The Quiet Capstone: Wiring the PCE Benchmark into cuzk's Proof Pipeline

[assistant] Now add the `run_pce_bench` function:
[edit] /home/theuser/curio/extern/cuzk/cuzk-bench/src/main.rs
Edit applied successfully.

On its surface, message [msg 1409] is almost absurdly unremarkable. A single sentence of natural language, followed by a tool call confirming an edit was applied. No reasoning blocks, no analysis, no debate. Yet this message sits at a critical juncture in a months-long engineering effort to re-architect the Filecoin PoRep proof generation pipeline. It is the moment when the Pre-Compiled Constraint Evaluator (PCE) — the centerpiece of Phase 5 — transitions from a theoretical construct into something measurable. The run_pce_bench function being added here is the instrumentation that will determine whether the entire Phase 5 bet pays off.

The Long Road to This Edit

To understand why this single edit matters, one must trace the arc that led to it. The cuzk project began as an attempt to build a pipelined SNARK proving engine for Filecoin's 32 GiB PoRep circuits, which demand roughly 200 GiB of peak memory and involve synthesizing tens of millions of R1CS constraints. Phase 4 had been a bruising, empirical ordeal: the team systematically tested nine optimization proposals, only to see most fail under real hardware. SmallVec caused an 8.5% IPC regression on Zen4 and was rejected. cudaHostRegister added 5.7 seconds of mlock overhead and was reverted. Vec pre-allocation showed zero measurable impact. The net result after all that work was a 13.4% improvement — 88.9 seconds down to 77.0 seconds — far short of the 2–3x target.

The Phase 4 post-mortem (documented in [chunk 16.0]) delivered a stark conclusion: the synthesis bottleneck, now consuming ~50.8 seconds, was purely computational. Field arithmetic and LinearCombination construction dominated. Allocation was not the problem. Cache misses were not the problem. The CPU was simply doing too much work.

This diagnosis directly motivated Phase 5. The core idea of the Pre-Compiled Constraint Evaluator is radical in its simplicity: instead of re-synthesizing the entire R1CS constraint system for every proof (rebuilding the A, B, and C matrices from scratch each time), extract the matrix structure once and then evaluate via a sparse matrix-vector multiply. If the circuit topology is fixed — which it is for Filecoin's proof types — then the constraints themselves are static. Only the witness values change between proofs. The PCE captures this invariant.

The Architecture Decision That Shaped This Message

Message [msg 1409] is the culmination of a careful architectural pivot that occurred earlier in the session. At [msg 1378], the assistant initially explored adding a dedicated PCE synthesis function directly to bellperson's supraseal.rs — the core proving library. But by [msg 1379], a cleaner design emerged:

"Instead of modifying bellperson further, the PCE path should be entirely in cuzk-core and cuzk-pce. The key insight is: we don't need to add a new function to bellperson — we can construct ProvingAssignment objects directly from PCE output, then call the existing prove_from_assignments()."

This was a critical architectural decision. By keeping the PCE logic outside of bellperson, the team preserved the existing proving pipeline's interface while inserting a new synthesis backend underneath. The ProvingAssignment::from_pce constructor (added at [msg 1379]) became the seam between old and new: it takes the a, b, c vectors produced by the PCE's MatVec evaluation and wraps them in the same data structure that the downstream proving code expects. The rest of the pipeline — the MSM computations, the NTTs, the GPU kernel launches — remains completely unchanged.

This decision minimized risk. If the PCE approach failed to deliver, rolling back meant reverting the synthesize_auto dispatch logic in pipeline.rs rather than untangling changes from a foreign codebase. It also meant the team could validate the PCE incrementally: first extract the circuit, then benchmark the MatVec, then compare against the traditional synthesis path.

