The Moment of Execution: Forking Bellperson for Pipelined Proving

In the long arc of engineering a pipelined SNARK proving daemon for Filecoin, there comes a moment when analysis must yield to action. Message [msg 413] is that moment. After dozens of messages spent tracing call chains, auditing visibility modifiers, debating fork strategies, and writing a 791-line design document, the assistant finally declares: "Now I'll apply the minimal changes to bellperson." The message lists four surgical modifications, then executes the first one — an edit to Cargo.toml. It is brief, almost anticlimactic. But beneath its terseness lies the culmination of an exhaustive investigative process that spanned multiple sub-sessions, task agents, and design iterations.

The Long Road to a Single Edit

To understand why this message matters, one must appreciate the journey that preceded it. The cuzk project (a pipelined SNARK proving daemon for Filecoin's Curio storage mining infrastructure) had already completed Phase 0 (gRPC daemon with priority scheduling) and Phase 1 (multi-GPU worker pool supporting all four Filecoin proof types). Phase 2's goal was transformative: split the monolithic Groth16 proof generation into two separately schedulable phases — CPU-intensive circuit synthesis and GPU-intensive proving — enabling a pipelined architecture that could keep GPUs continuously busy while CPUs prepared the next batch of work.

The assistant had spent the preceding messages conducting an extraordinarily thorough investigation. In [msg 398], it discovered that bellperson's supraseal prover already had a clean internal two-phase architecture: a private synthesize_circuits_batch() function handling the CPU work, followed by inline GPU code that packed raw pointers and called supraseal_c2::generate_groth16_proof(). The boundary was clean. The required fork changes were estimated at ~30 lines. But the assistant did not rush to implement.

Instead, it spawned a task agent ([msg 398]) to trace the full call chain from filecoin-proofs-api::seal_commit_phase2 down to bellperson, verifying that the split point was indeed clean and that no hidden coupling existed. It then debated two strategies ([msg 399]): fork only bellperson (minimal), or fork bellperson plus storage-proofs-core plus filecoin-proofs plus filecoin-proofs-api (four forks, too many). It spawned another task ([msg 400]) to check the visibility of CompoundProof::circuit(), StackedCompound, and related types — confirming they were public and usable without forking additional crates. It asked the user for guidance ([msg 400]), receiving the instruction to write a detailed design document first. It then produced cuzk-phase2-design.md, a 791-line document covering per-partition pipeline strategy, memory budget analysis (13.6 GiB intermediate state vs 136 GiB for all 10 partitions), SRS manager design, and a 7-step implementation plan. It verified constraint counts per circuit ([msg 404]) to refine the memory analysis. It updated the design document with those findings ([msg 405], [msg 406]). It committed the design document ([msg 407]).

Only then — after all that — does message [msg 413] arrive.

The Four Modifications

The message lists four changes, each the product of careful reasoning:

1. Make ProvingAssignment and its fields pub in prover/mod.rs. The ProvingAssignment<Scalar> struct was crate-private, meaning external code could not even name the type. It holds the a, b, and c evaluation vectors produced by circuit synthesis — the essential intermediate state that must be passed from the CPU synthesis phase to the GPU proving phase. Making it public is the foundational change; without it, no external code can receive or manipulate synthesis output.

2. Make synthesize_circuits_batch pub and make the supraseal module pub in prover/mod.rs. The synthesize_circuits_batch() function was private, callable only within the prover module. It takes a vector of circuit implementations, runs circuit.synthesize() on each in parallel via rayon, and returns the ProvingAssignment vectors along with input and auxiliary assignments. Making it public exposes the CPU phase as a standalone API. Making the supraseal module public is necessary because the function lives there (behind the cuda-supraseal feature flag).

3. Add prove_from_assignments() in prover/supraseal.rs. This is the only new code — approximately 70 lines extracted from the existing create_proof_batch_priority_inner function. It takes the synthesized assignments, packs them into raw pointer arrays, and calls supraseal_c2::generate_groth16_proof() to perform the NTT and MSM operations on CUDA. By separating this from the synthesis code, the assistant creates a clean API boundary: call synthesize_circuits_batch() on a CPU thread, queue the result, and later call prove_from_assignments() on a GPU worker.

4. Re-export in groth16/mod.rs. The public API must be visible at the bellperson::groth16 level, not buried in submodules. The re-export makes ProvingAssignment, synthesize_circuits_batch, and prove_from_assignments available to cuzk-core and any other downstream consumer.

The message then executes the first edit — to Cargo.toml. This is the version bump that makes the fork replace the upstream dependency. The assistant had initially set the version to 0.26.0-cuzk.1, but would discover in [msg 422] that semver pre-release versions don't satisfy "0.26.0" dependency requirements. The fix — reverting to 0.26.0 — is a subtle but critical detail that demonstrates the assistant's understanding of Cargo's dependency resolution semantics.

