The Moment the Profiler Told the Truth: Eliminating Temporary Allocations in Groth16 Synthesis

In the course of optimizing a Groth16 SNARK proof generation pipeline for Filecoin's Proof-of-Replication (PoRep), an AI assistant made a single, deceptively simple edit that crystallized an entire session of performance detective work. The message — message 1192 in the conversation — reads in its entirety:

[assistant] This one is trickier — all 3 closures use |_| and create fresh LCs. Let me rewrite them to use add_to_lc: [edit] /home/theuser/curio/extern/bellpepper-core/src/gadgets/boolean.rs Edit applied successfully.

Beneath this brief surface lies a rich story of profiling-driven optimization, mistaken assumptions corrected by data, and the moment when a developer finally understood the true bottleneck in a ~55-second synthesis workload processing 130 million constraints. This article unpacks that single message — what led to it, what it reveals about the optimization process, and what knowledge it produced.

The Road to Message 1192: A Trail of Disappointing Optimizations

To understand why this message was written, one must trace the optimization journey that preceded it. The assistant had been implementing a series of synthesis optimizations drawn from a formal proposal document (c2-optimization-proposal-4.md). The first round included four changes: a SmallVec optimization for the LC Indexer (A1), pre-sizing for ProvingAssignment (A2), parallelizing B_G2 CPU MSMs (A4), and pinning a/b/c vectors with cudaHostRegister (B1), plus per-MSM window tuning (D4). Benchmarking revealed a regression, and careful isolation showed that the SmallVec change (A1) was causing a 5–6 second slowdown — a counterintuitive result that sent the assistant back to the drawing board.

The next round of optimization was more targeted. Based on perf stat analysis showing ~34% of runtime spent on jemalloc alloc/dealloc in the enforce hot loop, the assistant implemented three changes: a Vec recycling pool (arena allocator) to reuse six Vec allocations per enforce call, an interleaved A+B eval loop for better instruction-level parallelism, and software prefetch intrinsics in the eval loops. The result was underwhelming: only ~1% improvement (54.9s vs 55.5s baseline). Worse, perf stat revealed that instructions dropped 4.1% but IPC (instructions per cycle) also fell from 2.60 to 2.53 — the interleaved eval's more complex control flow was hurting pipeline utilization. The assistant reverted the interleaved eval, keeping only the recycling pool and prefetch.

The Perf Profile That Changed Everything

At this point, the assistant ran a deeper perf record session with call-graph data, producing a 3.6 GB profile of 229,841 samples. The results were the smoking gun. The top function by self-time was ProvingAssignment::enforce at 11.14%, but the truly revealing data was in the callees. UInt32::addmany consumed 6.82% of runtime, and Boolean::lc() — a method that creates a fresh LinearCombination from a boolean variable — consumed 6.51% all by itself. Critically, a search for symbols related to the recycling pool (pool, take, give, recycle, zero_recycled) returned zero samples. The recycling pool was simply not being used.

This was the key insight. The assistant had assumed that the allocation bottleneck was the six Vec allocations inside enforce — the three LinearCombination objects (A, B, C) that the constraint system creates per constraint. The recycling pool addressed exactly those six Vecs. But the perf data told a different story: the real allocation hotspot was inside the closures that circuit code passes to enforce. These closures call Boolean::lc() to create temporary LinearCombination objects — dozens of them per constraint in the SHA-256 circuit — and those allocations dwarfed the six Vecs that the recycling pool was designed to reuse.

The Pattern: |_| Means "I Don't Need Your Recycling"

The assistant dispatched a subagent to analyze how the circuit code uses enforce closures. The report showed that 84% of closures do use the passed-in lc (starting from the recycled zero), but 16% ignore it entirely using the |_| pattern. More importantly, even the 84% that use lc typically add terms from Boolean::lc() calls, creating temporary LCs that the recycling pool cannot capture.

The fix was to add new methods to Boolean — add_to_lc and sub_from_lc — that directly add or subtract a boolean's term(s) to an existing LinearCombination without creating a temporary LC object. This eliminates the Vec allocation inside Boolean::lc() at the hottest call sites. The assistant also added add_lc to Num for similar reasons.

