Along with its plan to transition their Macintosh line from Intel CPUs to its own CPUs, dubbed Apple Silicon, Apple announced Rosetta 2, a binary translation software that aims to smooth out the process. Thanks to Rosetta 2, most x86 programs will be able to execute after an initial translation step.
Apple started to use binary translation technology for the first time in 2006, when they began switching from PowerPC CPUs to x86. Based on QuickTransit, a technology originally developed by Transitive Corporation, later acquired by IBM, Rosetta is mostly transparent to users. The only side-effect users may perceive is their apps launching or running more slowly at times.
A slow launch is mostly the case the first time an app is launched. Indeed, this is when Rosetta usually kicks in; when the OS detects that the binary only includes x86_64 instructions. According to Joe Rossignol, writing for Mac Rumors, Microsoft indicated that the first launch of any of its Mac apps took approximately 20 seconds, while subsequent launches were fast. To reduce the impact of this initial translation step, Rosetta 2 is also able to translate an app AOT when it is installed. It is not clear yet for which apps Rosetta AOT translation will be supported.
While it is true that macOS will prefer running an app's arm64 instructions when they are available, the user can override this behaviour on an app-by-app basis. This could be required, for example, to run an app that has already been ported to Apple Silicon except for some legacy plugins or other kinds of binary extensions that the user depends on. In fact, one limitation of Rosetta is it will not enable mixing X86_64 and arm64 instructions in the same process.
Based on past experience with Rosetta, Rosetta 2 will be of great help to smooth the transition to Apple Silicon for both end users and developers. This does not rule out, though, that in a number of cases users will need to wait until the software they need will be released with native support for arm64. In particular, Rosetta 2 will not be able to translate kernel extensions nor will it support virtualization of x86_64 platforms. The latter implies that Rosetta 2 will not translate virtual machines such as VMWare and VirtualBox, nor Docker.
One major concern with Rosetta is performance. With the transition from PPC to x86, one factor slowing down Rosetta was the different byte ordering used by the two platforms, with PowerPC being a big-endian architecture, and x86 little-endian. While byte ordering is not a problem for the transition from x86 to ARM, another issue related to memory, namely the memory consistency model total store ordering (TSO), could hamper performance in this case. To prevent this from happening, Apple added support for x86 memory ordering to the M1 CPU, as Robert Graham noted on Twitter.
4/ So Apple simply cheated. They added Intel's memory-ordering to their CPU. When running translated x86 code, they switch the mode of the CPU to conform to Intel's memory ordering.
— Robᵉʳᵗ Graham, provocateur (@ErrataRob) November 25, 2020
In addition to this, as Graham describes, Apple has been implementing a number of other "tricks" to improve their chip performance, including speeding up JavaScript execution, retaining and releasing memory faster, and so on.
One specific bit of Rosetta 2 that sparked the interest of several developers on Hacker News is its ability to also translate apps that contain just-in-time compilers. The exact mechanics about this are not publicly documented, but Apple might be using a page fault to detect when the code attempts to jump into a recently created code page -- that is, roughly, a page that was set to writable mode, then switched to read-only and executable. When the page fault is handled, Rosetta will translate that page content.
As a final note, apps have a chance to detect when they are run under Rosetta.