When using Clang org CC (possibly other compilers, too), you don’t need to use intrinsics. These compilers have some support for vector types (https://clang.llvm.org/docs/LanguageExtensions.html#vectors-..., https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html)

One can, for example, write:

  typedef int v4si __attribute__ ((vector_size (16)));

  v4si a, b, c;
  […]
  b += 3; // add 3 to each of b’s elements
  c = a + b; // pairwise addition

Using this gives up some control over assembly; it doesn’t guarantee vector instructions get used, but it makes it easier for the compiler to generate them.

Also, I’m fairly sure there are vector instructions not covered by this extension.

CPU intrinsics are just another way of doing inline Assembly.

IANA C/C++/ASM expert, but one thing I've been thinking of would greatly benefit from type-punning (overlaying 2 kinds of data and interpreting one as the other) because it can use a 64-bit register to parallelise 4 ops at once where speed really, really matters.

That's trivial in ASM I believe, but don't know how well it's possible in C/C++, if at all.

I fail to see why not just use a __m64 variable and do the conversion.

https://software.intel.com/sites/landingpage/IntrinsicsGuide...

You can also try to make a union whose one field is a __m64, but beware not to trigger UB conditions.

Type punning in general would be somewhat difficult to do here in a way that vectorized but was still defined by the standard. You’d need to put a branch with undefined behavior in it when alignment wasn’t satisfied, then do a memcpy into a vector type, and hope that the compiler understands what you’re doing and doesn’t deoptimize it.

False or vacuously true

That is still writing asm. You need to write it, the compiler cannot do it by itself.