1 /*
   2  * Copyright (c) 1997, 2009, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 // arrayOopDesc is the abstract baseclass for all arrays.  It doesn't
  26 // declare pure virtual to enforce this because that would allocate a vtbl
  27 // in each instance, which we don't want.
  28 
  29 // The layout of array Oops is:
  30 //
  31 //  markOop
  32 //  klassOop  // 32 bits if compressed but declared 64 in LP64.
  33 //  length    // shares klass memory or allocated after declared fields.
  34 
  35 
  36 class arrayOopDesc : public oopDesc {
  37   friend class VMStructs;
  38 
  39   // Interpreter/Compiler offsets
  40 
  41   // Header size computation.
  42   // The header is considered the oop part of this type plus the length.
  43   // Returns the aligned header_size_in_bytes.  This is not equivalent to
  44   // sizeof(arrayOopDesc) which should not appear in the code.
  45   static int header_size_in_bytes() {
  46     size_t hs = align_size_up(length_offset_in_bytes() + sizeof(int),
  47                               HeapWordSize);
  48 #ifdef ASSERT
  49     // make sure it isn't called before UseCompressedOops is initialized.
  50     static size_t arrayoopdesc_hs = 0;
  51     if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
  52     assert(arrayoopdesc_hs == hs, "header size can't change");
  53 #endif // ASSERT
  54     return (int)hs;
  55   }
  56 
  57  public:
  58   // The _length field is not declared in C++.  It is allocated after the
  59   // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
  60   // it occupies the second half of the _klass field in oopDesc.
  61   static int length_offset_in_bytes() {
  62     return UseCompressedOops ? klass_gap_offset_in_bytes() :
  63                                sizeof(arrayOopDesc);
  64   }
  65 
  66   // Returns the offset of the first element.
  67   static int base_offset_in_bytes(BasicType type) {
  68     return header_size(type) * HeapWordSize;
  69   }
  70 
  71   // Returns the address of the first element.
  72   void* base(BasicType type) const {
  73     return (void*) (((intptr_t) this) + base_offset_in_bytes(type));
  74   }
  75 
  76   // Tells whether index is within bounds.
  77   bool is_within_bounds(int index) const        { return 0 <= index && index < length(); }
  78 
  79   // Accessors for instance variable which is not a C++ declared nonstatic
  80   // field.
  81   int length() const {
  82     return *(int*)(((intptr_t)this) + length_offset_in_bytes());
  83   }
  84   void set_length(int length) {
  85     *(int*)(((intptr_t)this) + length_offset_in_bytes()) = length;
  86   }
  87 
  88   // Should only be called with constants as argument
  89   // (will not constant fold otherwise)
  90   // Returns the header size in words aligned to the requirements of the
  91   // array object type.
  92   static int header_size(BasicType type) {
  93     size_t typesize_in_bytes = header_size_in_bytes();
  94     return (int)(Universe::element_type_should_be_aligned(type)
  95       ? align_object_offset(typesize_in_bytes/HeapWordSize)
  96       : typesize_in_bytes/HeapWordSize);
  97   }
  98 
  99   // Return the maximum length of an array of BasicType.  The length can passed
 100   // to typeArrayOop::object_size(scale, length, header_size) without causing an
 101   // overflow.
 102   static int32_t max_array_length(BasicType type) {
 103     assert(type >= 0 && type < T_CONFLICT, "wrong type");
 104     assert(type2aelembytes(type) != 0, "wrong type");
 105     const int bytes_per_element = type2aelembytes(type);
 106     if (bytes_per_element < HeapWordSize) {
 107       return max_jint;
 108     }
 109 
 110     const int32_t max_words = align_size_down(max_jint, MinObjAlignment);
 111     const int32_t max_element_words = max_words - header_size(type);
 112     const int32_t words_per_element = bytes_per_element >> LogHeapWordSize;
 113     return max_element_words / words_per_element;
 114   }
 115 };