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.

> Qualcomm didn't implement TSO in the chips they made

I’m not sure they can do that.

Under Technology License Agreement, Qualcomm can build chips using ARM-designed CPU cores. Specifically, Qualcomm SQ1 uses ARM Cortex A76/A55 for the fast/slow CPU cores.

I don’t think using ARM designed cores is enough to implement TSO, need custom ARM cores instead of the stock ones. To design custom ARM cores, Qualcomm needs architecture license from ARM which was recently been cancelled.