Verifying State in Untracked Territory: A Diagnostic Pivot in the cuzk Proving Engine Optimization

In the middle of a deep-dive optimization campaign for the cuzk SNARK proving engine — a custom Filecoin proof generation pipeline targeting a Threadripper PRO 7995WX (Zen4) CPU and RTX 5070 Ti GPU — the assistant encountered an unexpected wrinkle. Message [msg 1044] is a short but critical diagnostic step: three grep commands probing the actual contents of modified source files after discovering that those files live in directories not tracked by git. This message is a moment of grounded verification, a reality check before proceeding with further benchmarking.

The Context: Phase 4 Optimization and the SmallVec Regression

The broader narrative is one of methodical, data-driven optimization. The cuzk project had already completed Phases 0 through 3: a pipelined SNARK proving engine supporting all four Filecoin proof types (PoRep, PoSt, SnapDeals) with cross-sector batching achieving 1.46x throughput improvement. Phase 4 targeted compute-level optimizations, drawing from a detailed proposal document (c2-optimization-proposal-4.md) that catalogued nine categories of potential improvements.

The assistant had been working through "Wave 1" quick wins. One of these, optimization A1, proposed replacing Vec<(usize, T)> with SmallVec in the Indexer struct used by the LinearCombination type in bellpepper-core. The intuition was sound: the enforce() hot loop creates and destroys six Indexer objects per call, and avoiding heap allocations for small vectors should reduce allocator overhead. The proposal predicted a 15–25% synthesis speedup.

But the data told a different story. A dedicated synth-only microbenchmark subcommand had been built into cuzk-bench specifically for this A/B testing. The results were unambiguous: SmallVec was consistently slower than Vec on Zen4. With inline capacity 1, synthesis was 8% slower (59.8s vs 55.4s). Capacity 2 was 2.4% slower. Capacity 4 was a whopping 10.5% slower. The assistant hypothesized that Zen4's jemalloc thread-local cache was so fast (~10–15ns per allocation) that the larger SmallVec stack frames (1020 bytes vs 144 bytes per enforce call) were causing L1d cache thrashing.

The user had explicitly requested perf stat hardware counter data to confirm this hypothesis before making a final decision. The assistant was preparing to run these benchmarks when it hit a surprising discovery.

The Discovery: Untracked Directories

In [msg 1042], the assistant checked the git state and found the diff was "smaller than expected." The git diff --stat showed changes only in extern/bellperson/ and extern/cuzk/ — but not in extern/bellpepper-core/ or extern/supraseal-c2/, where the SmallVec and CUDA optimizations lived. In [msg 1043], the assistant dug deeper and found the answer: git status reported those directories as untracked.

This was a significant finding. The project uses a dependency chain where cuzk-core depends on bellperson (patched to a local fork), which depends on bellpepper-core (also patched to a local fork). The CUDA proving backend is in a forked supraseal-c2. All three forks live under extern/ and are referenced via [patch.crates-io] in Cargo.toml. But somehow, bellpepper-core and supraseal-c2 were never added to the main repo's git tracking — they existed as untracked directories, likely because they were cloned or initialized separately rather than being committed as part of the feat/cuzk branch.

This meant that the assistant's mental model of "what changes are applied" might not match reality. The modifications to these files — the SmallVec change, the CUDA timing instrumentation, the parallel B_G2 loop, the per-MSM window split, the max_num_circuits bump — could theoretically have been lost or in a different state than expected. Before running any more benchmarks, the assistant needed to verify.

Message 1044: The Verification

The subject message consists of three grep commands, each probing a specific file for specific patterns that serve as markers for the applied optimizations:

Command 1: Checking bellpepper-core's SmallVec state

grep -n 'SmallVec\|smallvec\|INDEXER_INLINE_CAP' extern/bellpepper-core/src/lc.rs | head -20

The output confirms that the SmallVec change is still in place. Line 5 shows use smallvec::{smallvec, SmallVec}; — the import is present. Line 42 defines INDEXER_INLINE_CAP: usize = 2 — the inline capacity is set to 2 (the last tested configuration). Line 47 shows the Indexer struct using SmallVec<[(usize, T); INDEXER_INLINE_CAP]> as its values field. Lines 55 and 64 show SmallVec::new() and smallvec![(index, value)] being used in constructors.

