The Pre-Commit Ritual: Reading the Diff Before Phase 12 Lands

In the final moments of a long and intricate optimization session, the assistant issues a seemingly mundane command:

[assistant] [bash] git diff --cached --stat
 extern/bellperson/src/groth16/mod.rs              |   5 +-
 extern/bellperson/src/groth16/prover/supraseal.rs | 163 ++++-
 extern/cuzk/cuzk-core/src/engine.rs               | 731 +++++++++++++---------
 extern/cuzk/cuzk-core/src/pipeline.rs             |  71 ++-
 extern/supraseal-c2/cuda/groth16_cuda.cu          | 366 +++++++----
 extern/supraseal-c2/src/lib.rs                    | 378 +++++++++++
 6 files changed, 1275 insertions(+), 439 deletions(-)

This is message [msg 3050], the subject of this article. On its surface, it is nothing more than a developer checking what files are staged for commit — a routine pre-commit sanity check performed thousands of times a day by programmers worldwide. But in the context of this coding session, this single command represents a moment of culmination: the assistant is about to commit Phase 12 of a months-long optimization campaign targeting the SUPRASEAL_C2 Groth16 proof generation pipeline for Filecoin's Proof-of-Replication (PoRep). The git diff --cached --stat is the final verification before the work is permanently recorded in the repository's history.

Why This Message Was Written: The Pre-commit Ritual

The assistant had just executed git add on the five modified files and one new file ([msg 3049]), staging them for commit. Before actually running git commit, it pauses to inspect what is about to be committed. This is not an idle curiosity — it is a deliberate quality-control step. In the context of an AI-assisted coding session where changes span multiple interconnected subsystems (Rust FFI, C++ CUDA kernels, Go daemon orchestration), verifying the staged diff serves several critical purposes.

First, it confirms that only the intended files are included. The assistant had been working across the supraseal-c2 CUDA code, the bellperson Groth16 Rust implementation, the cuzk-core engine and pipeline, and the supraseal-c2 Rust FFI bindings. Staging the wrong file or missing a critical change could break the build or introduce subtle bugs. The --cached flag specifically shows what is staged, not the working tree — it answers the question "what will go into the commit?"

Second, the --stat flag provides a high-level summary without overwhelming detail. The assistant does not need to read every line of diff at this moment; it needs to verify that the magnitude and distribution of changes make sense. A 731-line change to engine.rs is expected given the extensive restructuring of the worker loop for the split API. A 378-line addition to lib.rs matches the new FFI functions added for start_groth16_proof and finalize_groth16_proof. The 366-line change to the CUDA file reflects the use-after-free fix and the provers_owned heap allocation. The numbers tell a coherent story.

Third, this check serves as a final sanity gate before the commit permanently alters the project's history. The assistant had just discovered and fixed a critical use-after-free bug ([msg 3028]-[msg 3031]) where the prep_msm_thread background thread captured a dangling reference to a stack-allocated array. That fix is included in these staged changes. Verifying the diff ensures that the fix, along with all other Phase 12 work, is properly captured.

What the Output Reveals: A Story in Numbers

The git diff --cached --stat output is terse but remarkably informative. Six files were changed, with a total of 1,275 insertions and 439 deletions — a net addition of 836 lines. The distribution tells a story about the nature of Phase 12.

extern/cuzk/cuzk-core/src/engine.rs — 731 lines changed (+++++-----): This is the heart of the Phase 12 restructuring. The engine's worker loop was rewritten to use the split API: instead of calling a monolithic generate_groth16_proof that blocks until the entire proof is ready, the engine now calls prove_start to begin GPU computation and prove_finalize to retrieve the result after the b_g2_msm background work completes. This required restructuring the async task graph, adding new state tracking for pending proofs, and integrating the finalizer task that polls for completion. The 731-line delta reflects this deep structural change.