Assumptions and Their Consequences

The message rests on several assumptions, most validated but one requiring correction:

The split is clean. The assistant assumes that synthesize_circuits_batch() and the GPU-phase code in create_proof_batch_priority_inner share no mutable state beyond the explicit return values. This was validated by reading both functions in their entirety — synthesis returns owned Vecs, and the GPU phase consumes them via pointer casts. No hidden coupling exists.

Making things public won't break internal callers. The assistant assumes that changing struct ProvingAssignment to pub struct ProvingAssignment and its fields from private to public won't affect existing code within bellperson. This is correct: Rust's visibility modifiers are additive — making something more visible never breaks code that could already see it.

The version patch will work. This assumption was incorrect. The assistant initially set the fork's version to 0.26.0-cuzk.1, following the convention of adding a suffix to distinguish the fork. But Cargo's [patch.crates-io] mechanism requires the patched version to satisfy the dependency requirement using semver compatibility rules. A pre-release version 0.26.0-cuzk.1 does not match "0.26.0" (which requires >=0.26.0, <0.27.0 in semver, but pre-release versions are excluded). The error message in [msg 421] — "Patch bellperson v0.26.0-cuzk.1 was not used in the crate graph" — triggered the fix in [msg 422], reverting to 0.26.0. This is a classic Cargo gotcha that even experienced Rust developers encounter.

The pub(super) visibility on create_proof_batch_priority_inner is safe. The assistant notes that create_proof_batch_priority_inner is pub(super) (visible within the prover module). Making the supraseal submodule pub doesn't change this — external code still cannot reach pub(super) items through a public module path. The assistant correctly verifies this reasoning.

Input and Output Knowledge

The input knowledge required to understand this message is substantial. One must understand: the Groth16 proving system and its two-phase structure (circuit synthesis followed by prover computation); the bellperson library's architecture (how it wraps bellpepper-core for constraint system building and uses supraseal for GPU acceleration); Cargo's workspace and patch mechanisms; Rust's visibility system (pub, pub(super), crate-private); the Filecoin proof pipeline (PoRep C2 with 10 partitions, ~136 GiB intermediate state); and the cuzk project's phased roadmap.

The output knowledge created by this message is equally significant. It establishes that the bellperson fork requires only ~130 lines of changes (not the hundreds or thousands that a rewrite would demand). It confirms that the existing internal architecture already supports the split — no restructuring is needed, only exposure. It validates the minimal-fork philosophy that guided the entire Phase 2 design: change as little as possible, expose what already works, and build the pipeline logic in cuzk-core rather than in forked dependencies. And it demonstrates that the fork compiles cleanly with zero new warnings and all 8 existing tests passing ([msg 424]).

The Thinking Process

The message's thinking process is visible in its structure. The assistant leads with a declarative statement of intent — "Now I'll apply the minimal changes" — signaling that the investigative phase is complete. It then enumerates the four modifications in numbered list form, each one corresponding to a specific file and a specific visibility or code change. The ordering is logical: first make the data type public, then make the synthesis function public, then add the new GPU-phase function, then re-export at the module boundary. The final line reports the result of the first edit, confirming execution.

What is not visible in this message is equally telling. There is no hesitation, no second-guessing, no exploration of alternatives. The assistant has already resolved all open questions in the preceding messages. The design document is written and committed. The call chains are traced. The visibility of every needed type is verified. The memory budget is calculated. The only remaining work is to execute — and execution is what this message does.

Significance

Message [msg 413] is the inflection point where Phase 2 transitions from design to implementation. The bellperson fork it initiates will be committed as f258e8c7 ([msg 428]), forming the foundation for the pipelined prover in cuzk-core. The changes are minimal — approximately 130 lines across three files — but their impact is architectural. They transform bellperson from a monolithic black box into a two-phase engine whose intermediate state can be queued, scheduled, and processed independently. This enables the per-partition pipeline strategy that reduces peak intermediate state from 136 GiB to 13.6 GiB, making it feasible to run multiple concurrent proving pipelines on a single 256 GiB machine.

The message also embodies a design philosophy that recurs throughout the cuzk project: understand deeply, change minimally. The assistant did not attempt to rewrite bellperson's internals, restructure its module hierarchy, or add new abstractions. It simply turned private things into public things and extracted existing code into a standalone function. This restraint is the mark of an engineer who respects existing code and trusts the architecture that years of upstream development have produced. The fork is not a fork in the sense of divergence — it is a fork in the sense of exposure, revealing capabilities that were already present but hidden.