*
* This is a Hash Chains (hc) based matchfinder.
*
- * The data structure is a hash table where each hash bucket contains a linked
- * list (or "chain") of sequences whose first 3 bytes share the same hash code.
- * Each sequence is identified by its starting position in the input buffer.
+ * The main data structure is a hash table where each hash bucket contains a
+ * linked list (or "chain") of sequences whose first 4 bytes share the same hash
+ * code. Each sequence is identified by its starting position in the input
+ * buffer.
*
* The algorithm processes the input buffer sequentially. At each byte
- * position, the hash code of the first 3 bytes of the sequence beginning at
+ * position, the hash code of the first 4 bytes of the sequence beginning at
* that position (the sequence being matched against) is computed. This
* identifies the hash bucket to use for that position. Then, this hash
* bucket's linked list is searched for matches. Then, a new linked list node
*
* Optimizations
*
+ * The main hash table and chains handle length 4+ matches. Length 3 matches
+ * are handled by a separate hash table with no chains. This works well for
+ * typical "greedy" or "lazy"-style compressors, where length 3 matches are
+ * often only helpful if they have small offsets. Instead of searching a full
+ * chain for length 3+ matches, the algorithm just checks for one close length 3
+ * match, then focuses on finding length 4+ matches.
+ *
* The longest_match() and skip_positions() functions are inlined into the
* compressors that use them. This isn't just about saving the overhead of a
* function call. These functions are intended to be called from the inner
#include "wimlib/lz_hash.h"
#include "wimlib/unaligned.h"
-#if MATCHFINDER_MAX_WINDOW_ORDER < 14
-# define HC_MATCHFINDER_HASH_ORDER 14
-#else
-# define HC_MATCHFINDER_HASH_ORDER 15
-#endif
-
#if MATCHFINDER_MAX_WINDOW_ORDER <= 16
typedef u16 pos_t;
#else
typedef u32 pos_t;
#endif
+#define HC_MATCHFINDER_HASH3_ORDER 14
+#define HC_MATCHFINDER_HASH4_ORDER 15
+
struct hc_matchfinder {
- pos_t hash_tab[1UL << HC_MATCHFINDER_HASH_ORDER];
+
+ /* The hash table for finding length 3 matches */
+ pos_t hash3_tab[1UL << HC_MATCHFINDER_HASH3_ORDER];
+
+ /* The hash table which contains the first nodes of the linked lists for
+ * finding length 4+ matches */
+ pos_t hash4_tab[1UL << HC_MATCHFINDER_HASH4_ORDER];
+
+ /* The "next node" references for the linked lists. The "next node" of
+ * the node for the sequence with position 'pos' is 'next_tab[pos]'. */
pos_t next_tab[];
};
* The matchfinder structure.
* @in_begin
* Pointer to the beginning of the input buffer.
- * @in_next
- * Pointer to the next byte in the input buffer to process. This is the
- * pointer to the sequence being matched against.
+ * @cur_pos
+ * The current position in the input buffer (the position of the sequence
+ * being matched against).
* @best_len
* Require a match longer than this length.
* @max_len
* Must be <= @max_len.
* @max_search_depth
* Limit on the number of potential matches to consider. Must be >= 1.
+ * @next_hashes
+ * The precomputed hash codes for the sequence beginning at @in_next.
+ * These will be used and then updated with the precomputed hashcodes for
+ * the sequence beginning at @in_next + 1.
* @offset_ret
* If a match is found, its offset is returned in this location.
*
* Return the length of the match found, or 'best_len' if no match longer than
* 'best_len' was found.
