The compiler can't autovectorize because it can't assume it's okay to read past the end of the string. Only the human or the machine can make that determination, using special knowledge about the environment, such as that when locating the NUL byte out-of-bounds reads are okay so long as they don't cross a page boundary. And "okay" is a stretch because tools like Valgrind ship with huge lists of manually maintained suppressions to account for hacks like this that violate the standard rules.