LLM agent produces runtime-compiled regex engine delivering 2x-4x gains on long ripgrep queries

The agent iterated for one month against the rebar benchmark suite before a holdout set forced generalization. The resulting FRE engine exposed a native code path that the agent then integrated into ripgrep via a background thread, switching matchers once compilation completed. Short queries paid a thread penalty while long searches showed the reported speedup on representative traces.

Data from the holdout run confirmed that native AOT variants outperformed the baseline matcher once search duration exceeded roughly eight seconds. Most complex patterns still required the original engine for the initial segment, limiting average gains. Cache locality improvements mirrored demoscene techniques previously documented in FFTW adaptive code generation.

Operationally this pattern extends to any workload where a narrow class of inputs dominates latency tails. Teams can now emit domain-specific JIT components without maintaining compiler expertise, shifting the cost from months of engineering to minutes of prompt iteration. Risks include overfitting to observed traces and increased binary surface for each generated matcher.

Next steps include automated verification of holdout performance before deployment and extension to other hot paths such as JSON parsers or compression codecs already targeted by similar agent loops.