Faster "obj is T[]" for sealed T

src/coreclr/jit/importer.cpp

@@ -12416,7 +12416,8 @@ GenTree* Compiler::impCastClassOrIsInstToTree(
     bool                  partialExpand   = false;
     const CorInfoHelpFunc helper          = info.compCompHnd->getCastingHelper(pResolvedToken, isCastClass);
 
-    GenTree* exactCls = nullptr;
+    GenTree* exactCls         = nullptr;
+    bool     isExpandingArray = false;
 
     // Legality check.
     //
@@ -12429,12 +12430,23 @@ GenTree* Compiler::impCastClassOrIsInstToTree(
             // Jit can only inline expand the normal CHKCASTCLASS helper.
             canExpandInline = (helper == CORINFO_HELP_CHKCASTCLASS);
         }
-        else
+        else if (helper == CORINFO_HELP_ISINSTANCEOFCLASS)
         {
-            if (helper == CORINFO_HELP_ISINSTANCEOFCLASS)
+            // If the class is exact, the jit can expand the IsInst check inline.
+            canExpandInline = isClassExact;
+        }
+
+        if (isClassExact && ((helper == CORINFO_HELP_ISINSTANCEOFARRAY) || (helper == CORINFO_HELP_CHKCASTARRAY)))
+        {
+            // We have "obj isinst/castclass T[]" and in case if T is sealed we can still perform
+            // the fast pMT check (in case if T[] -> obj.GetType is legal in jit-time)
+
+            CORINFO_CLASS_HANDLE elementCls = NO_CLASS_HANDLE;
+            info.compCompHnd->getChildType(pResolvedToken->hClass, &elementCls);
+            if ((elementCls != NO_CLASS_HANDLE) && impIsClassExact(elementCls))
             {
-                // If the class is exact, the jit can expand the IsInst check inline.
-                canExpandInline = isClassExact;
+                canExpandInline  = true;
+                isExpandingArray = true;
             }
         }
 
@@ -12565,7 +12577,7 @@ GenTree* Compiler::impCastClassOrIsInstToTree(
         //
         // use the special helper that skips the cases checked by our inlined cast
         //
-        const CorInfoHelpFunc specialHelper = CORINFO_HELP_CHKCASTCLASS_SPECIAL;
+        const CorInfoHelpFunc specialHelper = isExpandingArray ? helper : CORINFO_HELP_CHKCASTCLASS_SPECIAL;
 
         condTrue = gtNewHelperCallNode(specialHelper, TYP_REF, partialExpand ? op2 : op2Var, gtClone(op1));
     }