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.

My guess is that the sort of "legacy x86-forever" apps for Windows don't really need much in the way of performance. Think your classic Visual Basic 6 sort of thing that a business relies on for decades.

I'm also fairly certain that the TSO changes to the memory system are non-trivial, and it's possible that Qualcomm doesn't see it as a value-add in their chips - and they're probably right. Windows machines are such a hot mess that outside a relatively small group of users (who probably run Linux anyway, so aren't anyone's target market), nobody would know or care what TSO is. If it add costs and power and doesn't matter, why bother?

> My guess is that the sort of "legacy x86-forever" apps for Windows don't really need much in the way of performance.

Games are a pretty notable exception that demand high performance and for the most part will be stuck on x86 forever. Brand new games might start shipping native ARM Windows binaries if the platform gets enough momentum, but games have very limited support lifecycles so it's unlikely that many released before that point will ever be updated to ARM native.