What run_pce_bench Actually Does

The function being added in [msg 1409] is the validation harness for this entire architecture. It builds on two pieces of infrastructure created in the same session:

  1. The PceExtract subcommand (added at [msg 1407]), which defines the CLI interface for extracting a pre-compiled circuit from C1 data and running PCE-based synthesis.
  2. The extract_pce_from_c1 helper (added at [msg 1411]), a public function in cuzk-core/pipeline.rs that exposes PCE extraction from C1 data for use by the bench tool. The run_pce_bench function wires these together. It takes a circuit type (PoRep, WinningPoSt, WindowPoSt, SnapDeals), extracts the pre-compiled circuit using RecordingCS, then runs the PCE MatVec evaluation to produce ProvingAssignment objects. This allows the team to measure: - How long the one-time extraction takes (the cost of running RecordingCS to capture the matrix structure) - How long each subsequent PCE evaluation takes (the MatVec multiply) - How these times compare to the traditional synthesize_with_hint path - Whether the PCE output produces identical ProvingAssignment data (correctness validation)

The Thinking Process Visible in the Session

The assistant's reasoning throughout this session reveals a methodical, systems-level approach. Each step builds on the previous one with clear dependencies:

  1. Create the PCE crate (msgs 1361–1377): Build the foundational data structures — CsrMatrix, PreCompiledCircuit, RecordingCS, DensityTracker, and the multi-threaded evaluate_csr function. This is the mathematical engine.
  2. Add the from_pce constructor (msg 1379): Create the bridge between PCE output and the existing proving pipeline. This is the architectural seam.
  3. Add the PCE cache and synthesize_auto (msg 1382): Create the unified synthesis entry point that dispatches to either PCE or traditional synthesis based on whether a pre-compiled circuit is available. This is the runtime integration.
  4. Update all six synthesis call sites (msgs 1384–1394): Replace every synthesize_with_hint(...) call with synthesize_auto(...). This is the wiring.
  5. Add the benchmark command (msgs 1407–1409): Create the tooling to measure and validate the PCE path. This is the empirical feedback loop. The assistant's thinking is characterized by a strong preference for clean architecture over convenience. When faced with the choice of modifying bellperson (which would have been simpler in the short term), the assistant instead designed a more complex but cleaner separation of concerns. The comment at [msg 1379] — "Let me design this properly" — signals a deliberate pause to reconsider the approach.

Assumptions Embedded in This Message

Several assumptions underpin the work being finalized in [msg 1409]:

The circuit topology is truly static. The PCE approach only works if the R1CS constraint structure (which constraints exist, how they connect variables) is identical across proofs for the same circuit type. For Filecoin's PoRep circuits, this is believed to be true — the circuit is parameterized by the sector size and proof type, not by the specific data being proven. But this assumption has not yet been empirically validated for all edge cases.

The MatVec will be faster than synthesis. The entire Phase 5 bet rests on the premise that evaluating a pre-computed sparse matrix against a witness vector is cheaper than re-building the matrix from scratch via circuit synthesis. This is plausible — synthesis involves traversing a graph of gadgets, allocating LinearCombination objects, and performing field operations to build each constraint row-by-row. A MatVec, by contrast, is a tight loop over pre-computed indices and coefficients. But the magnitude of the speedup is unknown. The Phase 4 data showed synthesis at ~50.8 seconds; the PCE needs to significantly undercut this to justify the complexity.

The RecordingCS extraction is a one-time cost. The PCE design assumes that the circuit is extracted once (during the first proof of each type) and cached thereafter. If extraction is expensive — and for a 32 GiB PoRep circuit with millions of constraints, it likely is — this cost must be amortized over many proofs. The benchmark command will measure this directly.

What This Message Creates

Message [msg 1409] produces a specific output: the run_pce_bench function in cuzk-bench/src/main.rs. But the knowledge created extends beyond this function. The message completes the integration of the PCE into the cuzk proving engine, making it possible to:

The Broader Context: What Comes Next

The message immediately following [msg 1409] is [msg 1410], where the assistant adds the extract_pce_from_c1 helper function. This is the final piece needed for the bench tool to function. After that, the session moves toward compilation verification and, presumably, the first real PCE benchmark runs.

The stakes are high. Phase 4 demonstrated that incremental optimization of the existing synthesis path yields diminishing returns. The PCE represents a qualitatively different approach — not optimizing the existing process, but replacing it with a fundamentally cheaper one. If the MatVec evaluation delivers a 2–5x speedup over synthesis, the overall proof time could drop from 77 seconds to well under 40, potentially meeting the original throughput targets. If it does not, the team faces the uncomfortable question of what comes after Phase 5.

Message [msg 1409] is, in this context, the moment when the hypothesis becomes testable. The run_pce_bench function is the instrument that will deliver the verdict.