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This ranges between parity (for very small strings) and 5x faster (for 32-63B strings) in benchmarking on M1 MBP. For largeish strings it delivers a roughly 2x speedup; further increase in blocksize nets a small win in microbenchmarks that I do not expect would translate to real world usage due to codesize impact and the fact that most strings are smallish. There's some opportunity for further work here; in particular, if people start building Swift for a baseline of AVX2 or AVX512, we should have paths for that (and we should also implement them if/when we get better multiversioning dispatch machinery in the language). Span adoption would be interesting. It's likely we should have a dedicated "small core" implementation that uses only aligned accesses. Still, this is a significant improvement as-is, and we should land it. 