[dotnet] Support interpreter-only mode in the simulator

[lambdageek]

For .NET 6 hot reload to work, and for other scenarios, we need to be able to run apps using the interpreter on the simulator.

Currently calling the following code from AppDelegate.FinishedLaunching returns "JIT"

using System.Runtime.CompilerServices;

public string GetExecutionMode()
{
  if (!RuntimeFeature.IsDynamicCodeSupported)
    return "AOT";
  if (RutnimeFeature.IsDynamicCodeCompiled)
    return "JIT";
  return "Interpreter";
}

[spouliot]

Application.mtouch.cs is only used for mtouch, which is not used for dotnet (only legacy)

Are you seeing the MT0141 error ?

[lambdageek]

One way to test whether the interpreter is enabled is using

using System.Runtime.CompilerServices;

public string GetExecutionMode()
{
  if (!RuntimeFeature.IsDynamicCodeSupported)
    return "AOT";
  if (RutnimeFeature.IsDynamicCodeCompiled)
    return "JIT";
  return "Interpreter";
}

This test could be adjusted to (a) run, not just build; (b) try to use the above method to detect the execution engine.

https://github.com/xamarin/xamarin-macios/blob/5c12fdfac92277cd19cb4c73daa083ddc75d781b/tests/dotnet/MyInterpretedApp/MyInterpretedApp.csproj

[spouliot]

(a) run, not just build

No, those projects don't have a way to communicate success/failure back to the runner. They exists so the tooling, like msbuild can be verified, or to check for files expected presence (or absence), app bundle size...

Even if we added a success/failure execution report on run it would not tell us much. The interpreter could be enabled but could be crashing in various places that not part of the simple app.

To achieve more useful results we run existing unit tests under several configurations, including the interpreter (at least for legacy). That validates the build and that the feature works across a large set of tests.

If would be easy to add a dotnet specific test for RuntimeFeature so we know it returns the expected values under the different build configuration.

[spouliot]

Example from xharness: All configurations of monotouch-test for iOS Unified 64-bits - device

iOS Unified 64-bits - device (Ignored)Run all Build all
+AssemblyBuildTarget: dylib (debug) (Ignored) Run Build
+AssemblyBuildTarget: dylib (debug) [dotnet] (Ignored) Run Build
+AssemblyBuildTarget: dylib (debug, profiling) (Ignored) Run Build
+AssemblyBuildTarget: dylib (debug, profiling) [dotnet] (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (debug) (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (debug) [dotnet] (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (debug, profiling) (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (debug, profiling) [dotnet] (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (release) (Ignored) Run Build
+AssemblyBuildTarget: SDK framework (release) [dotnet] (Ignored) Run Build
+Debug (Ignored) Run Build
+Debug (all optimizations) (Ignored) Run Build
+Debug (all optimizations) [dotnet] (Ignored) Run Build
+Debug (dynamic registrar) (Ignored) Run Build
+Debug (dynamic registrar) [dotnet] (Ignored) Run Build
+Debug (interpreter -mscorlib) (Ignored) Run Build
+Debug (interpreter -mscorlib) [dotnet] (Ignored) Run Build
+Debug (interpreter) (Ignored) Run Build
+Debug (interpreter) [dotnet] (Ignored) Run Build
+Debug [dotnet] (Ignored) Run Build
+Debug: SGenConc (Ignored) Run Build
+Debug: SGenConc [dotnet] (Ignored) Run Build
+Release (Ignored) Run Build
+Release (all optimizations) (Ignored) Run Build
+Release (all optimizations) [dotnet] (Ignored) Run Build
+Release (interpreter -mscorlib) (Ignored) Run Build
+Release (interpreter -mscorlib) [dotnet] (Ignored) Run Build
+Release [dotnet] (Ignored) Run Build

[lambdageek]

Application.mtouch.cs is only used for mtouch, which is not used for dotnet (only legacy)

Are you seeing the MT0141 error ?

No I'm not. I guess it must be something else that is affecting the execution engine choice. Sorry. let me update the PR description with just the observable behavior and not my guesses.

[lambdageek]

Update MONO_AOT_MODE_INTERP is the right thing to use for iOS device and iOS Simulator (particularly on Apple Silicon macs).

---

Hmm... I have a sneaking suspicion that this is due to MONO_AOT_MODE_INTERP

https://github.com/xamarin/xamarin-macios/blob/b720b4482736b784105385bcf23f86dcb1d0c28a/tools/common/Target.cs#L764

I think that (as opposed to MONO_AOT_MODE_INTERP_ONLY) means "use interpreter as the fallback execution engine". which on simulator (since we're using the JIT) means "try to JIT; if it fails (it won't), use interpreter".

The mono tests in dotnet/runtime use MONO_AOT_MODE_INTERP_ONLY.

I'm going to try changing the runtime tests to use MONO_AOT_MODE_INTERP and check what behavior we get there.

[lambdageek]

I think that [MONO_AOT_MODE_INTERP] (as opposed to MONO_AOT_MODE_INTERP_ONLY) means "use interpreter as the fallback execution engine". which on simulator (since we're using the JIT) means "try to JIT; if it fails (it won't), use interpreter".

Hmm... actually I don't know that this is correct. I think it's possible the IsDynamicCodeSupported/Compiled intrinsics might not be telling me the right things. Need to check some actual dynamic code and see if it's JITed or interpreted

[lambdageek]

So here's what needs to happen to get the interpreter working on iOS simulator:

1. We need to compile at least System.Private.CoreLib.dll with mono --aot=interp and make the resulting aot image available to the runtime. It's ok to AOT other assemblies with --aot=interp, but it's not necessary. the S.P.C AOT image contains all the necessary interpreter trampolines that we care about.
2. Runtime should start with MONO_AOT_MODE_INTERP.

On iOSSimulator-arm64 we will run into W^X issues until a runtime pack that includes dotnet/runtime#53197 is used (I think .NET 6 Preview 6), but iOSSimulator-x64 should be ok now.

@spouliot I think the steps above are similar to what is used for the interpreter on ios devices (although maybe it uses mono --aot=interp,full, not mono --aot=interp) do you know what is the best way to fix enable the interp for the simulators too?