AArch64 is, effectively, a totally new instruction set. It was designed to allow for fully regular decoding, unlike AArch32 & Thumb, which is a big part of why

AArch64 is, effectively, a totally new instruction set. It was designed to allow for fully regular decoding, unlike AArch32 & Thumb, which is a big part of why getting those off of the chip is a big win – it's not just the decoders you clean up, you can now make your pipeline and speculation deeper because you can peek N instructions into the future without having to decode the N-1 instructions in front of it first to figure out where the Nth instruction lies. That's part of why you see AArch64-only designs like this and the A12+ cores with much larger reorder buffers.