src/hotspot/share/gc/x/xObjArrayAllocator.cpp

*** 48,11 ***
    // A max segment size of 64K was chosen because microbenchmarking
    // suggested that it offered a good trade-off between allocation
    // time and time-to-safepoint
    const size_t segment_max = XUtils::bytes_to_words(64 * K);
    const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
!   const size_t header = arrayOopDesc::header_size(element_type);
    const size_t payload_size = _word_size - header;
  
    if (payload_size <= segment_max) {
      // To small to use segmented clearing
      return ObjArrayAllocator::initialize(mem);
--- 48,21 ---
    // A max segment size of 64K was chosen because microbenchmarking
    // suggested that it offered a good trade-off between allocation
    // time and time-to-safepoint
    const size_t segment_max = XUtils::bytes_to_words(64 * K);
    const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
! 
+   // Clear leading 32 bits, if necessary.
+   int base_offset = arrayOopDesc::base_offset_in_bytes(element_type);
+   if (!is_aligned(base_offset, HeapWordSize)) {
+     assert(is_aligned(base_offset, BytesPerInt), "array base must be 32 bit aligned");
+     *reinterpret_cast<jint*>(reinterpret_cast<char*>(mem) + base_offset) = 0;
+     base_offset += BytesPerInt;
+   }
+   assert(is_aligned(base_offset, HeapWordSize), "remaining array base must be 64 bit aligned");
+ 
+   const size_t header = heap_word_size(base_offset);
    const size_t payload_size = _word_size - header;
  
    if (payload_size <= segment_max) {
      // To small to use segmented clearing
      return ObjArrayAllocator::initialize(mem);

*** 61,12 ***
    // Segmented clearing
  
    // The array is going to be exposed before it has been completely
    // cleared, therefore we can't expose the header at the end of this
    // function. Instead explicitly initialize it according to our needs.
!   arrayOopDesc::set_mark(mem, markWord::prototype());
!   arrayOopDesc::release_set_klass(mem, _klass);
    assert(_length >= 0, "length should be non-negative");
    arrayOopDesc::set_length(mem, _length);
  
    // Keep the array alive across safepoints through an invisible
    // root. Invisible roots are not visited by the heap itarator
--- 71,16 ---
    // Segmented clearing
  
    // The array is going to be exposed before it has been completely
    // cleared, therefore we can't expose the header at the end of this
    // function. Instead explicitly initialize it according to our needs.
!   if (UseCompactObjectHeaders) {
!     arrayOopDesc::release_set_mark(mem, _klass->prototype_header());
+   } else {
+     arrayOopDesc::set_mark(mem, markWord::prototype());
+     arrayOopDesc::release_set_klass(mem, _klass);
+   }
    assert(_length >= 0, "length should be non-negative");
    arrayOopDesc::set_length(mem, _length);
  
    // Keep the array alive across safepoints through an invisible
    // root. Invisible roots are not visited by the heap itarator