Popcount CPU instruction (2019)
On the PS3 SPU cores, the best "does everything" instruction was the single "rotate and mask" series of instructions which was the underlying implementation of all rotates, shifts, and bitfield extraction instructions. That architecture is still my favourite years on - every register was vectorised by default, and the instruction was the minimalistic set to achieve a lot of unusual operations that were incredibly useful for graphics or signal processing work. Often you couldn't find an exact instruction to do what you wanted, but if you studied the documentation long enough, you'd find one that generalised it and actually did do what you wanted, as well as gracefully handling loads of other edge cases you hadn't thought of - for instance the various instructions to create shuffle masks so that lots of byte/word/long re-ordering operations could be done with the single shuffle instruction, which could be used to emulate unaligned memory access using aligned-only memory accesses.Not exactly weird, but on the PowerPC side "eieio", is my most favourite name for an instruction on any architecture - without fail, every time I come across it, I start humming Old McDonald!