[runtime] Avoid intermediate copy in FlatString::substring - #503
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Summary
Easy win:
FlatString::substringwas first copying the substring to a temporaryVec, allocating the new string, and then copying the contents into the new string. Avoid this unnecessary intermediate copy by allocating an uninitializedFlatString(with a new canonical creation utility) and then copying the bytes over directly.Notes:
FlatString::substring_by_byte_index+8.8% improvement to Octane RegExp benchmark from this change, and ~+0.8% overall increase to Octane score.
Tests