8329994: Zap alignment padding bits for ArrayOops in non-release builds

Reviewed-by: ayang, sjohanss

src/hotspot/share/gc/shared/memAllocator.cpp

@@ -388,6 +388,7 @@ oop ObjArrayAllocator::initialize(HeapWord* mem) const {
   assert(_length >= 0, "length should be non-negative");
   if (_do_zero) {
     mem_clear(mem);
+    mem_zap_start_padding(mem);
     mem_zap_end_padding(mem);
   }
   arrayOopDesc::set_length(mem, _length);
@@ -395,6 +396,20 @@ oop ObjArrayAllocator::initialize(HeapWord* mem) const {
 }
 
 #ifndef PRODUCT
+void ObjArrayAllocator::mem_zap_start_padding(HeapWord* mem) const {
+  const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
+  const size_t base_offset_in_bytes = arrayOopDesc::base_offset_in_bytes(element_type);
+  const size_t header_size_in_bytes = arrayOopDesc::header_size_in_bytes();
+
+  const address base = reinterpret_cast<address>(mem) + base_offset_in_bytes;
+  const address header_end = reinterpret_cast<address>(mem) + header_size_in_bytes;
+
+  if (header_end < base) {
+    const size_t padding_in_bytes = base - header_end;
+    Copy::fill_to_bytes(header_end, padding_in_bytes, heapPaddingByteVal);
+  }
+}
+
 void ObjArrayAllocator::mem_zap_end_padding(HeapWord* mem) const {
   const size_t length_in_bytes = static_cast<size_t>(_length) << ArrayKlass::cast(_klass)->log2_element_size();
   const BasicType element_type = ArrayKlass::cast(_klass)->element_type();

src/hotspot/share/gc/shared/memAllocator.hpp

@@ -94,6 +94,7 @@ protected:
   const int  _length;
   const bool _do_zero;
 
+  void mem_zap_start_padding(HeapWord* mem) const PRODUCT_RETURN;
   void mem_zap_end_padding(HeapWord* mem) const PRODUCT_RETURN;
 
 public:

src/hotspot/share/gc/z/zObjArrayAllocator.cpp

@@ -139,6 +139,8 @@ oop ZObjArrayAllocator::initialize(HeapWord* mem) const {
     return true;
   };
 
+  mem_zap_start_padding(mem);
+
   if (!initialize_memory()) {
     // Re-color with 11 remset bits if we got intercepted by a GC safepoint
     const bool result = initialize_memory();