*/
-static inline unsigned
+static inline u32
hc_matchfinder_longest_match(struct hc_matchfinder * const restrict mf,
- const u8 * const in_begin,
- const u8 * const in_next,
- unsigned best_len,
- const unsigned max_len,
- const unsigned nice_len,
- const unsigned max_search_depth,
- unsigned *offset_ret)
+ const u8 * const restrict in_begin,
+ const ptrdiff_t cur_pos,
+ u32 best_len,
+ const u32 max_len,
+ const u32 nice_len,
+ const u32 max_search_depth,
+ u32 next_hashes[const restrict static 2],
+ u32 * const restrict offset_ret)
{
- unsigned depth_remaining = max_search_depth;
+ const u8 *in_next = in_begin + cur_pos;
+ u32 depth_remaining = max_search_depth;
const u8 *best_matchptr = best_matchptr; /* uninitialized */
+ pos_t cur_node3, cur_node4;
+ u32 hash3, hash4;
+ u32 next_seq3, next_seq4;
+ u32 seq4;
const u8 *matchptr;
- unsigned len;
- u32 first_3_bytes;
- u32 hash;
- pos_t cur_node;
+ u32 len;
- /* Insert the current sequence into the appropriate linked list. */
- if (unlikely(max_len < LOAD_U24_REQUIRED_NBYTES))
+ if (unlikely(max_len < 5)) /* can we read 4 bytes from 'in_next + 1'? */
goto out;
- first_3_bytes = load_u24_unaligned(in_next);
- hash = lz_hash(first_3_bytes, HC_MATCHFINDER_HASH_ORDER);
- cur_node = mf->hash_tab[hash];
- mf->next_tab[in_next - in_begin] = cur_node;
- mf->hash_tab[hash] = in_next - in_begin;
- if (unlikely(best_len >= max_len))
- goto out;
+ /* Get the precomputed hash codes. */
+ hash3 = next_hashes[0];
+ hash4 = next_hashes[1];
- /* Search the appropriate linked list for matches. */
+ /* From the hash buckets, get the first node of each linked list. */
+ cur_node3 = mf->hash3_tab[hash3];
+ cur_node4 = mf->hash4_tab[hash4];
- if (!cur_node)
- goto out;
+ /* Update for length 3 matches. This replaces the singleton node in the
+ * 'hash3' bucket with the node for the current sequence. */
+ mf->hash3_tab[hash3] = cur_pos;
+
+ /* Update for length 4 matches. This prepends the node for the current
+ * sequence to the linked list in the 'hash4' bucket. */
+ mf->hash4_tab[hash4] = cur_pos;
+ mf->next_tab[cur_pos] = cur_node4;
+
+ /* Compute the next hash codes. */
+ next_seq4 = load_u32_unaligned(in_next + 1);
+ next_seq3 = loaded_u32_to_u24(next_seq4);
+ next_hashes[0] = lz_hash(next_seq3, HC_MATCHFINDER_HASH3_ORDER);
+ next_hashes[1] = lz_hash(next_seq4, HC_MATCHFINDER_HASH4_ORDER);
+ prefetchw(&mf->hash3_tab[next_hashes[0]]);
+ prefetchw(&mf->hash4_tab[next_hashes[1]]);
+
+ if (best_len < 4) { /* No match of length >= 4 found yet? */
+
+ /* Check for a length 3 match if needed. */
+
+ if (!cur_node3)
+ goto out;
+
+ seq4 = load_u32_unaligned(in_next);
+
+ if (best_len < 3) {
+ matchptr = &in_begin[cur_node3];
+ if (load_u24_unaligned(matchptr) == loaded_u32_to_u24(seq4)) {
+ best_len = 3;
+ best_matchptr = matchptr;
+ }
+ }
+
+ /* Check for a length 4 match. */
+
+ if (!cur_node4)
+ goto out;
- if (best_len < 3) {
for (;;) {
- /* No length 3 match found yet.