This confirms that the A1 SmallVec optimization is still applied, with capacity 2. The assistant now knows the exact state of this file before deciding whether to revert it or run further benchmarks.

Command 2: Checking supraseal-c2's CUDA changes

grep -n 'CUZK_TIMING\|par_map\|msm_l\|msm_a\|msm_b\|cudaHostRegister' extern/supraseal-c2/cuda/groth16_cuda.cu | head -30

This is a multi-pattern grep checking for five distinct markers:

• CUZK_TIMING — the custom timing instrumentation added to measure GPU phases
• par_map — the parallel B_G2 loop (optimization A4)
• msm_l, msm_a, msm_b — the split MSM windows (optimization D4)
• cudaHostRegister — the B1 optimization that was already reverted The output shows lines 147–150 defining tail_msm_l_bases, tail_msm_a_bases, tail_msm_b_g1_bases, and tail_msm_b_g2_bases — confirming D4 is active. Line 163 shows groth16_pool.par_map(num_circuits, ...) — confirming A4 is active. Lines 259–261 show tail_msm_l_mask, tail_msm_a_mask, tail_msm_b_mask — more D4 evidence. Notably, cudaHostRegister does not appear in the output, confirming the B1 revert is intact. Command 3: Checking max_num_circuits

grep -n 'max_num_circuits' extern/supraseal-c2/cuda/groth16_srs.cuh | head -5

Line 64 shows static const int max_num_circuits = 30; — confirming the bump from 10 to 30 is in place.

The Thinking Process: Methodical Verification Under Uncertainty

What makes this message interesting is not the content of the grep output itself, but the reasoning that led to it. The assistant had been operating under an assumption — that all modified files in the workspace were tracked by git and would appear in git diff. When that assumption was falsified by the unexpectedly small diff in [msg 1042], the assistant didn't just shrug and move on. It investigated further ([msg 1043]), discovered the untracked directories, and then — crucially — verified the actual file contents before proceeding.

This is a textbook example of grounded engineering practice. The assistant could have assumed that because it had previously edited these files, they were still in the expected state. But the untracked directory discovery introduced uncertainty: had the files been modified in a different copy? Were they in a different branch? Could a build or checkout have overwritten them? The only way to resolve this uncertainty was to read the files directly.

The choice of grep over reading entire files is also telling. The assistant knows exactly which patterns to look for — each optimization leaves a distinctive fingerprint in the source code. INDEXER_INLINE_CAP for SmallVec, par_map for parallel B_G2, msm_l/msm_a/msm_b for split MSM windows, CUZK_TIMING for instrumentation, cudaHostRegister for the reverted optimization. These are not random searches; they are targeted probes for specific state markers. This reflects deep familiarity with the codebase and the optimization proposal.

Broader Implications

This message highlights a recurring challenge in multi-repository development workflows. The cuzk project uses Rust's [patch.crates-io] mechanism to override upstream dependencies with local forks. This is a powerful pattern — it allows modifying third-party libraries (bellpepper-core, bellperson, supraseal-c2) without waiting for upstream PRs. But it also creates a git tracking problem: these forked directories need to be managed as first-class citizens of the main repository, or their state can become opaque.

The untracked directory issue also raises questions about reproducibility. If the bellpepper-core and supraseal-c2 forks are not committed to the main repo, then checking out the feat/cuzk branch on a different machine would not reproduce the same working tree. The SmallVec changes, the CUDA timing instrumentation, the parallel B_G2 optimization — all of it would be missing. This is a latent risk that the assistant's verification step has surfaced.

What Comes Next

With the file contents confirmed, the assistant now has a solid foundation for the next steps: reverting A1 (SmallVec) back to Vec, running the perf stat benchmarks the user requested, and ultimately committing Phase 4 Wave 1 with the optimizations that actually help (A4 parallel B_G2, D4 per-MSM window, max_num_circuits bump, and the synth-bench tool). The verification in [msg 1044] ensures that these next steps operate on known, verified state rather than assumptions.

In the broader arc of the cuzk project, this message is a small but important moment — a pause for verification before proceeding with measurement. It demonstrates the kind of disciplined, uncertainty-aware engineering that characterizes the entire optimization campaign. The assistant doesn't just run benchmarks; it verifies its assumptions, checks its state, and ensures that every measurement is grounded in a known configuration. This is the difference between a black-box optimization and a transparent, reproducible one.