What's fun is that, as the use of SIMD in your example is useless, LLVM correctly completely removes it, and makes your "neon" and "fallback" versions exactly t

What's fun is that, as the use of SIMD in your example is useless, LLVM correctly completely removes it, and makes your "neon" and "fallback" versions exactly the same - without any SIMD (compiler explorer: https://godbolt.org/z/YWoMGoaxT https://godbolt.org/z/YWoMGoaxT). As an additional note, aarch64 always has NEON (similar to how x86-64 always has SSE2; extensions useful to dispatch would be SVE on aarch64 and AVX/AVX2/AVX-512 on x86-64), so no point dynamically checking for it.