When PCE Fast Path Fails: A GPU Assertion Crash in WindowPoSt Proving

Introduction

Message 163 in this opencode conversation is a crash report. The user, having just implemented Pre-Compiled Constraint Evaluator (PCE) extraction for all four proof types in the CuZK proving engine, ran the daemon to test the new code. The result was a study in contrasts: the first WindowPoSt proof succeeded beautifully, extracting a 39.9 GiB PCE in the background while the GPU proved the circuit. The second WindowPoSt proof, which attempted to use that freshly-extracted PCE via the "fast path," crashed with a GPU-side assertion failure, dumping core.

This message is a critical data point in the debugging journey. It reveals that while the PCE extraction path works correctly for WindowPoSt, the PCE consumption path — where a previously extracted PCE is used to accelerate subsequent proofs of the same type — contains a bug that manifests as a dimension mismatch in the CUDA proving kernel. The crash provides a wealth of diagnostic information: log lines tracing the full lifecycle of both proofs, timing data, dimension reports from the constraint systems, and the exact assertion that failed in the GPU code.

To understand this message fully, one needs to know the architecture of the CuZK proving engine, the role of PCEs in accelerating Groth16 proofs, and the recent history of constraint system harmonization that preceded this test run. The message itself creates new knowledge: it proves that the PCE extraction path is functional for WindowPoSt, it identifies a specific failure mode in the PCE fast path, and it provides concrete numerical data (input counts, constraint counts, matrix dimensions) that can be used to diagnose the root cause.

Context: The PCE Implementation Project

The work leading up to this message was a major feature addition to the CuZK proving engine. CuZK is a GPU-accelerated proving system for Filecoin proofs, supporting four proof types: PoRep (Proof of Replication), WinningPoSt, WindowPoSt, and SnapDeals. The Pre-Compiled Constraint Evaluator (PCE) is an optimization technique where the constraint system matrices for a given circuit are pre-computed and cached to disk, allowing subsequent proofs of the same type to skip constraint synthesis and instead perform a fast matrix-vector multiplication (MatVec evaluation) against a freshly-generated witness.

Prior to this work, PCE extraction was only enabled for PoRep C2 proofs. The project's goal was to extend PCE extraction to all four proof types, add a partitioned pipeline for SnapDeals (mirroring the existing PoRep partition architecture), and fix any bugs discovered along the way.

The implementation touched three core files:

The Message: Command and Environment

The user ran the daemon from /tmp/czk on a host named biryani, logged in as theuser. The command was:

CUZK_VALIDATE_PARTITIONS=1 ./cuzk -l "unix:///tmp/cuzk.sock" -c /tmp/cuzk-p12-pw16.toml

The environment variable CUZK_VALIDATE_PARTITIONS=1 is significant — it enables partition validation, which likely performs additional checks on the proof output. The daemon listens on a Unix socket at /tmp/cuzk.sock and loads configuration from /tmp/cuzk-p12-pw16.toml. The process exited with signal KILL(-9) and a core dump, as indicated by the shell prompt decoration.

The daemon was configured with 192 rayon threads, 32 GPU threads, and a single GPU (device 0) with 2 GPU workers per device. The SRS (Structured Reference String) for the porep-32g circuit was preloaded from disk (a 44 GiB file taking 15.4 seconds to load). The PCE for porep-32g was also preloaded from disk (25.7 GiB, 328 inputs, 130 million constraints). PCEs for winning-32g, wpost-32g, and snap-32g were not found on disk and would need to be extracted during proving.

The First Proof: Success and PCE Extraction

At 20:10:26, the first WindowPoSt proof request arrived:

Prove request_id=wdpost-180965-0-[9 196 174 168 218 102 3 200] proof_kind=3 input_size=0

The proof kind 3 corresponds to WindowPoSt. The request was immediately dequeued for proving and entered the monolithic (non-partitioned) synthesis path.

The synthesis log shows the circuit being built with 102 sectors (the number of sectors in this particular WindowPoSt challenge). The synthesis used the standard path (synthesize_with_hint) because no PCE was cached yet for wpost-32g. The "no capacity hint cached" message confirms this is the first synthesis for this circuit type.

Synthesis completed in 5.74 seconds, producing a circuit with:

RecordingCS: extracted pre-compiled circuit num_inputs=26036 num_aux=126902376 num_constraints=125305253 a_nnz=862416755 b_nnz=127398015 c_nnz=157401792 a_aux_density=96179805 b_input_density=1 b_aux_density=91925766

The PCE extraction produced 26,036 inputs — 196 more than the 25,840 reported by the standard synthesis path. This is the exact discrepancy that the RecordingCS/WitnessCS harmonization fix was supposed to address. The fact that it persists here suggests that either:

  1. The fix was applied to RecordingCS but not to ProvingAssignment (which is used by the standard synthesis path), or
  2. The fix changed RecordingCS to match WitnessCS (both producing 26,036 inputs), but ProvingAssignment was left at 25,840 inputs. The PCE was saved to disk at /data/zk/params/pce-window-post.bin (39.9 GiB, taking 35.6 seconds to write). The GPU proving completed in 65.9 seconds, and the first proof was successfully returned with a total wall time of 91.4 seconds. From the user's perspective, the first proof worked. The PCE was extracted, cached to disk, and the proof was valid. This would have been encouraging — the extraction path appeared functional.

