Zkcuda parallel witness

[siq1]

No description provided.

[siq1]

Please merge this PR after #163.

[gemini-code-assist]

Summary of Changes

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This pull request introduces parallel witness generation capabilities specifically within the Zkcuda backend. The core change involves refactoring the hint calling mechanism to be thread-safe, enabling the parallel evaluation of circuit components and hints during witness computation.

Highlights

• Hint Registry Update: The HintRegistry has been modified to store stateless function pointers (fn) instead of mutable closures (dyn FnMut). This change, along with adding the Clone derive, makes the registry compatible with being shared across threads.
• Parallel Witness Evaluation in Zkcuda: Within the Zkcuda Context, I've introduced parallel processing using the rayon crate for evaluating IR kernels and hint solvers. Inputs for parallel tasks are now collected and processed concurrently.
• API Signature Changes: Functions throughout the codebase that interact with a HintCaller (e.g., evaluation and witness solving methods) have been updated to accept an immutable reference (&impl HintCaller) instead of a mutable one (&mut impl HintCaller).
• Test Case Removal: A test case (test_300_closure) in to_binary_hint.rs that relied on mutable state captured by a closure (which is incompatible with the new stateless fn requirement for thread safety) has been removed.

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[gemini-code-assist]

Code Review

This pull request refactors the hint execution mechanism to support parallel witness generation. Key changes include modifying HintCaller methods to take &self, adding a Sync bound to HintCaller, and changing HintRegistry to use function pointers (fn) for hints instead of Box<dyn FnMut>. These changes enable HintRegistry to be Clone and Sync, which is essential for parallel processing. A test relying on stateful closures has been removed as it's incompatible with the new design. Parallel execution is implemented in zkcuda::Context using Rayon.