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