extern/supraseal-c2/src/lib.rs — 378 lines added (+++++++): This is the Rust FFI layer that bridges the C++ CUDA implementation to the Rust-side Groth16 prover. The split API required two new entry points — start_groth16_proof and finalize_groth16_proof — along with the PendingGpuProof handle type, the SynthesisCapacityHint struct, and the glue code to manage the lifecycle of in-flight GPU proofs. This file is entirely new in the staged diff (378 insertions, 0 deletions), reflecting the fact that the split API is a new capability, not a modification of existing code.

extern/supraseal-c2/cuda/groth16_cuda.cu — 366 lines changed (+++++----): The CUDA implementation received the use-after-free fix (copying the provers array into pp->provers_owned for the background thread) and the early deallocation of NTT evaluation vectors (a, b, c) after the GPU kernel region completes. These changes are critical for correctness and memory efficiency, respectively.

extern/bellperson/src/groth16/prover/supraseal.rs — 163 lines changed (+++--): This file implements the Rust-side Groth16 prover that calls into the FFI. The changes here adapt the prover to use the new split API, managing the PendingProofHandle and coordinating the start/finalize lifecycle.

extern/cuzk/cuzk-core/src/pipeline.rs — 71 lines changed (++--): The pipeline module received the PendingGpuProof type alias and supporting infrastructure for the split API integration.

extern/bellperson/src/groth16/mod.rs — 5 lines changed (+-): A minimal change, likely a re-export or trait adjustment needed to make the split API work from the bellperson Groth16 module.

Assumptions Embedded in This Message

The assistant makes several assumptions when issuing this command. It assumes that git diff --cached accurately reflects what will be committed — that no unstaged changes exist that should also be included. It assumes that the diff statistics are a reliable proxy for the nature and correctness of the changes. It assumes that a quick visual scan of the file list and line counts is sufficient to catch obvious problems before committing.

These assumptions are reasonable in a normal development workflow, but they are not foolproof. A change that touches exactly the right files with plausible line counts could still contain logical errors. The assistant's pre-commit check is a heuristic, not a proof.

Knowledge Required and Created

To fully understand this message, one needs knowledge of Git's staging workflow (git add, git diff --cached, --stat), the structure of the cuzk project (the extern/ directory layout, the Rust/C++/CUDA split), and the Phase 12 optimization context (the split API, the use-after-free bug, the memory capacity wall). Without this context, the command appears to be a trivial git invocation.

The message creates knowledge about the scope of Phase 12. Any developer reading this commit summary later can immediately see that Phase 12 touched six files, added 1,275 lines, removed 439, and involved deep changes to the engine, the FFI layer, and the CUDA kernel code. This becomes part of the project's historical record.

The Thinking Process Visible in the Surrounding Messages

The surrounding messages reveal the assistant's thought process leading up to this moment. In [msg 3046], the assistant reflects on the OOM results: "pw=10 peaked at 367 GiB, pw=12 peaked at 668 GiB. That's 301 GiB difference for just 2 extra workers. That's way more than 2 × 13 GiB = 26 GiB. Something else is going on..." This shows the assistant reasoning about the non-linear memory scaling and concluding it's a systemic capacity issue, not a leak.

In [msg 3047], the assistant cleans up by killing the daemon and RSS monitor processes. In [msg 3048], it checks git status to see what files are dirty. In [msg 3049], it stages the files with git add. And finally, in the subject message, it verifies the staged diff before committing.

This sequence reveals a disciplined workflow: diagnose → fix → benchmark → clean up → stage → verify → commit. The assistant is not just writing code; it is managing the entire software engineering lifecycle, including the often-overlooked ritual of pre-commit verification.

Conclusion

Message [msg 3050] is, on its surface, a trivial git command. But in the context of this coding session, it represents the culmination of a complex optimization effort spanning multiple subsystems, languages, and paradigms. The git diff --cached --stat output tells a story of 1,275 new lines of code, 439 deleted, across six files — a snapshot of the Phase 12 split GPU proving API as it prepares to enter the project's permanent record. It is a moment of pause before the commit, a final check that everything is in order, and a quiet acknowledgment that the work is complete.