1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
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  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).
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  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  24 
  25 #ifndef SHARE_GC_CMS_FREECHUNK_HPP
  26 #define SHARE_GC_CMS_FREECHUNK_HPP
  27 
  28 #include "memory/memRegion.hpp"
  29 #include "oops/markWord.hpp"
  30 #include "runtime/mutex.hpp"
  31 #include "runtime/orderAccess.hpp"
  32 #include "utilities/debug.hpp"
  33 #include "utilities/globalDefinitions.hpp"
  34 #include "utilities/ostream.hpp"
  35 
  36 //
  37 // Free block maintenance for Concurrent Mark Sweep Generation
  38 //
  39 // The main data structure for free blocks are
  40 // . an indexed array of small free blocks, and
  41 // . a dictionary of large free blocks
  42 //
  43 
  44 // No virtuals in FreeChunk (don't want any vtables).
  45 
  46 // A FreeChunk is merely a chunk that can be in a doubly linked list
  47 // and has a size field. NOTE: FreeChunks are distinguished from allocated
  48 // objects in two ways (by the sweeper), depending on whether the VM is 32 or
  49 // 64 bits.
  50 // In 32 bits or 64 bits without CompressedOops, the second word (prev) has the
  51 // LSB set to indicate a free chunk; allocated objects' klass() pointers
  52 // don't have their LSB set. The corresponding bit in the CMSBitMap is
  53 // set when the chunk is allocated. There are also blocks that "look free"
  54 // but are not part of the free list and should not be coalesced into larger
  55 // free blocks. These free blocks have their two LSB's set.
  56 
  57 class FreeChunk {
  58   friend class VMStructs;
  59   // For 64 bit compressed oops, the markWord encodes both the size and the
  60   // indication that this is a FreeChunk and not an object.
  61   volatile size_t   _size;
  62   FreeChunk* _prev;
  63   FreeChunk* _next;
  64 
  65   markWord mark()     const volatile { return markWord((uintptr_t)_size); }
  66   void set_mark(markWord m)          { _size = (size_t)m.value(); }
  67 
  68  public:
  69   NOT_PRODUCT(static const size_t header_size();)
  70 
  71   // Returns "true" if the address indicates that the block represents
  72   // a free chunk.
  73   static bool indicatesFreeChunk(const HeapWord* addr) {
  74     // Force volatile read from addr because value might change between
  75     // calls.  We really want the read of _mark and _prev from this pointer
  76     // to be volatile but making the fields volatile causes all sorts of
  77     // compilation errors.
  78     return ((volatile FreeChunk*)addr)->is_free();
  79   }
  80 
  81   bool is_free() const volatile {
  82     LP64_ONLY(if (UseCompressedOops) return mark().is_cms_free_chunk(); else)
  83     return (((intptr_t)_prev) & 0x1) == 0x1;
  84   }
  85   bool cantCoalesce() const {
  86     assert(is_free(), "can't get coalesce bit on not free");
  87     return (((intptr_t)_prev) & 0x2) == 0x2;
  88   }
  89   void dontCoalesce() {
  90     // the block should be free
  91     assert(is_free(), "Should look like a free block");
  92     _prev = (FreeChunk*)(((intptr_t)_prev) | 0x2);
  93   }
  94   FreeChunk* prev() const {
  95     return (FreeChunk*)(((intptr_t)_prev) & ~(0x3));
  96   }
  97 
  98   debug_only(void* prev_addr() const { return (void*)&_prev; })
  99   debug_only(void* next_addr() const { return (void*)&_next; })
 100   debug_only(void* size_addr() const { return (void*)&_size; })
 101 
 102   size_t size() const volatile {
 103     LP64_ONLY(if (UseCompressedOops) return mark().get_size(); else )
 104     return _size;
 105   }
 106   void set_size(size_t sz) {
 107     LP64_ONLY(if (UseCompressedOops) set_mark(markWord::set_size_and_free(sz)); else )
 108     _size = sz;
 109   }
 110 
 111   FreeChunk* next()   const { return _next; }
 112 
 113   void link_after(FreeChunk* ptr) {
 114     link_next(ptr);
 115     if (ptr != NULL) ptr->link_prev(this);
 116   }
 117   void link_next(FreeChunk* ptr) { _next = ptr; }
 118   void link_prev(FreeChunk* ptr) {
 119     LP64_ONLY(if (UseCompressedOops) _prev = ptr; else)
 120     _prev = (FreeChunk*)((intptr_t)ptr | 0x1);
 121   }
 122   void clear_next()              { _next = NULL; }
 123   void markNotFree() {
 124     // Set _prev (klass) to null before (if) clearing the mark word below
 125     _prev = NULL;
 126 #ifdef _LP64
 127     if (UseCompressedOops) {
 128       OrderAccess::storestore();
 129       set_mark(markWord::prototype());
 130     }
 131 #endif
 132     assert(!is_free(), "Error");
 133   }
 134 
 135   // Return the address past the end of this chunk
 136   uintptr_t* end() const { return ((uintptr_t*) this) + size(); }
 137 
 138   // debugging
 139   void verify()             const PRODUCT_RETURN;
 140   void verifyList()         const PRODUCT_RETURN;
 141   void mangleAllocated(size_t size) PRODUCT_RETURN;
 142   void mangleFreed(size_t size)     PRODUCT_RETURN;
 143 
 144   void print_on(outputStream* st);
 145 };
 146 
 147 extern size_t MinChunkSize;
 148 
 149 
 150 #endif // SHARE_GC_CMS_FREECHUNK_HPP