GitHub engineers explain how they achieved >45 GiB/s case-folding throughput for their Blackbird code search engine, which indexes 480TB of source code. The key insight is counterintuitive: removing an early-exit branch (that stopped at the first non-ASCII byte) and making the loop entirely branch-free enables LLVM to auto-vectorize it, yielding a 15x speedup over the naive approach. The post details the full design: a branch-free ASCII sweep using byte-space arithmetic, a compact 1776-byte Unicode lookup table using a page bitmap and run-length encoding, and a novel decode-free fold technique that performs UTF-8 case folding as direct byte arithmetic without ever decoding to a code point. The result is open-sourced as the 'casefold' Rust crate.
