If you want to use instructions from an extension (for example SIMD), you either: provide 2 versions of the function, or just won't run in some CPUs. It would be weird for someone that doesn't know about that to compile it for x86 and then have it not run on another x86 machine. I don't think compilers use those instructions if you don't tell them too.

Anyway, the SIMD the compilers will do is nowhere near the amount that it's possible. If you manually use SIMD intrinsics/inline SIMD assembly, chances are that it will be faster than what the compiler would do. Especially because you are reducing the % of CPUs your program can run on.