1 /* 2 * Copyright (c) 2001, 2010, 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 // 26 // Free block maintenance for Concurrent Mark Sweep Generation 27 // 28 // The main data structure for free blocks are 29 // . an indexed array of small free blocks, and 30 // . a dictionary of large free blocks 31 // 32 33 // No virtuals in FreeChunk (don't want any vtables). 34 35 // A FreeChunk is merely a chunk that can be in a doubly linked list 36 // and has a size field. NOTE: FreeChunks are distinguished from allocated 37 // objects in two ways (by the sweeper), depending on whether the VM is 32 or 38 // 64 bits. 39 // In 32 bits or 64 bits without CompressedOops, the second word (prev) has the 40 // LSB set to indicate a free chunk; allocated objects' klass() pointers 41 // don't have their LSB set. The corresponding bit in the CMSBitMap is 42 // set when the chunk is allocated. There are also blocks that "look free" 43 // but are not part of the free list and should not be coalesced into larger 44 // free blocks. These free blocks have their two LSB's set. 45 46 class FreeChunk VALUE_OBJ_CLASS_SPEC { 47 friend class VMStructs; 48 // For 64 bit compressed oops, the markOop encodes both the size and the 49 // indication that this is a FreeChunk and not an object. 50 volatile size_t _size; 51 FreeChunk* _prev; 52 FreeChunk* _next; 53 54 markOop mark() const volatile { return (markOop)_size; } 55 void set_mark(markOop m) { _size = (size_t)m; } 56 57 public: 58 NOT_PRODUCT(static const size_t header_size();) 59 60 // Returns "true" if the address indicates that the block represents 61 // a free chunk. 62 static bool indicatesFreeChunk(const HeapWord* addr) { 63 // Force volatile read from addr because value might change between 64 // calls. We really want the read of _mark and _prev from this pointer 65 // to be volatile but making the fields volatile causes all sorts of 66 // compilation errors. 67 return ((volatile FreeChunk*)addr)->isFree(); 68 } 69 70 bool isFree() const volatile { 71 LP64_ONLY(if (UseCompressedOops) return mark()->is_cms_free_chunk(); else) 72 return (((intptr_t)_prev) & 0x1) == 0x1; 73 } 74 bool cantCoalesce() const { 75 assert(isFree(), "can't get coalesce bit on not free"); 76 return (((intptr_t)_prev) & 0x2) == 0x2; 77 } 78 void dontCoalesce() { 79 // the block should be free 80 assert(isFree(), "Should look like a free block"); 81 _prev = (FreeChunk*)(((intptr_t)_prev) | 0x2); 82 } 83 FreeChunk* prev() const { 84 return (FreeChunk*)(((intptr_t)_prev) & ~(0x3)); 85 } 86 87 debug_only(void* prev_addr() const { return (void*)&_prev; }) 88 debug_only(void* next_addr() const { return (void*)&_next; }) 89 debug_only(void* size_addr() const { return (void*)&_size; }) 90 91 size_t size() const volatile { 92 LP64_ONLY(if (UseCompressedOops) return mark()->get_size(); else ) 93 return _size; 94 } 95 void setSize(size_t sz) { 96 LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::set_size_and_free(sz)); else ) 97 _size = sz; 98 } 99 100 FreeChunk* next() const { return _next; } 101 102 void linkAfter(FreeChunk* ptr) { 103 linkNext(ptr); 104 if (ptr != NULL) ptr->linkPrev(this); 105 } 106 void linkAfterNonNull(FreeChunk* ptr) { 107 assert(ptr != NULL, "precondition violation"); 108 linkNext(ptr); 109 ptr->linkPrev(this); 110 } 111 void linkNext(FreeChunk* ptr) { _next = ptr; } 112 void linkPrev(FreeChunk* ptr) { 113 LP64_ONLY(if (UseCompressedOops) _prev = ptr; else) 114 _prev = (FreeChunk*)((intptr_t)ptr | 0x1); 115 } 116 void clearPrev() { _prev = NULL; } 117 void clearNext() { _next = NULL; } 118 void markNotFree() { 119 // Set _prev (klass) to null before (if) clearing the mark word below 120 _prev = NULL; 121 #ifdef _LP64 122 if (UseCompressedOops) { 123 OrderAccess::storestore(); 124 set_mark(markOopDesc::prototype()); 125 } 126 #endif 127 assert(!isFree(), "Error"); 128 } 129 130 // Return the address past the end of this chunk 131 HeapWord* end() const { return ((HeapWord*) this) + size(); } 132 133 // debugging 134 void verify() const PRODUCT_RETURN; 135 void verifyList() const PRODUCT_RETURN; 136 void mangleAllocated(size_t size) PRODUCT_RETURN; 137 void mangleFreed(size_t size) PRODUCT_RETURN; 138 139 void print_on(outputStream* st); 140 }; 141 142 extern size_t MinChunkSize; 143