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/vendor/gc/doc/barrett_diagram

http://github.com/feyeleanor/RubyGoLightly
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  1This is an ASCII diagram of the data structure used to check pointer
  2validity.  It was provided by Dave Barrett <barrett@asgard.cs.colorado.edu>,
  3and should be of use to others attempting to understand the code.
  4The data structure in GC4.X is essentially the same.   -HB
  5
  6
  7
  8
  9		Data Structure used by GC_base in gc3.7:
 10			      21-Apr-94
 11			 
 12			
 13
 14
 15    63                  LOG_TOP_SZ[11]  LOG_BOTTOM_SZ[10]   LOG_HBLKSIZE[13]
 16   +------------------+----------------+------------------+------------------+
 17 p:|                  |   TL_HASH(hi)  |                  |   HBLKDISPL(p)   |
 18   +------------------+----------------+------------------+------------------+
 19    \-----------------------HBLKPTR(p)-------------------/
 20    \------------hi-------------------/ 
 21                      \______ ________/ \________ _______/ \________ _______/
 22                             V                   V                  V
 23                             |                   |                  |
 24           GC_top_index[]    |                   |                  | 
 25 ---      +--------------+   |                   |                  |  
 26  ^       |              |   |                   |                  |   
 27  |       |              |   |                   |                  |   
 28 TOP      +--------------+<--+                   |                  |      
 29 _SZ   +-<|      []      | *                     |                  |     
 30(items)|  +--------------+  if 0 < bi< HBLKSIZE  |                  |    
 31  |    |  |              | then large object     |                  |    
 32  |    |  |              | starts at the bi'th   |                  |    
 33  v    |  |              | HBLK before p.        |             i    |    
 34 ---   |  +--------------+                       |          (word-  |    
 35       v                                         |         aligned) |    
 36   bi= |GET_BI(p){->hash_link}->key==hi          |                  |   
 37       v                                         |                  |    
 38       |   (bottom_index)  \ scratch_alloc'd     |                  |    
 39       |   ( struct  bi )  / by get_index()      |                  |    
 40 ---   +->+--------------+                       |                  |    
 41  ^       |              |                       |                  |
 42  ^       |              |                       |                  |
 43 BOTTOM   |              |   ha=GET_HDR_ADDR(p)  |                  |
 44_SZ(items)+--------------+<----------------------+          +-------+
 45  |   +--<|   index[]    |                                  |         
 46  |   |   +--------------+                      GC_obj_map: v              
 47  |   |   |              |              from      / +-+-+-----+-+-+-+-+  --- 
 48  v   |   |              |              GC_add   < 0| | |     | | | | |   ^  
 49 ---  |   +--------------+             _map_entry \ +-+-+-----+-+-+-+-+   |  
 50      |   |   asc_link   |                          +-+-+-----+-+-+-+-+ MAXOBJSZ
 51      |   +--------------+                      +-->| | |  j  | | | | |  +1   
 52      |   |     key      |                      |   +-+-+-----+-+-+-+-+   |  
 53      |   +--------------+                      |   +-+-+-----+-+-+-+-+   | 
 54      |   |  hash_link   |                      |   | | |     | | | | |   v 
 55      |   +--------------+                      |   +-+-+-----+-+-+-+-+  ---
 56      |                                         |   |<--MAX_OFFSET--->|   
 57      |                                         |         (bytes)
 58HDR(p)| GC_find_header(p)                       |   |<--MAP_ENTRIES-->| 
 59      |                           \ from        |    =HBLKSIZE/WORDSZ   
 60      |    (hdr) (struct hblkhdr) / alloc_hdr() |    (1024 on Alpha)
 61      +-->+----------------------+              |    (8/16 bits each)
 62GET_HDR(p)| word   hb_sz (words) |              |          
 63          +----------------------+              |     
 64          | struct hblk *hb_next |              |
 65          +----------------------+              |       
 66          |mark_proc hb_mark_proc|              |
 67          +----------------------+              |
 68          | char * hb_map        |>-------------+
 69          +----------------------+           
 70          | ushort hb_obj_kind   |           
 71          +----------------------+           
 72          |   hb_last_reclaimed  |           
 73 ---      +----------------------+                
 74  ^       |                      |
 75 MARK_BITS|       hb_marks[]     | *if hdr is free, hb_sz + DISCARD_WORDS
 76_SZ(words)|                      |  is the size of a heap chunk (struct hblk)
 77  v       |                      |  of at least MININCR*HBLKSIZE bytes (below),
 78 ---      +----------------------+  otherwise, size of each object in chunk.
 79
 80Dynamic data structures above are interleaved throughout the heap in blocks of 
 81size MININCR * HBLKSIZE bytes as done by gc_scratch_alloc which cannot be
 82freed; free lists are used (e.g. alloc_hdr).  HBLKs's below are collected.
 83
 84	      (struct hblk)      
 85 ---      +----------------------+ < HBLKSIZE ---         ---          DISCARD_
 86  ^       |garbage[DISCARD_WORDS]|   aligned   ^           ^ HDR_BYTES WORDS
 87  |       |                      |             |           v (bytes)   (words)
 88  |       +-----hb_body----------+ < WORDSZ    |          ---   ---   
 89  |       |                      |   aligned   |           ^     ^
 90  |       |      Object 0        |             |           hb_sz |
 91  |       |                      |           i |(word-    (words)|
 92  |       |                      |      (bytes)|aligned)   v     |
 93  |       + - - - - - - - - - - -+ ---         |          ---    |
 94  |       |                      |  ^          |           ^     |
 95  n *     |                      |  j (words)  |          hb_sz BODY_SZ 
 96 HBLKSIZE |      Object 1        |  v          v           |   (words)
 97 (bytes)  |                      |---------------          v   MAX_OFFSET
 98  |       + - - - - - - - - - - -+                        ---  (bytes)
 99  |       |                      | !All_INTERIOR_PTRS      ^     |
100  |       |                      | sets j only for       hb_sz   |
101  |       |      Object N        | valid object offsets.   |     |
102  v       |                      | All objects WORDSZ      v     v
103 ---      +----------------------+ aligned.               ---   ---
104
105DISCARD_WORDS is normally zero.  Indeed the collector has not been tested
106with another value in ages.