1 /*

     2  * Copyright 2001-2005 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 //

    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 

    89   size_t size() const volatile {

    90     LP64_ONLY(if (UseCompressedOops) return mark()->get_size(); else )

    91     return _size;

    92   }

    93   void setSize(size_t sz) {

    94     LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::set_size_and_free(sz)); else )

    95     _size = sz;

    96   }

    97 

    98   FreeChunk* next()   const { return _next; }

    99 

   100   void linkAfter(FreeChunk* ptr) {

   101     linkNext(ptr);

   102     if (ptr != NULL) ptr->linkPrev(this);

   103   }

   104   void linkAfterNonNull(FreeChunk* ptr) {

   105     assert(ptr != NULL, "precondition violation");

   106     linkNext(ptr);

   107     ptr->linkPrev(this);

   108   }

   109   void linkNext(FreeChunk* ptr) { _next = ptr; }

   110   void linkPrev(FreeChunk* ptr) {

   111      LP64_ONLY(if (UseCompressedOops) _prev = ptr; else)

   112      _prev = (FreeChunk*)((intptr_t)ptr | 0x1);

   113   }

   114   void clearPrev()              { _prev = NULL; }

   115   void clearNext()              { _next = NULL; }

   116   void markNotFree() {

   117    LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::prototype());)

   118    // Also set _prev to null

   119    _prev = NULL;

   120   }

   121 

   122   // Return the address past the end of this chunk

   123   HeapWord* end() const { return ((HeapWord*) this) + size(); }

   124 

   125   // debugging

   126   void verify()             const PRODUCT_RETURN;

   127   void verifyList()         const PRODUCT_RETURN;

   128   void mangleAllocated(size_t size) PRODUCT_RETURN;

   129   void mangleFreed(size_t size)     PRODUCT_RETURN;

   130 };

   131 

   132 // Alignment helpers etc.

   133 #define numQuanta(x,y) ((x+y-1)/y)

   134 enum AlignmentConstants {

   135   MinChunkSize = numQuanta(sizeof(FreeChunk), MinObjAlignmentInBytes) * MinObjAlignment

   136 };

   137