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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.
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