1 /*
   2  * Copyright 1997-2006 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
  20  * CA 95054 USA or visit www.sun.com if you need additional information or
  21  * have any 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, except here.
  45   static int header_size_in_bytes() {
  46     size_t hs = UseCompressedOops ?
  47             sizeof(arrayOopDesc) :
  48             align_size_up(sizeof(arrayOopDesc) + sizeof(int), HeapWordSize);
  49 #ifdef ASSERT
  50     // make sure it isn't called before UseCompressedOops is initialized.
  51     static size_t arrayoopdesc_hs = 0;
  52     if (arrayoopdesc_hs == 0) arrayoopdesc_hs = hs;
  53     assert(arrayoopdesc_hs == hs, "header size can't change");
  54 #endif // ASSERT
  55     return (int)hs;
  56   }
  57 
  58  public:
  59   // The _length field is not declared in C++.  It is allocated after the
  60   // declared nonstatic fields in arrayOopDesc if not compressed, otherwise
  61   // it occupies the second half of the _klass field in oopDesc.
  62   static int length_offset_in_bytes() {
  63     return UseCompressedOops ? klass_gap_offset_in_bytes() :
  64                                sizeof(arrayOopDesc);
  65   }
  66 
  67   // Returns the offset of the first element.
  68   static int base_offset_in_bytes(BasicType type) {
  69     return header_size(type) * HeapWordSize;
  70   }
  71 
  72   // Returns the address of the first element.
  73   void* base(BasicType type) const {
  74     return (void*) (((intptr_t) this) + base_offset_in_bytes(type));
  75   }
  76 
  77   // Tells whether index is within bounds.
  78   bool is_within_bounds(int index) const        { return 0 <= index && index < length(); }
  79 
  80   // Accessors for instance variable which is not a C++ declared nonstatic
  81   // field.
  82   int length() const {
  83     return *(int*)(((intptr_t)this) + length_offset_in_bytes());
  84   }
  85   void set_length(int length) {
  86     *(int*)(((intptr_t)this) + length_offset_in_bytes()) = length;
  87   }
  88 
  89   // Should only be called with constants as argument
  90   // (will not constant fold otherwise)
  91   // Returns the header size in words aligned to the requirements of the
  92   // array object type.
  93   static int header_size(BasicType type) {
  94     size_t typesize_in_bytes = header_size_in_bytes();
  95     return (int)(Universe::element_type_should_be_aligned(type)
  96       ? align_object_size(typesize_in_bytes/HeapWordSize)
  97       : typesize_in_bytes/HeapWordSize);
  98   }
  99 
 100   // This method returns the  maximum length that can passed into
 101   // typeArrayOop::object_size(scale, length, header_size) without causing an
 102   // overflow. We substract an extra 2*wordSize to guard against double word
 103   // alignments.  It gets the scale from the type2aelembytes array.
 104   static int32_t max_array_length(BasicType type) {
 105     assert(type >= 0 && type < T_CONFLICT, "wrong type");
 106     assert(type2aelembytes(type) != 0, "wrong type");
 107     // We use max_jint, since object_size is internally represented by an 'int'
 108     // This gives us an upper bound of max_jint words for the size of the oop.
 109     int32_t max_words = (max_jint - header_size(type) - 2);
 110     int elembytes = type2aelembytes(type);
 111     jlong len = ((jlong)max_words * HeapWordSize) / elembytes;
 112     return (len > max_jint) ? max_jint : (int32_t)len;
 113   }
 114 
 115 };