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24 
25 #ifndef SHARE_VM_GC_CMS_FREECHUNK_HPP
26 #define SHARE_VM_GC_CMS_FREECHUNK_HPP
27 
28 #include "memory/memRegion.hpp"
29 #include "oops/markOop.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 markOop 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 
mark() const65   markOop mark()     const volatile { return (markOop)_size; }
set_mark(markOop m)66   void set_mark(markOop m)          { _size = (size_t)m; }
67 
68  public:
NOT_PRODUCT(static const size_t header_size ();)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 
is_free() const81   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   }
cantCoalesce() const85   bool cantCoalesce() const {
86     assert(is_free(), "can't get coalesce bit on not free");
87     return (((intptr_t)_prev) & 0x2) == 0x2;
88   }
dontCoalesce()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   }
prev() const94   FreeChunk* prev() const {
95     return (FreeChunk*)(((intptr_t)_prev) & ~(0x3));
96   }
97 
98   debug_only(void* prev_addr() const { return (void*)&_prev; })
debug_only(void * next_addr ()const{ return (void*)&_next; })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   }
set_size(size_t sz)106   void set_size(size_t sz) {
107     LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::set_size_and_free(sz)); else )
108     _size = sz;
109   }
110 
next() const111   FreeChunk* next()   const { return _next; }
112 
link_after(FreeChunk * ptr)113   void link_after(FreeChunk* ptr) {
114     link_next(ptr);
115     if (ptr != NULL) ptr->link_prev(this);
116   }
link_next(FreeChunk * ptr)117   void link_next(FreeChunk* ptr) { _next = ptr; }
link_prev(FreeChunk * ptr)118   void link_prev(FreeChunk* ptr) {
119     LP64_ONLY(if (UseCompressedOops) _prev = ptr; else)
120     _prev = (FreeChunk*)((intptr_t)ptr | 0x1);
121   }
clear_next()122   void clear_next()              { _next = NULL; }
markNotFree()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(markOopDesc::prototype());
130     }
131 #endif
132     assert(!is_free(), "Error");
133   }
134 
135   // Return the address past the end of this chunk
end() const136   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_VM_GC_CMS_FREECHUNK_HPP
151