[mono][wasm] Add interpreter support for calls to UnmanagedCallersOnly EntryPoint without managed delegate reference

[lewing]

#94615 (comment)

#94615 added EntryPoint support to UCO on wasm (it was missing before)

 
        [UnmanagedCallersOnly(EntryPoint = "display_meaning")]
        public static void DisplayMeaningExport(int meaning)
        {
            Console.WriteLine($"The meaning of life is {meaning}");
        }

but there is currently an issue with when invoking the export from unmanaged when there is no managed delegate pointing to it and something akin to the following workaround is required:

   Intptr workaround = (IntPtr)(delegate* unmanaged<int,void>)&display_meaning;  // needs to be in method loaded by interp prior to unmanaged call 

This is because get_native_to_interp must be called before attempting to invoke the unmanaged entry point or the call will fail. Without the call wasm_native_to_interp_ftndescs is never initialized. For the workaround to work the delegate * does not need to be reached/reachable, the interpreter simply needs to resolve the method prior to calling it from unmanaged.

[ghost]

Tagging subscribers to 'arch-wasm': @lewing
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Issue Details

---

#94615 (comment)

#94615 added EntryPoint support to UCO

 
        [UnmanagedCallersOnly(EntryPoint = "display_meaning")]
        public static void DisplayMeaningExport(int meaning)
        {
            Console.WriteLine($"The meaning of life is {meaning}");
        }

but there is currently an issue with when invoking the export from unmanaged when there is no managed delegate pointing to it.

   Intptr workaround = (IntPtr)(delegate* unmanaged<int,void>)&display_meaning;  // needs to be in method loaded by interp prior to unmanaged call 

Author:	lewing
Assignees:	-
Labels:	arch-wasm, area-VM-meta-mono
Milestone:	9.0.0

[ghost]

Tagging subscribers to this area: @BrzVlad, @kotlarmilos
See info in area-owners.md if you want to be subscribed.

Issue Details

---

#94615 (comment)

#94615 added EntryPoint support to UCO

 
        [UnmanagedCallersOnly(EntryPoint = "display_meaning")]
        public static void DisplayMeaningExport(int meaning)
        {
            Console.WriteLine($"The meaning of life is {meaning}");
        }

but there is currently an issue with when invoking the export from unmanaged when there is no managed delegate pointing to it.

   Intptr workaround = (IntPtr)(delegate* unmanaged<int,void>)&display_meaning;  // needs to be in method loaded by interp prior to unmanaged call 

unless get_native_to_interp is called before attempting to invoke the unmanaged entry point the call will fail because the n2m entry slot is not initialized.

Author:	lewing
Assignees:	lewing
Labels:	arch-wasm, area-Codegen-Interpreter-mono, area-VM-meta-mono, os-wasi, os-browser
Milestone:	9.0.0

[lewing]

@maraf this is essentially the same issue you are seeing with the JSExport for nativeAOT changes, by which I mean for mono we could drastically simplify the current initialization code if we fix the fallback case here

[lewing]

cc @vargaz @BrzVlad

[lewing]

@steveisok @vitek-karas this is blocking wasi work

[vargaz]

The generator probably needs to generate some extra code to lookup the MonoMethod*, and call something like mono_jit_compile_method () on it to have the interpreter initialize the pointers.

[vargaz]

Something like:

MonoMethod *
mono_marshal_get_managed_wrapper (MonoMethod *method, MonoClass *delegate_klass, MonoGCHandle target_handle, MonoError *error);

	MonoError error;
	mono_error_init (&error);
	MonoMethod* m = lookup_dotnet_method("Wasi.Console.Sample", "", "Test", "DisplayMeaningExport", 1);
	MonoMethod *w = mono_marshal_get_managed_wrapper (m, NULL, 0, &error);
	mono_compile_method (w);

Will force the pointers to be initialized. This can be done either on startup at the end of mono_wasm_load_runtime (), or on demand in the generated pinvoke func, i.e.

void display_meaning (int arg0) {
   if (!wasm_native_to_interp_ftndescs [2].func) <INIT>
  ((WasmInterpEntrySig_2)wasm_native_to_interp_ftndescs [2].func) ((int*)&arg0, wasm_native_to_interp_ftndescs [2].arg);
}

[knutwannheden]

@lewing Do you have any more details (or even better an example I can look at) on the mentioned workaround? I tried adding some code like that to the Main method, but that didn't really help.

[maraf]

Do you have any more details (or even better an example I can look at) on the mentioned workaround? I tried adding some code like that to the Main method, but that didn't really help.

@knutwannheden It's something that can't be done in user. I needs to happen in generate C code that works as a trampoline to call managed code

[knutwannheden]

Excellent news! Looking forward to the next release containing this fix ❤️