Why is Apple Rosetta 2 fast? (2022)

Post got the big one: Total Store Ordering (TSO).

The rest are all techniques in reasonably common use, but unless you have hardware support for x86's strong memory ordering, you cannot get very good x86-on-ARM performance, because it's by no means clear when strong memory ordering matters, and when it doesn't, inspecting existing code - so you have to liberally sprinkle memory barriers around, which really kill performance.

The huge and fast L1I/L1D cache doesn't hurt things either... emulation tends cache-intensive.

It's surprising that (AFAIK) Qualcomm didn't implement TSO in the chips they made for the recent-ish Windows ARM machines. If anything they need fast x86 emulation even more than Apple does since Windows has a much longer tail of software support than macOS, there's going to be important Windows apps that stubbornly refuse to support native ARM basically forever.

Microsoft's AoT+JiT techniques still pull off impressive performance (90+% in almost every case, 96-99% in the majority).

But yes, if they were actually serious about Windows on ARM, they would have implemented TSO in their "custom" Qualcomm SQ1/SQ2 chips.

Last time I checked, the default behavior for Microsoft's translation was to pretend that the hardware is doing TSO, and hope it works out. So that should obviously be fast, but occasionally wrong.