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+This is an ASCII diagram of the data structure used to check pointer
+validity. It was provided by Dave Barrett <barrett@asgard.cs.colorado.edu>,
+and should be of use to others attempting to understand the code.
+The data structure in GC4.X is essentially the same. -HB
+
+
+
+
+ Data Structure used by GC_base in gc3.7:
+ 21-Apr-94
+
+
+
+
+ 63 LOG_TOP_SZ[11] LOG_BOTTOM_SZ[10] LOG_HBLKSIZE[13]
+ +------------------+----------------+------------------+------------------+
+ p:| | TL_HASH(hi) | | HBLKDISPL(p) |
+ +------------------+----------------+------------------+------------------+
+ \-----------------------HBLKPTR(p)-------------------/
+ \------------hi-------------------/
+ \______ ________/ \________ _______/ \________ _______/
+ V V V
+ | | |
+ GC_top_index[] | | |
+ --- +--------------+ | | |
+ ^ | | | | |
+ | | | | | |
+ TOP +--------------+<--+ | |
+ _SZ +-<| [] | * | |
+(items)| +--------------+ if 0 < bi< HBLKSIZE | |
+ | | | | then large object | |
+ | | | | starts at the bi'th | |
+ v | | | HBLK before p. | i |
+ --- | +--------------+ | (word- |
+ v | aligned) |
+ bi= |GET_BI(p){->hash_link}->key==hi | |
+ v | |
+ | (bottom_index) \ scratch_alloc'd | |
+ | ( struct bi ) / by get_index() | |
+ --- +->+--------------+ | |
+ ^ | | | |
+ ^ | | | |
+ BOTTOM | | ha=GET_HDR_ADDR(p) | |
+_SZ(items)+--------------+<----------------------+ +-------+
+ | +--<| index[] | |
+ | | +--------------+ GC_obj_map: v
+ | | | | from / +-+-+-----+-+-+-+-+ ---
+ v | | | GC_add < 0| | | | | | | | ^
+ --- | +--------------+ _map_entry \ +-+-+-----+-+-+-+-+ |
+ | | asc_link | +-+-+-----+-+-+-+-+ MAXOBJSZ
+ | +--------------+ +-->| | | j | | | | | +1
+ | | key | | +-+-+-----+-+-+-+-+ |
+ | +--------------+ | +-+-+-----+-+-+-+-+ |
+ | | hash_link | | | | | | | | | | v
+ | +--------------+ | +-+-+-----+-+-+-+-+ ---
+ | | |<--MAX_OFFSET--->|
+ | | (bytes)
+HDR(p)| GC_find_header(p) | |<--MAP_ENTRIES-->|
+ | \ from | =HBLKSIZE/WORDSZ
+ | (hdr) (struct hblkhdr) / alloc_hdr() | (1024 on Alpha)
+ +-->+----------------------+ | (8/16 bits each)
+GET_HDR(p)| word hb_sz (words) | |
+ +----------------------+ |
+ | struct hblk *hb_next | |
+ +----------------------+ |
+ |mark_proc hb_mark_proc| |
+ +----------------------+ |
+ | char * hb_map |>-------------+
+ +----------------------+
+ | ushort hb_obj_kind |
+ +----------------------+
+ | hb_last_reclaimed |
+ --- +----------------------+
+ ^ | |
+ MARK_BITS| hb_marks[] | *if hdr is free, hb_sz + DISCARD_WORDS
+_SZ(words)| | is the size of a heap chunk (struct hblk)
+ v | | of at least MININCR*HBLKSIZE bytes (below),
+ --- +----------------------+ otherwise, size of each object in chunk.
+
+Dynamic data structures above are interleaved throughout the heap in blocks of
+size MININCR * HBLKSIZE bytes as done by gc_scratch_alloc which cannot be
+freed; free lists are used (e.g. alloc_hdr). HBLKs's below are collected.
+
+ (struct hblk)
+ --- +----------------------+ < HBLKSIZE --- --- DISCARD_
+ ^ |garbage[DISCARD_WORDS]| aligned ^ ^ HDR_BYTES WORDS
+ | | | | v (bytes) (words)
+ | +-----hb_body----------+ < WORDSZ | --- ---
+ | | | aligned | ^ ^
+ | | Object 0 | | hb_sz |
+ | | | i |(word- (words)|
+ | | | (bytes)|aligned) v |
+ | + - - - - - - - - - - -+ --- | --- |
+ | | | ^ | ^ |
+ n * | | j (words) | hb_sz BODY_SZ
+ HBLKSIZE | Object 1 | v v | (words)
+ (bytes) | |--------------- v MAX_OFFSET
+ | + - - - - - - - - - - -+ --- (bytes)
+ | | | !All_INTERIOR_PTRS ^ |
+ | | | sets j only for hb_sz |
+ | | Object N | valid object offsets. | |
+ v | | All objects WORDSZ v v
+ --- +----------------------+ aligned. --- ---
+
+DISCARD_WORDS is normally zero. Indeed the collector has not been tested
+with another value in ages.