Decode complexity is a real cost to x86-64 but it's not a huge one. Well, for medium devices in the Pentium range it can be pretty significant but for modern l

Decode complexity is a real cost to x86-64 but it's not a huge one. Well, for medium devices in the Pentium range it can be pretty significant but for modern large cores it'll only end up costing 5% or so of the total power budget on x86. And there are some design effects where ARM cores will tend to make the decode unit big enough that it's never the limiting factor whereas x86 cores tend to balance the size of the decoder against the other stages more. More significant than that, I think, is that modern 64 bit ARM is actually tend to produce denser code than modern 64 bit x86 - which is really weird when you consider that ARM-64 is fixed width and x86-64 is variable width but backwards compatibility can do that. And more important than that, however, is that x86-64 has a very strict memory model and ARM a more relaxed one which can cause problems for programmers who aren't careful but has huge implications for the design of the memory backend.