macOS Big Sur status report

[Guillaume Piolat]

Hello,

I'd like to post here to get you in touch with the future support 
of macOS Big Sur.



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1. Context


Future Mac comes with "Apple Silicon" CPU that are natively arm64.

On these system (out end of 2020), you can run both x86_64 and 
arm64 executables.

As of today, a D program works unchanged under macOS Big Sur 
(11.0). It is emulated. The new processors are really fast and 
it's surprisingly painless.

So there is no real urgency for transitioning, apart from keeping 
native performance. For maximum compatibility one would 
distribute "fat binaries", a arm64 executable stitched to a 
x86_64 executable with the "lipo" tool.


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2. druntime support

The triple is: arm64-apple-macos and currently it needs tweaks to

A druntime built for macOS 11.0 doesn't exist yet but is in the 
works.
druntime needs to be a bit adapted for Big Sur. It's mostly C 
symbols that used to have OSX-specific names; it requires 
extra-care.

You can thankfully hack around the current lack of runtime with 
LDC by linking with the druntime built for iOS.

See progress here: 
https://github.com/ldc-developers/ldc/issues/3559

I'm just unaware which druntime needs to be touched: the LDC one 
or upstream?


--------------

3. DUB support

It appears nothing needs to be done since DUB supports triple in 
the -a flag.

$ dub -a arm64-apple-macos


---------------------------

4. intel-intrinsics support

The idea is to keep the x86 semantics of intel-intrinsics and get 
it to work on arm64, at a slight performance loss. The extent of 
the impedence mismatch is unknown yet.

This is a similar idea to the "simde" library on Github.

For now the goal has been to get to the same semantics through 
the generic intrinsic implementations, and to get back 
performance later.

For now the goal was to find a way to respect the rounding 
semantics.

ARM have no MXCSR, rounding mode and flush-to-zero mode are put 
into the ARM's FPCSR control word instead.

As there is no new ARM instruction to round using the current 
rounding mode (which is definately a good design), we dispatch 
after reading that control word.

What is unsupported for now is:
- FP exceptions flags and FP exceptions masks
- most intrinsics should be really slow. It will need detailed 
work to ensure the fastest implementation of each and every x86 
intrinsic.


Thanks for reading and consider donating to LDC!