The Second Proof: Crash on PCE Fast Path

At 20:13:36, approximately 99 seconds after the first proof completed, a second WindowPoSt proof request arrived:

Prove request_id=wdpost-180965-0-[87 72 110 251 86 218 87 171] proof_kind=3 input_size=0

This time, the synthesis path was different. The log shows:

using PCE fast path for synthesis circuit_id=wpost-32g

The PCE that was extracted during the first proof is now available. Instead of performing full constraint synthesis, the engine uses the PCE fast path: it generates a witness using WitnessCS (synthesis without constraints, just to compute the assignment), then evaluates the pre-computed PCE matrices against that witness using MatVec multiplication.

The witness generation completed in 3.11 seconds, and the PCE MatVec evaluation took 1.04 seconds. Total synthesis time was 4.15 seconds — a meaningful improvement over the 5.74 seconds of the standard path, though not the dramatic speedup one might expect (the bottleneck appears to be witness generation, not constraint synthesis).

The synthesized proof was sent to GPU worker 0. And then the crash:

cuzk: cuda/groth16_cuda.cu:426: RustError::by_value generate_groth16_proofs_start_c(const Assignment<bls12_381::fr_t>*, size_t, const bls12_381::fr_t*, const bls12_381::fr_t*, SRS&, std::mutex*, void**): Assertion `points_a.size() == p.inp_assignment_size + p.a_aux_popcount' failed.

The process died with an IOT instruction (core dumped).

Anatomy of the Crash

The assertion failure is in CUDA C++ code, specifically in the generate_groth16_proofs_start_c function in groth16_cuda.cu at line 426. This is the entry point for GPU-accelerated Groth16 proving.

The assertion is:

points_a.size() == p.inp_assignment_size + p.a_aux_popcount

Let's break down each term:

Interpreting the Failure

The crash occurs specifically on the PCE fast path, not on the standard synthesis path. This narrows the search for the root cause considerably.

In the PCE fast path, the data flow is:

  1. Generate a witness using WitnessCS → produces an assignment with a certain number of input variables
  2. Load the PCE from cache → contains pre-computed A, B, C matrices with known dimensions
  3. Evaluate the PCE matrices against the witness → produces evaluated matrices
  4. Send the evaluated matrices to the GPU for proving The assertion failure indicates that at step 4, the evaluated matrices don't satisfy the structural invariant. This could happen if: - The witness has a different number of input variables than the PCE expects - The PCE's A matrix has a different structure than what the GPU code assumes - The MatVec evaluation produced incorrectly-sized output The most likely culprit is a mismatch between the witness's input count and the PCE's expected input count. Recall that the PCE was extracted with num_inputs=26036 (from RecordingCS), while the standard synthesis path produced num_inputs=25840 (from ProvingAssignment). If the PCE fast path generates a witness using WitnessCS (which produces 26,036 inputs) but the PCE was built with data from RecordingCS (also 26,036 inputs after the fix), the input counts should match. But the assertion checks points_a.size() against inp_assignment_size + a_aux_popcount, not just the input count. The a_aux_popcount is derived from the PCE data, not from the witness. If the PCE's A matrix has a certain number of aux entries, and the witness has a certain number of aux variables, these need to be consistent. The RecordingCS::extend() implementation, which handles CSR column index remapping when merging child constraint systems, could have a bug that produces incorrect aux popcount metadata. Another possibility is that the PCE extraction and the PCE fast path use different definitions of "input." The RecordingCS might count inputs differently than WitnessCS, even after the harmonization fix. The 196-input discrepancy that was supposedly fixed might still manifest in subtle ways — perhaps in how the ONE variable is counted, or in how child CS inputs are merged during extend().

The 25,840 vs 26,036 Discrepancy: A Lingering Issue

The log data reveals a telling detail. The first proof's standard synthesis reported a "cached synthesis capacity hint" of 25,840 inputs. But the PCE extraction (running on the same circuit) reported 26,036 inputs. This 196-input gap is the original bug that the RecordingCS/WitnessCS harmonization was supposed to fix.

