The root cause of this issue is in the way Verilator 5.038 handles the >>> operator for signed values. Instead of propagating the sign bit, it fills the shifted bits with zeros, effectively treating the operation as an unsigned shift. This discrepancy leads to the incorrect result observed in the test case. The implications of this bug can be significant, especially in designs that heavily rely on signed arithmetic operations. Incorrect shifts can lead to miscalculations, logical errors, and ultimately, the failure of the design to function as intended.

How to Reproduce the Error

To reproduce the error, you can use the provided Verilog code and the following command:

verilator --binary -Wno-lint --timing test.v && ./obj_dir/Vtest

This command first compiles the Verilog code using Verilator with the --binary flag to generate an executable, -Wno-lint to suppress linting warnings, and --timing to enable timing simulation. Then, it executes the compiled simulation. The output clearly shows the error:

- V e r i l a t i o n   R e p o r t: Verilator 5.038 2025-07-08 rev v5.038
- Verilator: Built from 0.026 MB sources in 2 modules, into 0.020 MB in 7 C++ files needing 0.000 MB
- Verilator: Walltime 0.215 s (elab=0.000, cvt=0.003, bld=0.204); cpu 0.010 s on 1 threads; alloced 21.023 MB
%Error: test.v:23:  got=4075 exp=4294967275
%Error: test.v:23:  got=00000000000000000000111111101011 exp=11111111111111111111111111101011
%Error: test.v:23: Verilog $stop
Aborting...
Aborted (core dumped)

As you can see, the simulation produces an error because the actual result (got) does not match the expected result (exp). The binary representation of the results further confirms that Verilator performed a logical right shift instead of an arithmetic right shift.

This consistent and easily reproducible error underscores the importance of verifying arithmetic operations in your designs, especially when using different versions or tools. The provided test case serves as a valuable tool for quickly identifying this specific issue in Verilator 5.038.

The Incorrect Output

The Verilator output clearly demonstrates the issue:

%Error: test.v:23:  got=4075 exp=4294967275
%Error: test.v:23:  got=00000000000000000000111111101011 exp=11111111111111111111111111101011
%Error: test.v:23: Verilog $stop
Aborting...
Aborted (core dumped)

This output shows that Verilator calculated out35 as 4075 (binary 00000000000000000000111111101011), while the expected value is 4294967275 (binary 11111111111111111111111111101011). The discrepancy in the binary representation clearly indicates that Verilator performed a logical right shift (filling with zeros) instead of the correct arithmetic right shift (filling with the sign bit).

Root Cause Analysis and Potential Impact

The arithmetic right shift (>>>) is crucial for signed number operations in Verilog. It ensures that the sign of the number is preserved during the shift, which is essential for accurate calculations. The incorrect behavior in Verilator 5.038 can lead to significant errors in designs that rely on signed arithmetic. Imagine a scenario where you are designing a digital signal processing (DSP) system, and you use arithmetic right shifts for scaling or normalization. If Verilator performs a logical shift instead, the results will be completely wrong, potentially leading to system malfunction.

The issue stems from how Verilator's internal representation and handling of signed numbers interact with the shift operator. It's possible that a specific optimization or transformation within Verilator's code generation process is inadvertently causing the incorrect shift operation. Understanding the root cause requires a deep dive into Verilator's source code and the specific implementation of the >>> operator for signed types.

This bug can have a cascading effect, impacting not just the immediate result of the shift operation but also subsequent calculations that depend on it. Debugging such issues can be challenging, as the source of the error may not be immediately obvious. Therefore, it is imperative to use a reliable version of Verilator and to thoroughly test designs that utilize signed arithmetic.

Similar Issues and Incomplete Fixes

Interestingly, a similar issue seems to have been addressed in a previous bug report (#5994). This suggests that the problem might be a recurring one or that the previous fix was not comprehensive enough to cover all scenarios. It highlights the complexity of ensuring correct behavior for all operators and data types in a hardware description language simulator like Verilator.

The fact that the issue resurfaces even after a previous attempt to fix it underscores the importance of robust testing and regression suites. These suites should include a wide range of test cases that specifically target signed arithmetic operations and boundary conditions. This will help catch similar bugs in future releases and ensure the long-term reliability of Verilator.

System Details

This issue has been observed in the following environment:

• Verilator 5.038 2025-07-08 rev v5.038
• Also tried Verilator 5.041 devel rev v5.040-251-gb4d064d16
• OS: Ubuntu 24.04.2 LTS

It's important to note that the issue persists even in a more recent development version (5.041), indicating that the fix has not yet been implemented in the main branch. This information is crucial for users who might be considering upgrading to the latest version, as they should be aware of this potential bug.

Workarounds and Recommendations

So, what can you do if you're affected by this bug? Here are a few recommendations:

1. Verify your results: If you're using Verilator 5.038 or a similar version, carefully verify the results of any arithmetic right shift operations, especially when dealing with signed numbers. You can use test benches and assertions to check the output against expected values.
2. Use a different version: Consider using a different version of Verilator where this bug is not present. You might try an older version or a more recent development build that includes a fix. However, always test thoroughly when switching versions.
3. Implement a workaround: If you need to stick with Verilator 5.038, you can implement a workaround by manually performing the arithmetic right shift. This might involve checking the sign bit and manually filling the vacated bits with ones or zeros as needed. This can add complexity to your code but can ensure correct behavior.
4. Report the bug: If you encounter this bug, consider reporting it to the Verilator developers. This helps them track and prioritize issues, and it contributes to the overall improvement of the tool. You can report the bug on the Verilator GitHub repository.

Conclusion

The arithmetic right shift bug in Verilator 5.038 is a serious issue that can lead to incorrect simulation results. It's crucial to be aware of this bug and take appropriate measures to mitigate its impact. By verifying your results, using a different version, implementing a workaround, and reporting the bug, you can help ensure the accuracy of your Verilog designs. Stay vigilant, guys, and happy simulating!

For more in-depth information on Verilog and its operators, check out this IEEE Standard for Verilog