Reading the Bellperson Fork: The Critical Knowledge-Gathering Step Before Building Phase 2

In the middle of implementing Phase 2 of the cuzk proving engine — a pipelined architecture that separates CPU circuit synthesis from GPU proving to eliminate idle GPU time — the assistant pauses to read the bellperson fork source code. Message 434 captures this moment of deliberate research: three file reads targeting the Groth16 prover module in the forked bellperson library. The message is brief in form but dense in purpose, representing a critical knowledge-gathering step before the assistant can write the core pipeline code.

The Message

The assistant writes:

Now let me look at the bellperson fork to understand what's already been done there:

It then issues three [read] commands targeting files in /home/theuser/curio/extern/bellperson/src/groth16/:

1. mod.rs — the top-level Groth16 module, showing the module structure including prover, params, proof, generator, and the conditional compilation for cuda-supraseal
2. prover/supraseal.rs — the supraseal prover implementation, showing imports from bellpepper_core, ff, pairing, and rayon
3. prover/mod.rs — the prover module declaration, showing the conditional compilation between native and supraseal provers Each file read returns the full file contents, which the assistant will use in the next round to understand the API surface it needs to call.

Why This Message Was Written

The message sits at a pivotal moment in the Phase 2 implementation. Steps 1 and 2 of Phase 2 were already completed in earlier rounds: creating a minimal bellperson fork that exposes the synthesis/GPU split point, and wiring that fork into the cuzk workspace. What remains are steps 3 through 7 — the actual implementation of the pipelined proving engine, which includes creating an SRS manager module, implementing per-partition pipelining, refactoring the engine for pipeline mode, and implementing split synthesis/GPU prover functions.

Before the assistant can write any of that code, it must understand exactly what API the bellperson fork provides. The Phase 2 design document calls for calling synthesize_circuits_batch() to perform CPU-side circuit synthesis and prove_from_assignments() to perform GPU-side NTT+MSM proving, but these functions were not part of the original bellperson public API — they were exposed specifically by the fork. The assistant needs to verify that the fork actually exposes these functions, understand their signatures, and see how they interact with the supraseal (CUDA) backend.

This is not idle browsing. The assistant is looking for specific things:

• The supraseal.rs file structure: This is where the split synthesis/GPU functions would live. The assistant needs to see the function signatures, the type parameters (especially E: MultiMillerLoop for the pairing engine), and how the supraseal backend manages GPU state.
• The module visibility: Whether the supraseal module is pub and whether the key functions are publicly accessible from outside the bellperson crate.
• The conditional compilation guards: The #[cfg(feature = "cuda-supraseal")] attributes that control when the supraseal backend is used versus the native CPU backend.
• The dependency chain: What types from bellpepper_core, ff, pairing, and ec-gpu-gen are involved, since the pipeline code will need to import and use these types.

Input Knowledge Required

To understand this message, one needs substantial context about the overall cuzk project. The reader must know that:

• cuzk is a pipelined SNARK proving daemon for Filecoin, designed to replace the monolithic supraseal-c2 proof generation pipeline
• Phase 1 implemented a multi-GPU worker pool with priority scheduling, but each proof still called the monolithic seal_commit_phase2 function, meaning the GPU sat idle while CPU synthesis ran
• Phase 2 aims to split the monolithic prover into two stages: CPU synthesis (circuit evaluation) and GPU proving (NTT+MSM), so they can overlap
• The bellperson fork was created specifically to expose this split point — the original bellperson library only exposes a combined prove() function that does both synthesis and GPU work in one call
• The supraseal backend is a C++/CUDA implementation of the Groth16 prover that handles the GPU-side operations (NTT and MSM)
• The workspace is at /home/theuser/curio/extern/cuzk/ and the bellperson fork is at /home/theuser/curio/extern/bellperson/ Without this context, the message reads as a developer casually browsing code. With it, the message reveals itself as a precise, targeted investigation driven by a clear implementation goal.