The fact that it still appears in this test run means one of two things:

  1. The fix was applied to RecordingCS (making it produce 26,036 inputs to match WitnessCS), but ProvingAssignment (used by the standard synthesis path) was not changed, so it still produces 25,840 inputs.
  2. The fix was incomplete or incorrect, and the harmonization didn't actually work. Either way, the mismatch persists. For the PCE fast path, this means: - The PCE was extracted with 26,036 inputs (from RecordingCS) - The witness is generated with 26,036 inputs (from WitnessCS) - But the GPU expects a structure consistent with... what? The assertion failure suggests that even though the input counts might match between RecordingCS and WitnessCS, something deeper is inconsistent. The a_aux_popcount — the number of non-zero A entries for aux variables — might differ between the PCE data and what the witness generation produces.

Implications for the PCE Architecture

This crash reveals a fundamental challenge in the PCE approach: the constraint system used for extraction (RecordingCS) and the constraint system used for fast-path witness generation (WitnessCS) must produce structurally identical circuits. Every dimension — number of inputs, number of aux variables, number of constraints, and crucially, the sparsity pattern of the matrices — must match exactly.

The harmonization fix addressed the input count, but the crash suggests that other dimensions might also be mismatched. The RecordingCS::extend() implementation, which handles the complex task of remapping column indices when merging child constraint systems, is a prime candidate for bugs. CSR (Compressed Sparse Row) matrix manipulation is notoriously error-prone, and a subtle mistake in index remapping could produce incorrect matrix metadata while still appearing to work for the extraction itself.

The fact that the first proof (which uses standard synthesis) succeeds while the second proof (which uses PCE fast path) crashes is strong evidence that the PCE data itself is correct — it was extracted from a valid circuit and saved to disk without corruption. The problem lies in how the PCE is used during fast-path proving, specifically in the interaction between the cached PCE matrices and the freshly-generated witness.

What This Message Teaches Us

Message 163 is a textbook example of a regression crash in a complex system. It provides several valuable pieces of information:

  1. PCE extraction works for WindowPoSt. The background extraction completed successfully, producing a 39.9 GiB PCE with 26,036 inputs and 125 million constraints. The PCE was saved to disk and loaded correctly on the next proof.
  2. The PCE fast path is exercised. The second proof correctly detected the cached PCE and used the fast path (witness generation + MatVec evaluation), confirming that the PCE cache lookup and routing logic is functional.
  3. The crash is deterministic and reproducible. It happens on the second WindowPoSt proof, every time, at the same point (GPU assertion). This makes it amenable to debugging.
  4. The crash is in GPU code, not CPU code. The Rust-side synthesis (witness generation + PCE evaluation) completes without errors. The failure is in the CUDA kernel that performs the actual Groth16 proving. This suggests the data passed to the GPU is structurally invalid, even though it passes Rust-side validation.
  5. The harmonization fix needs revisiting. The 25,840 vs 26,036 input discrepancy between standard synthesis and PCE extraction is still present, indicating that the fix didn't fully harmonize all three constraint system types (RecordingCS, WitnessCS, and ProvingAssignment).

Diagnostic Value of the Log Data

The log output in this message is exceptionally detailed. It includes:

Conclusion

Message 163 captures a pivotal moment in the PCE implementation project. The user has deployed the new code to a real test environment, run it against actual WindowPoSt proof requests, and observed both success and failure. The first proof demonstrates that the PCE extraction pipeline is functional end-to-end: it synthesizes the circuit, extracts the PCE matrices, caches them to disk, and produces a valid Groth16 proof. The second proof demonstrates that the PCE fast path — the very optimization that motivated this entire feature — has a critical bug that manifests as a GPU assertion failure.

The crash is not a random glitch or an environmental issue. It is a deterministic failure caused by a structural mismatch between the PCE data and the witness data in the fast path. The root cause is almost certainly related to the constraint system harmonization that was the focus of the previous debugging session. While the RecordingCS extensibility fix addressed the most visible symptom (the input count mismatch), it appears to have left deeper inconsistencies in the matrix structure.

For the developer reading this message, the path forward is clear: investigate the full structural parity between RecordingCS, WitnessCS, and ProvingAssignment — not just the input count, but the entire matrix sparsity pattern, the aux variable layout, and the behavior of the extend() method across all constraint system types. The crash provides a concrete, reproducible test case: run two WindowPoSt proofs in sequence, and the second will trigger the assertion. Fixing this bug will require ensuring that the PCE fast path produces data that satisfies the GPU's structural invariants, which in turn requires perfect harmonization of all three constraint system implementations.

In the broader narrative of this opencode session, message 163 serves as the reality check that follows a period of intensive implementation. The code compiles, the extraction works, but the fast path crashes. It's a reminder that in systems programming — especially in performance-critical GPU code — correctness is not guaranteed by compilation alone. Every optimization path must be tested against real data, and every invariant must be verified at runtime.