Yeah, modern CPUs are great at executing garbage code reasonably fast. Getting binary-translated code to come close to native performance is still difficult.

Apple obviously had an advantage as they also control the hardware (and Arm helped by adding some extensions to simplify translation); significantly easing two difficult parts of translating from x86: TSO and status flags. AVX is somewhat annoying (256-bit regs -> 128-bit regs, frequent merging of scalar values), but manageable.

> You would like to be able to precisely handle exceptions (especially ones caused by invalid memory accesses) while presenting a userspace exception handler with an architecturally valid state for the source program.

This is absolutely annoying and makes many optimizations much more difficult as a lot of additional state needs to be kept around, either for real or in metadata for reconstruction (including weird status flags, fun with partially written flags (inc/dec), maybe-written flags (shift/rotate), etc.). Does Rosetta 2 always have precise status flags (including PF/AF) at every possibly-faulting memory access? (This should be rarely needed in practice, so I've never implemented flag recovery in my own binary translators (primarily for research, Instrew but also non-public).)