8249945: Improve ARRAY_SIZE()

Make ARRAY_SIZE type-safe.

Reviewed-by: tschatzl, lfoltan, dholmes

src/hotspot/share/utilities/globalDefinitions.hpp

@@ -1095,7 +1095,11 @@ template<class T> static void swap(T& a, T& b) {
   b = tmp;
 }
 
-#define ARRAY_SIZE(array) (sizeof(array)/sizeof((array)[0]))
+// array_size_impl is a function that takes a reference to T[N] and
+// returns a reference to char[N].  It is not ODR-used, so not defined.
+template<typename T, size_t N> char (&array_size_impl(T (&)[N]))[N];
+
+#define ARRAY_SIZE(array) sizeof(array_size_impl(array))
 
 //----------------------------------------------------------------------------------------------------
 // Sum and product which can never overflow: they wrap, just like the

test/hotspot/gtest/utilities/test_globalDefinitions.cpp

@@ -223,3 +223,30 @@ TEST(globalDefinitions, log2) {
   EXPECT_EQ_LOG2(log2_uint, uint);
   EXPECT_EQ_LOG2(log2_jlong, jlong);
 }
+
+TEST(globalDefinitions, array_size) {
+  const size_t test_size = 10;
+
+  {
+    int test_array[test_size] = {};
+    static_assert(test_size == ARRAY_SIZE(test_array), "must be");
+  }
+
+  {
+    double test_array[test_size] = {};
+    static_assert(test_size == ARRAY_SIZE(test_array), "must be");
+  }
+
+  struct ArrayElt { int x; };
+
+  {
+    ArrayElt test_array[test_size] = {};
+    static_assert(test_size == ARRAY_SIZE(test_array), "must be");
+  }
+
+  {
+    const ArrayElt test_array[] = { {0}, {1}, {2}, {3}, {4}, {5} };
+    static_assert(6 == ARRAY_SIZE(test_array), "must be");
+  }
+
+}