- * Check the first 3 bytes. */
- matchptr = &in_begin[cur_node];
+ /* No length 4 match found yet. Check the first 4 bytes. */
+ matchptr = &in_begin[cur_node4];
- if (load_u24_unaligned(matchptr) == first_3_bytes)
+ if (load_u32_unaligned(matchptr) == seq4)
break;
- /* The first 3 bytes did not match. Keep trying. */
- cur_node = mf->next_tab[cur_node];
- if (!cur_node || !--depth_remaining)
+ /* The first 4 bytes did not match. Keep trying. */
+ cur_node4 = mf->next_tab[cur_node4];
+ if (!cur_node4 || !--depth_remaining)
goto out;
}
- /* Found a match of length >= 3. Extend it to its full length. */
+ /* Found a match of length >= 4. Extend it to its full length. */
best_matchptr = matchptr;
- best_len = lz_extend(in_next, best_matchptr, 3, max_len);
+ best_len = lz_extend(in_next, best_matchptr, 4, max_len);
if (best_len >= nice_len)
goto out;
- cur_node = mf->next_tab[cur_node];
- if (!cur_node || !--depth_remaining)
+ cur_node4 = mf->next_tab[cur_node4];
+ if (!cur_node4 || !--depth_remaining)
+ goto out;
+ } else {
+ if (!cur_node4 || best_len >= nice_len)
goto out;
}
+ /* Check for matches of length >= 5. */
+
for (;;) {
for (;;) {
- matchptr = &in_begin[cur_node];
+ matchptr = &in_begin[cur_node4];
- /* Already found a length 3 match. Try for a longer
+ /* Already found a length 4 match. Try for a longer
* match; start by checking either the last 4 bytes and
* the first 4 bytes, or the last byte. (The last byte,
* the one which would extend the match length by 1, is
#endif
break;
- cur_node = mf->next_tab[cur_node];
- if (!cur_node || !--depth_remaining)
+ /* Continue to the next node in the list. */
+ cur_node4 = mf->next_tab[cur_node4];
+ if (!cur_node4 || !--depth_remaining)
goto out;
}
#endif
len = lz_extend(in_next, matchptr, len, max_len);
if (len > best_len) {
+ /* This is the new longest match. */
best_len = len;
best_matchptr = matchptr;
if (best_len >= nice_len)
goto out;
}
- cur_node = mf->next_tab[cur_node];
- if (!cur_node || !--depth_remaining)
+
+ /* Continue to the next node in the list. */
+ cur_node4 = mf->next_tab[cur_node4];
+ if (!cur_node4 || !--depth_remaining)
goto out;
}
out:
* The matchfinder structure.
* @in_begin
* Pointer to the beginning of the input buffer.
- * @in_next
- * Pointer to the next byte in the input buffer to process.
- * @in_end
- * Pointer to the end of the input buffer.
+ * @cur_pos
+ * The current position in the input buffer (the position of the sequence
+ * being matched against).
+ * @end_pos
+ * The length of the input buffer.
+ * @next_hashes
+ * The precomputed hash codes for the sequence beginning at @in_next.
+ * These will be used and then updated with the precomputed hashcodes for
+ * the sequence beginning at @in_next + @count.
* @count
* The number of bytes to advance. Must be > 0.
+ *
+ * Returns @in_next + @count.
*/
-static inline void
-hc_matchfinder_skip_positions(struct hc_matchfinder * restrict mf,
- const u8 *in_begin,
- const u8 *in_next,
- const u8 *in_end,
- unsigned count)
+static inline const u8 *
+hc_matchfinder_skip_positions(struct hc_matchfinder * const restrict mf,
+ const u8 * const restrict in_begin,
+ const ptrdiff_t cur_pos,
+ const ptrdiff_t end_pos,
+ const u32 count,
+ u32 next_hashes[const restrict static 2])
{
- u32 hash;
+ const u8 *in_next = in_begin + cur_pos;
+ const u8 * const stop_ptr = in_next + count;
+
+ if (likely(count + 5 <= end_pos - cur_pos)) {
+ u32 hash3, hash4;
+ u32 next_seq3, next_seq4;
- if (unlikely(in_next + count >= in_end - LZ_HASH3_REQUIRED_NBYTES))
- return;
+ hash3 = next_hashes[0];
+ hash4 = next_hashes[1];
+ do {
+ mf->hash3_tab[hash3] = in_next - in_begin;
+ mf->next_tab[in_next - in_begin] = mf->hash4_tab[hash4];
+ mf->hash4_tab[hash4] = in_next - in_begin;
+
+ next_seq4 = load_u32_unaligned(++in_next);
+ next_seq3 = loaded_u32_to_u24(next_seq4);
+ hash3 = lz_hash(next_seq3, HC_MATCHFINDER_HASH3_ORDER);
+ hash4 = lz_hash(next_seq4, HC_MATCHFINDER_HASH4_ORDER);
+
+ } while (in_next != stop_ptr);
+
+ prefetchw(&mf->hash3_tab[hash3]);
+ prefetchw(&mf->hash4_tab[hash4]);
+ next_hashes[0] = hash3;
+ next_hashes[1] = hash4;
+ }
- do {
- hash = lz_hash_3_bytes(in_next, HC_MATCHFINDER_HASH_ORDER);
- mf->next_tab[in_next - in_begin] = mf->hash_tab[hash];
- mf->hash_tab[hash] = in_next - in_begin;
- in_next++;
- } while (--count);
+ return stop_ptr;
}
#endif /* _HC_MATCHFINDER_H */
git://wimlib.net / wimlib / blobdiff