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As with other SIMD-accelerated functions in php-src, the new UTF-16 encoding and decoding routines can be compiled either with AVX2 acceleration "always on", "always off", or else with runtime detection of AVX2 support. With the new UTF-16 decoder/encoder, conversion of extremely short strings (as in several bytes) has the same performance as before, and conversion of medium-length (~100 character) strings is about 65% faster, but conversion of long (~10,000 character) strings is around 6 times faster. Many other mbstring functions will also be faster now when handling UTF-16; for example, mb_strlen is almost 3 times faster on medium strings, and almost 9 times faster on long strings. (Why does mb_strlen benefit more from AVX2 acceleration than mb_convert_encoding? It's because mb_strlen only needs to decode, but not re-encode, the input string, and the UTF-16 decoder benefits much more from SIMD acceleration than the UTF-16 encoder.)