Chunk 29.1

In this chunk, the assistant dives deep into implementing the Phase 12 split API designed to hide `b_g2_msm` latency by decoupling the GPU worker's critical path from CPU post-processing. The core task is refactoring `groth16_cuda.cu` to allocate a persistent `groth16_pending_proof` handle early, aliasing all necessary state (split flags, vectors, results) into it so the handle can outlive the GPU worker's critical path. The assistant encounters and fixes compilation errors, including an ordering issue where `pp` was referenced before allocation and a `mult_pippenger` type mismatch caused by const/non-const pointer ambiguity in a ternary expression. Following the C++ changes, the Rust FFI boundary is updated, adding `generate_groth16_proofs_start_c` and `finalize_groth16_proof` declarations in `lib.rs`, creating `PendingProofHandle` and `prove_start`/`prove_finish` functions in `bellperson`'s `supraseal.rs`, and exporting the new types from the `groth16` module. The assistant then integrates the split API into the higher-level proving pipeline and engine. In `pipeline.rs`, `gpu_prove_start` and `gpu_prove_finish` wrappers are added to interface with the new FFI functions. The most significant architectural change begins in `engine.rs`, where the monolithic GPU worker loop is restructured. The plan is for the worker to call `gpu_prove_start`, spawn a separate tokio task to handle the finalization (`gpu_prove_finish`) and the complex result-processing logic (tracker updates, partition assembly, error handling), and then immediately loop back to pick up the next synthesis job. To manage the complexity of this refactoring without massive code duplication, the assistant begins designing helper functions (`process_partition_result`, `process_monolithic_result`) to encapsulate the existing result-handling logic. The overarching theme remains latency hiding through critical-path optimization, driven by the detailed memory bandwidth analysis from Phase 11. The assistant's methodology is highly iterative, cycling through editing C++/Rust code, building, diagnosing type or linking errors, and fixing them. This chunk highlights the inherent complexity of cross-language FFI development, where a single logical change (splitting a function) requires coordinated modifications across C++ structs, Rust wrappers, and application-level orchestration. The work represents a pragmatic trade-off: accepting increased architectural complexity to unblock the GPU worker and improve throughput, while carefully preserving the existing result-processing machinery to avoid introducing bugs. The final integration into the engine loop is the last major hurdle before benchmarking the effectiveness of the split API.

From Memory Bandwidth to Latency Hiding: The Phase 12 Split API Refactoring 2121 words

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