Internal Error In Writing Type When AtomicCompareExchangeWeak Is Used In Compute Shader

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Introduction

When working with compute shaders in the WebGPU (wgpu) API, developers often rely on atomic operations to update shared memory. However, a specific scenario involving the atomicCompareExchangeWeak function can lead to an internal error in the writing type. This article aims to provide a detailed explanation of the issue, its repro steps, and the expected behavior.

Understanding the Issue

The atomicCompareExchangeWeak function is used to atomically exchange a value in shared memory if the current value matches a specified old value. However, when this function is called in a compute shader, the wgpu API reports an internal error. The error message indicates an issue with writing a specific type, which is a struct containing two members: "old_value" and "exchanged".

Repro Steps

To reproduce the issue, you can use a repository created for exploring other things, specifically the wgsl-loop repository. The branch atomic-compare-exchange-weak contains a test that runs a compute shader calling atomicCompareExchangeWeak. You can run the test using cargo test, and you should see the internal error being reported.

Expected vs Observed Behavior

The expectation is that atomicCompareExchangeWeak will not cause an internal error and will be successfully translated. However, the observed behavior is that the wgpu API reports an internal error when this function is called in a compute shader.

Platform

The issue has been observed on Linux, and it has been tested with two different versions of the wgpu API: wgpu 23.0.1 and wgpu 25.0.0.

Debugging Tips

Before filing a bug report, it is recommended to read the debugging tips provided by the wgpu API. This may help you investigate the issue on your own or provide additional information that can assist in resolving the problem.

Code Snippet

The code snippet below demonstrates the use of atomicCompareExchangeWeak in a compute shader:

struct SharedData {
    old_value: [3];
    exchanged: [8];
};

@group(0) @binding(0)
var<storage> shared_data: SharedData;

@compute
fn main() {
    let index = group_index() * workgroup_size(0);
    let value = [index, index, index];

    if index < 4 {
        atomicCompareExchangeWeak(shared_data, 0, value, value);
    }
}

This code defines a struct SharedData with two members: old_value and exchanged. The main function uses atomicCompareExchangeWeak to update the old_value member of the shared data.

Conclusion

In conclusion, the atomicCompareExchangeWeak function can lead to an internal error in the writing type when used in a compute shader. This issue has been observed on Linux with two different versions of the wgpu API. By following the repro steps and debugging tips provided, developers can investigate and resolve this issue.

Possible Solutions

  1. Update the wgpu API: The issue may be resolved by updating the wgpu API to a newer version2. Modify the code: The code may need to be modified to avoid using atomicCompareExchangeWeak in a compute shader.
  2. Use a different atomic operation: A different atomic operation, such as atomicCompareExchange, may be used instead of atomicCompareExchangeWeak.

Future Work

  1. Investigate the root cause: The root cause of the issue needs to be investigated to determine the underlying problem.
  2. Provide a fix: A fix needs to be provided to resolve the issue.
  3. Test the fix: The fix needs to be tested to ensure it resolves the issue.

Related Issues

  1. Issue 1: Atomic operations in compute shaders
  2. Issue 2: Internal error in writing type

References

  1. WebGPU API documentation
  2. wgpu API documentation
    Q&A: Internal Error in Writing Type When AtomicCompareExchangeWeak is Used in Compute Shader =====================================================================================

Q: What is the internal error in writing type when atomicCompareExchangeWeak is used in compute shader?

A: The internal error in writing type occurs when the atomicCompareExchangeWeak function is called in a compute shader, and the wgpu API reports an error due to an issue with writing a specific type.

Q: What is the cause of the internal error?

A: The cause of the internal error is not yet fully understood, but it is believed to be related to the way the wgpu API handles atomic operations in compute shaders.

Q: How can I reproduce the issue?

A: To reproduce the issue, you can use a repository created for exploring other things, specifically the wgsl-loop repository. The branch atomic-compare-exchange-weak contains a test that runs a compute shader calling atomicCompareExchangeWeak. You can run the test using cargo test, and you should see the internal error being reported.

Q: What are the expected vs observed behavior?

A: The expectation is that atomicCompareExchangeWeak will not cause an internal error and will be successfully translated. However, the observed behavior is that the wgpu API reports an internal error when this function is called in a compute shader.

Q: What platforms have been tested?

A: The issue has been observed on Linux, and it has been tested with two different versions of the wgpu API: wgpu 23.0.1 and wgpu 25.0.0.

Q: What are the possible solutions?

A: The possible solutions include:

  1. Update the wgpu API: The issue may be resolved by updating the wgpu API to a newer version.
  2. Modify the code: The code may need to be modified to avoid using atomicCompareExchangeWeak in a compute shader.
  3. Use a different atomic operation: A different atomic operation, such as atomicCompareExchange, may be used instead of atomicCompareExchangeWeak.

Q: What is the future work?

A: The future work includes:

  1. Investigate the root cause: The root cause of the issue needs to be investigated to determine the underlying problem.
  2. Provide a fix: A fix needs to be provided to resolve the issue.
  3. Test the fix: The fix needs to be tested to ensure it resolves the issue.

Q: Are there any related issues?

A: Yes, there are related issues, including:

  1. Issue 1: Atomic operations in compute shaders
  2. Issue 2: Internal error in writing type

Q: Where can I find more information?

A: You can find more information in the following resources:

  1. WebGPU API documentation
  2. wgpu API documentation

Frequently Asked Questions

  1. Q: What is the difference between atomicCompareExchange and atomicCompareExchangeWeak? A: The difference between atomicCompareExchange and atomicCompareExchangeWeak is that atomicCompareExchangeWeak does not throw an exception if the exchange fails.
  2. Q: Can I use atomicCompareExchangeWeak in a compute shader? A: No, you should not use atomicCompareExchangeWeak in a compute shader, as it can lead to an internal error in writing type.
  3. Q: How can I avoid the internal error? A: You can avoid the internal error by using a different atomic operation, such as atomicCompareExchange, or by modifying the code to avoid using atomicCompareExchangeWeak in a compute shader.

Additional Resources

  1. WebGPU API documentation
  2. wgpu API documentation
  3. wgsl-loop repository