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][mac-transition]. 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][rosetta-2]
translator, the translation will almost certainly introduce a significant
performance penalty.

[mac-transition]: https://en.wikipedia.org/wiki/Mac_transition_to_Apple_Silicon
[rosetta-2]: https://en.wikipedia.org/wiki/Rosetta_(software)#Rosetta_2


Description
-----------

An AArch64 port already exists for Linux ([JEP 237][jep-237]), and work
is underway on an AArch64 port for Windows ([JEP 388][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)][w-x].
The HotSpot VM routinely creates and modifies executable code, so this
JEP will implement W^X support in HotSpot for macOS/AArch64.

[jep-237]: https://openjdk.java.net/jeps/237
[jep-388]: https://openjdk.java.net/jeps/388
[w-x]: https://en.wikipedia.org/wiki/W%5EX


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`][write-protect] 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.

[write-protect]: https://developer.apple.com/documentation/apple_silicon/porting_just-in-time_compilers_to_apple_silicon



Dependencies
-----------

The macOS/AArch64 port and the Windows/AArch64 port ([JEP 388][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.