Output Knowledge Created

By reading these files, the assistant gains several pieces of knowledge that directly inform the next implementation steps:

1. The module structure confirms the fork's layout: The prover module contains supraseal as a public submodule (visible in prover/mod.rs), meaning the split functions are accessible from outside the crate. The conditional compilation with #[cfg(feature = "cuda-supraseal")] means the pipeline code must also be conditionally compiled or handle the case where the GPU backend is not available.
2. The supraseal prover's dependencies are visible: The imports in supraseal.rs show it uses bellpepper_core types (Circuit, ConstraintSystem, SynthesisError), ff traits (Field, PrimeField), pairing::MultiMillerLoop, and rayon for parallelism. The pipeline code will need to work with these same types.
3. The function signatures are revealed: Although the full function bodies aren't shown in the snippet, the module structure and imports give the assistant enough information to understand how to call into the supraseal prover. The synthesize_circuits_batch() function would take a batch of circuits and produce assignments, while prove_from_assignments() would take those assignments plus the SRS parameters and produce the proof.
4. The relationship between native and supraseal paths: The prover/mod.rs shows that when cuda-supraseal is not enabled, the native CPU prover is used instead. This tells the assistant that the pipeline mode might need to handle both paths, or at least be aware of the feature flag.

Assumptions and Potential Pitfalls

The assistant makes several assumptions in this message:

• That the fork's API is stable: The assistant assumes the functions it needs (synthesize_circuits_batch, prove_from_assignments) exist and have the expected signatures. If the fork exposed them differently, the pipeline code would need adjustment.
• That reading the source is sufficient: The assistant assumes that reading the file contents gives it enough information to write correct calling code. In practice, the type signatures might involve complex generic parameters (lifetimes, trait bounds) that require careful handling.
• That the supraseal module is the right place: The assistant assumes the split synthesis/GPU functions live in the supraseal module specifically, rather than being exposed at a higher level in the prover module or in a separate module. One subtle assumption is that the supraseal.rs file contains the exact functions needed. The file content shown in the message is truncated — it shows the imports and module structure but not the full function bodies. The assistant is making a judgment call that the visible structure is enough to proceed, and that the full details will become clear when writing the calling code.

The Thinking Process

The message reveals a methodical, research-first approach to implementation. The assistant does not jump into writing code. Instead, it:

1. Identifies what it needs to know: Before writing the pipeline module, it needs to understand the bellperson fork's API surface
2. Locates the relevant source files: It targets mod.rs for the module structure and prover/supraseal.rs for the implementation details
3. Reads systematically: It reads the module declaration first (to understand the structure), then the implementation file (to understand the API), then the prover module (to understand visibility and conditional compilation)
4. Defers action: Rather than trying to write code immediately, it reads first and will act in the next round when it has the information This is visible in the conversational flow: the previous message (msg 432) reviewed the Phase 2 design document and existing source files. This message (msg 434) reads the bellperson fork. The next message (msg 435, not shown) will presumably contain the actual implementation code. Each round builds on the knowledge gathered in the previous round.

Significance for the Project

This message, though small, is a microcosm of the entire Phase 2 effort. The Phase 2 design document describes a sophisticated pipelined architecture, but the actual implementation depends entirely on the bellperson fork's API. If the fork doesn't expose the right functions, the entire Phase 2 plan needs revision. By reading the fork source code, the assistant is validating a critical assumption of the design before committing to implementation.

The message also demonstrates the opencode session's iterative nature. The assistant cannot see the file contents without issuing read commands, and it cannot act on those contents until the next round. This creates a natural rhythm of research → implementation → testing that mirrors real-world software development workflows.

In the broader context of the cuzk project, this message represents the transition from planning to building. The design documents have been written, the fork has been created, and now the assistant is gathering the final piece of information needed before writing the pipeline code. The success of Phase 2 — and by extension, the entire cuzk proving daemon — depends on getting this step right.