The BDWGC conservative garbage collector uses a 2-level tree data structure to aid in fast pointer identification. This data structure is described in a bit more detail here, since
The high and middle bits are used to look up an entry in the table described here. The resulting table entry consists of either a block descriptor (struct hblkhdr * or hdr *) identifying the layout of objects in the block, or an indication that this address range corresponds to the middle of a large block, together with a hint for locating the actual block descriptor. Such a hint consist of a displacement that can be subtracted from the middle bits of the candidate pointer without leaving the object.
In either case, the block descriptor (struct hblkhdr) refers to a table of object starting addresses (the hb_map field). The starting address table is indexed by the low bits if the candidate pointer. The resulting entry contains a displacement to the beginning of the object, or an indication that this cannot be a valid object pointer. (If all interior pointer are recognized, pointers into large objects are handled specially, as appropriate.)
The rest of this discussion focuses on the two level data structure used to map the high and middle bits to the block descriptor.
The high bits are used as an index into the GC_top_index (really GC_arrays._top_index) array. Each entry points to a bottom_index data structure. This structure in turn consists mostly of an array index indexed by the middle bits of the candidate pointer. The index array contains the actual hdr pointers.
Thus a pointer lookup consists primarily of a handful of memory references, and can be quite fast:
In order to conserve space, not all GC_top_index entries in fact point to distinct bottom_index structures. If no address with the corresponding high bits is part of the heap, then the entry points to GC_all_nils, a single bottom_index structure consisting only of NULL hdr pointers.
Bottom_index structures contain slightly more information than just hdr pointers. The asc_link field is used to link all bottom_index structures in ascending order for fast traversal. This list is pointed to be GC_all_bottom_indices. It is maintained with the aid of key field that contains the high bits corresponding to the bottom_index.
In the case of 64 bit addresses, this picture is complicated slightly by the fact that one of the index structures would have to be huge to cover the entire address space with a two level tree. We deal with this by turning GC_top_index into a chained hash table, instead of a simple array. This adds a hash_link field to the bottom_index structure.
The "hash function" consists of dropping the high bits. This is cheap to compute, and guarantees that there will be no collisions if the heap is contiguous and not excessively large.
The following is an ASCII diagram of the data structure. This was contributed by Dave Barrett several years ago.
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
_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). HBLK's below are collected.
(struct hblk) HDR_BYTES
--- +----------------------+ < HBLKSIZE --- (bytes)
^ +-----hb_body----------+ (and WORDSZ) ^ --- ---
| | | aligned | ^ ^
| | | | hb_sz |
| | | | (words) |
| | Object 0 | | | |
| | | i |(word- v |
| + - - - - - - - - - - -+ --- (bytes)|aligned) --- |
| | | ^ | ^ |
| | | j (words) | | |
n * | Object 1 | v v hb_sz BODY_SZ
HBLKSIZE | |--------------- | (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. --- ---