Message 1191 (immediately preceding our target message) showed the code that triggered the realization. The assistant read the sha256_maj function in boolean.rs:

cs.enforce(
    || "maj computation",
    |_| {
        bc.lc(CS::one(), Scalar::ONE) + &bc.lc(CS::one(), Scalar::ONE)
            - &b.lc(CS::one(), Scalar::ONE)
            - &c.lc(CS::one(), Scalar::ONE)
    },
    ...

All three closures in this enforce call use |_| — they completely ignore the recycled lc that the constraint system passes in. Instead, they create four fresh LinearCombination objects via Boolean::lc(), then perform arithmetic that creates even more temporary LCs. This is the pattern that the recycling pool cannot touch.

Message 1192: The Recognition and the Response

Message 1192 is the assistant's direct response to seeing this code. The language is telling: "This one is trickier — all 3 closures use |_| and create fresh LCs." The word "trickier" acknowledges that the previous patches (to UInt32::addmany, Num::add_bool_with_coeff, sha256_ch) were simpler because those closures at least started from the passed-in lc. Here, the closures don't even accept the recycled buffer.

The assistant's decision is immediate and confident: "Let me rewrite them to use add_to_lc." This is not a speculative change — it's a direct application of the newly created add_to_lc/sub_from_lc API to the specific pattern that perf profiling identified as the bottleneck. The edit is applied, and the assistant moves on to patch lookup.rs (message 1193) and add sub_from_lc to Boolean (message 1195).

Assumptions, Correct and Incorrect

This message reveals several assumptions, some validated and some corrected by data:

Incorrect assumption: The assistant initially assumed that the allocation bottleneck was the six Vecs per enforce call (the A, B, C LinearCombination objects). The recycling pool was built on this assumption. Perf data proved it wrong — the real bottleneck was the temporary LCs created inside closures.

Correct assumption: The assistant assumed that Boolean::lc() was a major allocation source and that eliminating its temporary LCs would yield performance gains. This was validated by perf data showing 6.51% self-time for Boolean::lc().

Implicit assumption: The assistant assumed that the |_| pattern (ignoring the passed-in lc) was common enough in hot code to warrant patching. The sha256_maj function confirmed this — all three closures use |_|.

Assumption about the fix: The assistant assumed that add_to_lc/sub_from_lc methods would be sufficient to replace the temporary LC creation pattern. This required that the boolean's contribution could be expressed as a simple addition/subtraction to an existing LC, which is true for the linear constraints used in Groth16 synthesis.

Input and Output Knowledge

Input knowledge required to understand this message includes: understanding of Groth16 SNARK constraint systems (the enforce method creates three linear combinations A, B, C per constraint); familiarity with Rust's LinearCombination type and its Vec-backed storage; knowledge of the SHA-256 circuit structure (the sha256_ch and sha256_maj functions are the core of SHA-256's compression function); awareness of the Boolean::lc() method and its allocation behavior; and understanding of the |_| closure pattern in Rust.

Output knowledge created by this message is the specific patch to the sha256_maj function's enforce closures, replacing temporary LC creation with in-place add_to_lc/sub_from_lc calls. More broadly, it confirms the optimization strategy: targeted elimination of temporary allocations at the hottest call sites, rather than generic pooling. The message also produces a pattern for future patches — when you see |_| closures creating fresh LCs, that's the place to apply add_to_lc.

The Thinking Process

The assistant's thinking process, visible in the surrounding messages, follows a clear arc: hypothesis → implement → measure → analyze → new hypothesis. The recycling pool was hypothesis 1. Perf data disproved it. The Boolean::lc() hotspot was hypothesis 2. The add_to_lc API was the implementation. Message 1192 is the application of hypothesis 2 to the most stubborn code pattern — the |_| closures that completely bypass the recycling infrastructure.

The phrase "This one is trickier" reveals real-time cognition: the assistant recognizes that the sha256_maj pattern is qualitatively different from the previously patched sites. It's not just a matter of substituting bit.lc(...) with bit.add_to_lc(...) — the closures here build complex expressions from multiple lc() calls. The assistant's decision to "rewrite them" rather than attempt a mechanical substitution shows an understanding that the fix requires restructuring the closure logic.

This message, for all its brevity, represents the culmination of a data-driven optimization cycle: measure, identify the true bottleneck, create the tool to fix it, and apply that tool to the most impactful locations. It's a small edit with a large story behind it.