JEP 391: macOS/AArch64 Port

Summary

Port the JDK to macOS/AArch64.

Non-Goals

• It is not a goal to implement all optional components (e.g., compiler intrinsics), even if they are implemented in other AArch64 ports.

• It is not a goal to support the write-xor-execute (W^X) memory-protection policy for targets other than macOS/AArch64.

Motivation

Apple has announced a long-term plan to transition their line of Macintosh computers from x64 to AArch64. We therefore expect to see broad demand for a macOS/AArch64 port of the JDK.

Although it will be possible to run a macOS/x64 build of the JDK on AArch64-based systems via macOS's built-in Rosetta 2 translator, the translation will almost certainly introduce a significant performance penalty.

Description

An AArch64 port already exists for Linux (JEP 237), and work is underway on an AArch64 port for Windows (JEP 388). We expect to reuse existing AArch64 code from these ports by employing conditional compilation — as is usual in ports of the JDK — to accommodate differences in low-level conventions such as the application binary interface (ABI) and the set of reserved processor registers.

macOS/AArch64 forbids memory segments from being executable and writeable at the same time, a policy known as write-xor-execute (W^X). The HotSpot VM routinely creates and modifies executable code, so this JEP will implement W^X support in HotSpot for macOS/AArch64.

Testing

Testing will include, but not be limited to, compatibility testing with the TCK, regression testing with jtreg, and validation with applications. The execution environment will include development platforms available from Apple as well as consumer hardware, once it becomes available.

Risks and Assumptions

• The changes for macOS/AArch64 risk breaking the existing Linux/AArch64, Windows/AArch64, and macOS/x64 ports. This risk will be reduced via extensive pre-integration testing.

• We expect to be able to implement the new ABI convention with reasonably small changes in the shared AArch64 code. We expect the footprint of the macOS-specific code to be small.

• We expect the macOS/AArch64 and Windows/AArch64 ports to be similar in some ways, allowing some code to be shared across these ports and further reducing the macOS-specific AArch64 code.

• We assume that the new version of macOS will not differ substantially from past versions, so that the amount of code change required for the new version will be small.

• We expect that supporting the W^X policy will be aided by operating-system services such as the pthread_jit_write_protect_np system call. If not, we will develop alternative approaches. The first implementation will target correctness with a possible performance penalty in uncommon cases, such as deoptimizations.

Dependencies

The macOS/AArch64 port and the Windows/AArch64 port (JEP 388) will likely share some code. Some parts of this JEP will depend upon the integration of JEP 388, while other parts can be developed in parallel.