Make Type.GetGenericTypeDefinition() an intrinsic

src/coreclr/jit/compiler.h

@@ -4430,6 +4430,7 @@ class Compiler
     void impImportLeave(BasicBlock* block);
     void impResetLeaveBlock(BasicBlock* block, unsigned jmpAddr);
     GenTree* impTypeIsAssignable(GenTree* typeTo, GenTree* typeFrom);
+    GenTree* impGetGenericTypeDefinition(GenTree* type);
 
     // Mirrors StringComparison.cs
     enum StringComparison

src/coreclr/jit/fgbasic.cpp

@@ -1369,6 +1369,7 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
                             case NI_System_Type_get_IsPrimitive:
                             case NI_System_Type_get_IsByRefLike:
                             case NI_System_Type_GetTypeFromHandle:
+                            case NI_System_Type_GetGenericTypeDefinition:
                             case NI_System_String_get_Length:
                             case NI_System_Buffers_Binary_BinaryPrimitives_ReverseEndianness:
 #if defined(FEATURE_HW_INTRINSICS)

src/coreclr/jit/importer.cpp

@@ -2486,6 +2486,27 @@ GenTree* Compiler::impTypeIsAssignable(GenTree* typeTo, GenTree* typeFrom)
     return nullptr;
 }
 
+
+GenTree* Compiler::impGetGenericTypeDefinition(GenTree* type)
+{
+    // this intrinsic requires the first arg to be some `typeof()` expression,
+    // ie. it applies to cases such as `typeof(...).GetGenericTypeDefinition()`.
+    CORINFO_CLASS_HANDLE hClassType = NO_CLASS_HANDLE;
+    if (gtIsTypeof(type, &hClassType))
+    {
+        CORINFO_CLASS_HANDLE hClassResult = info.compCompHnd->getTypeDefinition(hClassType);
+
+        GenTree* retNode = gtNewIconEmbClsHndNode(hClassResult);
+
+        // Drop the typeof(T) node
+        impPopStack();
+
+        return retNode;
+    }
+
+    return nullptr;
+}
+
 /*****************************************************************************
  * 'logMsg' is true if a log message needs to be logged. false if the caller has
  *   already logged it (presumably in a more detailed fashion than done here)

src/coreclr/jit/importercalls.cpp

@@ -2719,6 +2719,9 @@ GenTree* Compiler::impIntrinsic(GenTree*                newobjThis,
             case NI_System_Type_op_Equality:
             case NI_System_Type_op_Inequality:
 
+            // This allows folding "typeof(...).GetGenericTypeDefinition() == typeof(...)"
+            case NI_System_Type_GetGenericTypeDefinition:
+
             // These may lead to early dead code elimination
             case NI_System_Type_get_IsValueType:
             case NI_System_Type_get_IsPrimitive:
@@ -3221,6 +3224,14 @@ GenTree* Compiler::impIntrinsic(GenTree*                newobjThis,
                 break;
             }
 
+            case NI_System_Type_GetGenericTypeDefinition:
+            {
+                GenTree* type = impStackTop(0).val;
+
+                retNode = impGetGenericTypeDefinition(type);
+                break;
+            }
+
             case NI_System_Type_op_Equality:
             case NI_System_Type_op_Inequality:
             {
@@ -9018,6 +9029,10 @@ NamedIntrinsic Compiler::lookupNamedIntrinsic(CORINFO_METHOD_HANDLE method)
                         {
                             result = NI_System_Type_GetTypeFromHandle;
                         }
+                        else if (strcmp(methodName, "GetGenericTypeDefinition") == 0)
+                        {
+                            result = NI_System_Type_GetGenericTypeDefinition;
+                        }
                         else if (strcmp(methodName, "IsAssignableFrom") == 0)
                         {
                             result = NI_System_Type_IsAssignableFrom;

src/coreclr/jit/namedintrinsiclist.h

@@ -84,6 +84,7 @@ enum NamedIntrinsic : unsigned short
     NI_System_Type_op_Equality,
     NI_System_Type_op_Inequality,
     NI_System_Type_GetTypeFromHandle,
+    NI_System_Type_GetGenericTypeDefinition,
     NI_System_Array_Clone,
     NI_System_Array_GetLength,
     NI_System_Array_GetLowerBound,

src/libraries/System.Private.CoreLib/src/System/Type.cs

@@ -74,6 +74,7 @@ public bool IsInterface
 
         public virtual int GetArrayRank() => throw new NotSupportedException(SR.NotSupported_SubclassOverride);
 
+        [Intrinsic]
         public virtual Type GetGenericTypeDefinition() => throw new NotSupportedException(SR.NotSupported_SubclassOverride);
         public virtual Type[] GenericTypeArguments => (IsGenericType && !IsGenericTypeDefinition) ? GetGenericArguments() : EmptyTypes;
         public virtual Type[] GetGenericArguments() => throw new NotSupportedException(SR.NotSupported_SubclassOverride);