Chunk 22.1

In this implementation chunk, the assistant solidifies the decision to pursue Option B—direct synthesis into pinned memory—and elaborates on its necessity, noting that under heavy memory pressure from concurrent synthesis threads, even a staged memcpy would be severely bottlenecked by contested host memory bandwidth. The architecture is carefully designed: a `PinnedPool` in `cuzk-core` manages `cudaHostAlloc`'d buffers with a free list and budget integration, while `bellperson`'s `ProvingAssignment` is extended with a `PinnedBacking` struct to safely hold and release these buffers without triggering undefined behavior from standard `Vec` deallocation. The assistant proceeds to implement the core components. The `PinnedPool` struct is written in `cuzk-core/src/pinned_pool.rs`, featuring exact-size allocation, a free list, and integration with the `MemoryBudget` via `try_acquire`. In `bellperson`, the `ProvingAssignment` struct is modified to include a `pinned_backing` field, a `new_with_pinned` constructor, and a `release_abc` method that uses `std::mem::take` and `ManuallyDrop::take` to safely forget the Vec's contents before returning the buffer to the pool. A custom `Drop` implementation is added as a safety net, and the `prove_start` function in the supraseal module is updated to call `release_abc` after extracting pointers. The chunk concludes with the assistant running `cargo check` to validate the changes. The compilation reveals only a pre-existing visibility error unrelated to the new code, confirming that the core pinned memory integration compiles successfully. This positions the team to wire the pool into the engine startup and evictor, and pass it through the synthesis dispatch, moving from the design phase into full integration and testing of the zero-copy pipeline that promises to eliminate the H2D bottleneck and dramatically improve GPU utilization.

Building the Zero-Copy Bridge: Implementing a Pinned Memory Pool for GPU Proving 2628 words

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