Ensure syncblock is cleared with GC_ALLOC_ZEROING_OPTIONAL (dotnet#97174

)

Fixes dotnet#96790

src/coreclr/vm/gchelpers.cpp

@@ -434,7 +434,8 @@ OBJECTREF AllocateSzArray(MethodTable* pArrayMT, INT32 cElements, GC_ALLOC_FLAGS
             // dummy object.
             // If the GC gives us a 8 byte aligned address, we use it for the array and place the dummy
             // object after the array, otherwise we put the dummy object first, shifting the base of
-            // the array to an 8 byte aligned address.
+            // the array to an 8 byte aligned address. Also, we need to make sure that the syncblock of the
+            // second object is zeroed. GC won't take care of zeroing it out with GC_ALLOC_ZEROING_OPTIONAL.
             //
             // Note: on 64 bit platforms, the GC always returns 8 byte aligned addresses, and we don't
             // execute this code because DATA_ALIGNMENT < sizeof(double) is false.
@@ -447,14 +448,24 @@ OBJECTREF AllocateSzArray(MethodTable* pArrayMT, INT32 cElements, GC_ALLOC_FLAGS
             orArray = (ArrayBase*)Alloc(totalSize + MIN_OBJECT_SIZE, flags);
 
             Object* orDummyObject;
-            if ((size_t)orArray % sizeof(double))
+            if (((size_t)orArray % sizeof(double)) != 0)
             {
                 orDummyObject = orArray;
                 orArray = (ArrayBase*)((size_t)orArray + MIN_OBJECT_SIZE);
+                if (flags & GC_ALLOC_ZEROING_OPTIONAL)
+                {
+                    // clean the syncblock of the aligned array.
+                    *(((void**)orArray)-1) = 0;
+                }
             }
             else
             {
                 orDummyObject = (Object*)((size_t)orArray + totalSize);
+                if (flags & GC_ALLOC_ZEROING_OPTIONAL)
+                {
+                    // clean the syncblock of the dummy object.
+                    *(((void**)orDummyObject)-1) = 0;
+                }
             }
             _ASSERTE(((size_t)orArray % sizeof(double)) == 0);
             orDummyObject->SetMethodTable(g_pObjectClass);