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In this sub-session, the systematic diagnosis of the Phase 4 performance regression continued. The A2 pre-sizing optimization was fully reverted from the remaining call site in pipeline.rs, and the build was cleaned to force CUDA recompilation. After discovering that CUDA printf output was lost due to buffering, fflush was added to collect the first CUZK_TIMING breakdown, which identified B1 (cudaHostRegister) as the primary GPU culprit, adding 5.7 seconds of overhead. B1 was reverted, reducing total proof time from 101.3s to 94.4s. To isolate the remaining synthesis regression, a synth-only microbenchmark was built, and A/B testing of the A1 SmallVec change across four configurations (Vec, SmallVec cap=1/2/4) conclusively showed that SmallVec causes a ~5–6s regression in synthesis time regardless of inline capacity. The sub-session concluded with preparations to gather low-level perf stat hardware counters to understand why SmallVec is slower on the AMD Zen4 system.

The Regression That Taught a Thousand Lessons: Systematic Diagnosis of the Phase 4 Optimization Wave in the cuzk SNARK Prover 3353 words

Chunks

• Systematic Regression Diagnosis: How Instrumentation, Microbenchmarks, and the Discipline to Revert Saved the Phase 4 Optimization Campaign
  2141 words
• The First CUZK_TIMING Breakdown: How Printf Buffering Almost Derailed a Performance Diagnosis
  2455 words
• The Anatomy of a Counterintuitive Regression: How Systematic Diagnosis Uncovered SmallVec's Hidden Cost on AMD Zen4
  2420 words