Look into ReadyToRun

[rolfbjarne]

We should look into restoring support for (partial) AOT for macOS in .NET.

It looks like this is called ReadyToRun in the CoreCLR world.

Doc: https://learn.microsoft.com/en-us/dotnet/core/deploying/ready-to-run

[filipnavara]

Note that the framework assemblies are already shipped with precompiled R2R images. That's why assemblies like System.Collections.dll from the framework end up different for osx-x64 and osx-arm64 builds (mostly visible in universal app).

[rolfbjarne]

It doesn't look like we'll have time to look into this for .NET 7, so I'm postponing until .NET 8.

[rolfbjarne]

This seems relatively trivial to implement, I got a very simple hello world working like this:

https://gist.github.com/rolfbjarne/6a185790ab8fbc4fb6663aaac9d31fdd

There are two issues that prevents it from working out-of-the-box:

1. The ready to run targets get confused with assemblies whose RelativePath metadata is set to a path that includes any directory components, because the actual compilation target writes output files to the publish directory, but then there is other logic that uses RelativePath to compute the expected output path - and things break when those don't match.
2. We have to update the RelativePath of the *.r2r.dll file the ready to run compilation produces.

[rolfbjarne]

I've filed dotnet/runtime#79902 to see if the issue with RelativePath metadata can be fixed, or if we'll have to come up with a workaround.