Chunk 30.0

In this chunk, the assistant resolved the remaining compilation errors in the Phase 12 split GPU proving API implementation. Several Rust/C++ FFI issues were systematically fixed: a missing `SynthesisCapacityHint` struct was added, an unused generic parameter was removed from the `start_groth16_proof` FFI, the `PendingGpuProof` type alias was added to `pipeline.rs`, and the `process_partition_result`/`process_monolithic_result` helper functions were extracted from the engine's inline result-processing code. A trait bound mismatch in `prove_start` was resolved by changing its parameter to take `params: P` by value, and `continue` statements inside async blocks were corrected to `return` to properly exit the spawned finalizer task. After achieving a clean build, benchmarking revealed a memory capacity ceiling. Attempts to increase synthesis parallelism (partition_workers=12/15) led to OOM errors, with RSS peaking at 668 GiB, confirming the 755 GiB system is at its limit with the current concurrency model. More critically, a serious use-after-free bug was discovered in the C++ CUDA code: the background `prep_msm_thread` (which runs `b_g2_msm`) captured a dangling reference to the stack-allocated `provers` array after `generate_groth16_proofs_start_c` returned. This was fixed by copying the provers array into the heap-allocated `groth16_pending_proof` struct (`provers_owned`), ensuring the background thread always accesses stable memory. The final Phase 12 implementation was successfully committed to the `feat/cuzk` branch. Benchmarking with the optimal configuration (gw=2, pw=10, gt=32, j=15) yielded a throughput of **37.1s/proof**, a ~2.4% improvement over the Phase 11 baseline of 38.0s. The key achievement is a working split API that offloads `b_g2_msm` from the GPU worker's critical path, delivering a tangible performance gain while fixing a critical concurrency bug. The memory capacity bottleneck identified here sets the stage for future work on reducing per-partition memory footprint.

From Compilation to Capacity: The Full Arc of Phase 12's Split GPU Proving API 2853 words

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