Using Covariant returns for RuntimeType crashes during crossgen

[benaadams]

This PR #45355 crashes in crossgen

Using WinDebug it crashed on this line

runtime/src/coreclr/src/vm/array.cpp

Line 290 in 29e9b5b

DWORD numVirtuals = pParentClass->GetNumVirtuals();

Due to g_pArrayClass being a null ptr

runtime/src/coreclr/src/vm/array.cpp

Lines 270 to 271 in 29e9b5b

	// this is the base for every array type
	MethodTable *pParentClass = g_pArrayClass;

/cc @janvorli

[janvorli]

@benaadams it seems that the fix here is simple. With the change below, I was able to successfully build runtime:

diff --git a/src/coreclr/src/vm/appdomain.cpp b/src/coreclr/src/vm/appdomain.cpp
index f703b4214ce..87cc347fe10 100644
--- a/src/coreclr/src/vm/appdomain.cpp
+++ b/src/coreclr/src/vm/appdomain.cpp
@@ -1448,13 +1448,13 @@ void SystemDomain::LoadBaseSystemClasses()
     g_pEnumClass = CoreLibBinder::GetClass(CLASS__ENUM);
     _ASSERTE(!g_pEnumClass->IsValueType());

+    // Load Array class
+    g_pArrayClass = CoreLibBinder::GetClass(CLASS__ARRAY);
+
     // Load System.RuntimeType
     g_pRuntimeTypeClass = CoreLibBinder::GetClass(CLASS__CLASS);
     _ASSERTE(g_pRuntimeTypeClass->IsFullyLoaded());

-    // Load Array class
-    g_pArrayClass = CoreLibBinder::GetClass(CLASS__ARRAY);
-
     // Calling a method on IList<T> for an array requires redirection to a method on
     // the SZArrayHelper class. Retrieving such methods means calling
     // GetActualImplementationForArrayGenericIListMethod, which calls FetchMethod for

[janvorli]

I am running coreclr tests with your change and this fix to verify everything is fine.

[benaadams]

Is a suggestion some of the types might get linked out #45355 (comment); which would be a different reason if the tests fail :)

[janvorli]

So unfortunately the fix doesn't work at runtime. Attempt to run anything crashes. I need to figure out why.

[benaadams]

Might be ILLink step removing too many things?

[janvorli]

It didn't seem so. It looked like a problem with the swapped order of those two class loads, but it was a checked build and so some details were obscured by the compiler optimizations. I am building a debug build now to be able to debug with comfort.

[benaadams]

Jit diff now works with your fix #45356 (comment) 🙂

Which I was trying to use to evaluate if covariant returns made much difference, and indeed it looks to allow devirtualization: