187
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1 /*
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2 * Copyright 2001-2005 Sun Microsystems, Inc. All Rights Reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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20 * CA 95054 USA or visit www.sun.com if you need additional information or
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21 * have any questions.
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22 *
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23 */
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24
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25 //
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26 // Free block maintenance for Concurrent Mark Sweep Generation
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27 //
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28 // The main data structure for free blocks are
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29 // . an indexed array of small free blocks, and
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30 // . a dictionary of large free blocks
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31 //
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32
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33 // No virtuals in FreeChunk (don't want any vtables).
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34
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35 // A FreeChunk is merely a chunk that can be in a doubly linked list
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36 // and has a size field. NOTE: FreeChunks are distinguished from allocated
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37 // objects in two ways (by the sweeper), depending on whether the VM is 32 or
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38 // 64 bits.
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39 // In 32 bits or 64 bits without CompressedOops, the second word (prev) has the
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40 // LSB set to indicate a free chunk; allocated objects' klass() pointers
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41 // don't have their LSB set. The corresponding bit in the CMSBitMap is
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42 // set when the chunk is allocated. There are also blocks that "look free"
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43 // but are not part of the free list and should not be coalesced into larger
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44 // free blocks. These free blocks have their two LSB's set.
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45
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46 class FreeChunk VALUE_OBJ_CLASS_SPEC {
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47 friend class VMStructs;
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48 // For 64 bit compressed oops, the markOop encodes both the size and the
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49 // indication that this is a FreeChunk and not an object.
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50 volatile size_t _size;
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51 FreeChunk* _prev;
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52 FreeChunk* _next;
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53
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54 markOop mark() const volatile { return (markOop)_size; }
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55 void set_mark(markOop m) { _size = (size_t)m; }
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56
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57 public:
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58 NOT_PRODUCT(static const size_t header_size();)
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59
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60 // Returns "true" if the address indicates that the block represents
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61 // a free chunk.
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62 static bool indicatesFreeChunk(const HeapWord* addr) {
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63 // Force volatile read from addr because value might change between
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64 // calls. We really want the read of _mark and _prev from this pointer
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65 // to be volatile but making the fields volatile causes all sorts of
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66 // compilation errors.
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67 return ((volatile FreeChunk*)addr)->isFree();
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68 }
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69
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70 bool isFree() const volatile {
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71 LP64_ONLY(if (UseCompressedOops) return mark()->is_cms_free_chunk(); else)
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72 return (((intptr_t)_prev) & 0x1) == 0x1;
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73 }
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74 bool cantCoalesce() const {
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75 assert(isFree(), "can't get coalesce bit on not free");
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76 return (((intptr_t)_prev) & 0x2) == 0x2;
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77 }
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78 void dontCoalesce() {
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79 // the block should be free
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80 assert(isFree(), "Should look like a free block");
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81 _prev = (FreeChunk*)(((intptr_t)_prev) | 0x2);
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82 }
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83 FreeChunk* prev() const {
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84 return (FreeChunk*)(((intptr_t)_prev) & ~(0x3));
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85 }
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86
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87 debug_only(void* prev_addr() const { return (void*)&_prev; })
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88
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89 size_t size() const volatile {
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90 LP64_ONLY(if (UseCompressedOops) return mark()->get_size(); else )
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91 return _size;
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92 }
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93 void setSize(size_t sz) {
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94 LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::set_size_and_free(sz)); else )
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95 _size = sz;
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96 }
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97
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98 FreeChunk* next() const { return _next; }
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99
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100 void linkAfter(FreeChunk* ptr) {
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101 linkNext(ptr);
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102 if (ptr != NULL) ptr->linkPrev(this);
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103 }
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104 void linkAfterNonNull(FreeChunk* ptr) {
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105 assert(ptr != NULL, "precondition violation");
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106 linkNext(ptr);
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107 ptr->linkPrev(this);
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108 }
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109 void linkNext(FreeChunk* ptr) { _next = ptr; }
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110 void linkPrev(FreeChunk* ptr) {
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111 LP64_ONLY(if (UseCompressedOops) _prev = ptr; else)
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112 _prev = (FreeChunk*)((intptr_t)ptr | 0x1);
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113 }
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114 void clearPrev() { _prev = NULL; }
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115 void clearNext() { _next = NULL; }
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116 void markNotFree() {
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117 LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::prototype());)
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118 // Also set _prev to null
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119 _prev = NULL;
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120 }
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121
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122 // Return the address past the end of this chunk
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123 HeapWord* end() const { return ((HeapWord*) this) + size(); }
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124
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125 // debugging
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126 void verify() const PRODUCT_RETURN;
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127 void verifyList() const PRODUCT_RETURN;
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128 void mangleAllocated(size_t size) PRODUCT_RETURN;
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129 void mangleFreed(size_t size) PRODUCT_RETURN;
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130 };
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131
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132 // Alignment helpers etc.
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133 #define numQuanta(x,y) ((x+y-1)/y)
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134 enum AlignmentConstants {
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135 MinChunkSize = numQuanta(sizeof(FreeChunk), MinObjAlignmentInBytes) * MinObjAlignment
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136 };
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137
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