Chunk 15.0

In this chunk, the assistant completed the Phase 4 synthesis hot path optimizations by benchmarking and validating the Boolean::add_to_lc/sub_from_lc methods. Microbenchmarks showed synthesis time dropped from ~55.4s to ~50.9s (8.3% improvement), with `perf stat` confirming 91 billion fewer instructions (-15.3%) and 18.6 billion fewer branches (-26.7%). The assistant then ran a full E2E proof, which completed in 87.5s total, but revealed a GPU wrapper regression (36.0s vs 34.0s baseline) despite identical CUDA internal timing (~26s). Investigating the 10-second GPU timing gap, the assistant instrumented the C++ code and discovered the root cause: synchronous destructor overhead from freeing ~37 GB of C++ vectors (split_vectors, tail_msm bases) and ~130 GB of Rust Vecs (ProvingAssignment a/b/c) after GPU proving. The fix was to move these large allocations into detached threads on both the C++ and Rust sides, allowing the function to return immediately while deallocation happens in the background. After this fix, the GPU wrapper time dropped to 26.2s (matching CUDA internal time exactly), and the total E2E time improved to 77.2s — a 13.2% reduction from the 88.9s baseline. The themes of this chunk are profiling-driven performance optimization, identifying and eliminating hidden overheads (both synthesis allocations and destructor blocking), and the importance of instrumenting timing at every layer to pinpoint regressions. All Phase 4 changes were committed as `2da2a901`, including Boolean::add_to_lc, async deallocation, Vec recycling pool, software prefetch, and the A4/D4 CUDA changes.

The Two-Front War: How Surgical Synthesis Optimization and Async Deallocation Delivered 13.2% in Phase 4 2398 words

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