tdb2: remove truncated bit.

There was an idea that we would use a bit to indicate that we didn't have
the full hash value; this would allow us to move records down when we
expanded a hash without rehashing them.

There's little evidence that rehashing in this case is particularly
expensive, so remove the bit.  We use that bit simply to indicate that
an offset refers to a subhash instead.

ccan/tdb2/hash.c

@@ -89,10 +89,6 @@ static bool match(struct tdb_context *tdb,
 	const unsigned char *rkey;
 	tdb_off_t off;
 
-	/* FIXME: Handle hash value truncated. */
-	if (bits(val, TDB_OFF_HASH_TRUNCATED_BIT, 1))
-		abort();
-
 	/* Desired bucket must match. */
 	if (h->home_bucket != (val & TDB_OFF_HASH_GROUP_MASK))
 		return false;
@@ -125,10 +121,9 @@ static tdb_off_t hbucket_off(tdb_off_t group_start, unsigned bucket)
 		+ (bucket % (1 << TDB_HASH_GROUP_BITS)) * sizeof(tdb_off_t);
 }
 
-/* Truncated hashes can't be all 1: that's how we spot a sub-hash */
 bool is_subhash(tdb_off_t val)
 {
-	return val >> (64-TDB_OFF_UPPER_STEAL) == (1<<TDB_OFF_UPPER_STEAL) - 1;
+	return (val >> TDB_OFF_UPPER_STEAL_SUBHASH_BIT) & 1;
 }
 
 /* FIXME: Guess the depth, don't over-lock! */
@@ -241,8 +236,8 @@ fail:
 /* I wrote a simple test, expanding a hash to 2GB, for the following
  * cases:
  * 1) Expanding all the buckets at once,
- * 2) Expanding the most-populated bucket,
- * 3) Expanding the bucket we wanted to place the new entry ito.
+ * 2) Expanding the bucket we wanted to place the new entry into.
+ * 3) Expanding the most-populated bucket,
  *
  * I measured the worst/average/best density during this process.
  * 1) 3%/16%/30%
@@ -331,7 +326,6 @@ static int add_to_subhash(struct tdb_context *tdb, tdb_off_t subhash,
 	if (hash_used + TDB_SUBLEVEL_HASH_BITS > 64)
 		abort();
 
-	/* FIXME: Do truncated hash bits if we can! */
 	h.h = hash_record(tdb, off);
 	gnum = use_bits(&h, TDB_SUBLEVEL_HASH_BITS-TDB_HASH_GROUP_BITS);
 	h.group_start = subhash	+ sizeof(struct tdb_used_record)
@@ -380,7 +374,7 @@ static int expand_group(struct tdb_context *tdb, struct hash_info *h)
 	/* assert(num_vals); */
 
 	/* Overwrite expanded bucket with subhash pointer. */
-	h->group[bucket] = subhash | ~((1ULL << (64 - TDB_OFF_UPPER_STEAL))-1);
+	h->group[bucket] = subhash | (1ULL << TDB_OFF_UPPER_STEAL_SUBHASH_BIT);
 
 	/* Put values back. */
 	for (i = 0; i < num_vals; i++) {

ccan/tdb2/private.h

@@ -100,11 +100,11 @@ typedef uint64_t tdb_off_t;
 /* We steal this many upper bits, giving a maximum offset of 64 exabytes. */
 #define TDB_OFF_UPPER_STEAL 8
 #define   TDB_OFF_UPPER_STEAL_EXTRA 7
-#define   TDB_OFF_UPPER_STEAL_TRUNCBIT 1
-/* If this is set, hash is truncated (only 1 bit is valid). */
-#define TDB_OFF_HASH_TRUNCATED_BIT 56
-/* The bit number where we store next level of hash. */
+/* The bit number where we store extra hash bits. */
 #define TDB_OFF_HASH_EXTRA_BIT 57
+#define TDB_OFF_UPPER_STEAL_SUBHASH_BIT 56
+
+/* The bit number where we store the extra hash bits. */
 /* Convenience mask to get actual offset. */
 #define TDB_OFF_MASK \
 	(((1ULL << (64 - TDB_OFF_UPPER_STEAL)) - 1) - TDB_OFF_HASH_GROUP_MASK)