OK. (The one time I encountered SIMD in C++ was a decade ago; we had to use assembly then. And even then, we weren't playing with an up-to-the-minute compiler. (In my experience, your comments elsewhere on this article about embedded toolchains were accurate.)

Are the intrinsics standard? If so, what version of the standard were they in? How good is compiler support for them?

Not standard, unless you count "supported with the same semantics in multiple compilers" as good enough.

They are not portable between CPU architectures either way

For reference, C got real alignment support in C11 and C++ in C++17. Without this, even starting on SIMD is risky.

That said, you can go really far with a good C++ math library like Eigen which has singe vectorization in the operations themselves.