Refactor initial hash lookup in dynahash.c

The same pattern is used three times in dynahash.c to retrieve a bucket
number and a hash bucket from a hash value.  This has popped up while
discussing improvements for the type cache, where this piece of
refactoring would become useful.

Note that hash_search_with_hash_value() does not need the bucket number,
just the hash bucket.

Author: Teodor Sigaev
Reviewed-by: Aleksander Alekseev, Michael Paquier
Discussion: https://postgr.es/m/5812a6e5-68ae-4d84-9d85-b443176966a1@sigaev.ru

src/backend/utils/hash/dynahash.c

@@ -273,6 +273,8 @@ static void hdefault(HTAB *hashp);
 static int	choose_nelem_alloc(Size entrysize);
 static bool init_htab(HTAB *hashp, long nelem);
 static void hash_corrupted(HTAB *hashp);
+static uint32 hash_initial_lookup(HTAB *hashp, uint32 hashvalue,
+								  HASHBUCKET **bucketptr);
 static long next_pow2_long(long num);
 static int	next_pow2_int(long num);
 static void register_seq_scan(HTAB *hashp);
@@ -972,10 +974,6 @@ hash_search_with_hash_value(HTAB *hashp,
 	HASHHDR    *hctl = hashp->hctl;
 	int			freelist_idx = FREELIST_IDX(hctl, hashvalue);
 	Size		keysize;
-	uint32		bucket;
-	long		segment_num;
-	long		segment_ndx;
-	HASHSEGMENT segp;
 	HASHBUCKET	currBucket;
 	HASHBUCKET *prevBucketPtr;
 	HashCompareFunc match;
@@ -1008,17 +1006,7 @@ hash_search_with_hash_value(HTAB *hashp,
 	/*
 	 * Do the initial lookup
 	 */
-	bucket = calc_bucket(hctl, hashvalue);
-
-	segment_num = bucket >> hashp->sshift;
-	segment_ndx = MOD(bucket, hashp->ssize);
-
-	segp = hashp->dir[segment_num];
-
-	if (segp == NULL)
-		hash_corrupted(hashp);
-
-	prevBucketPtr = &segp[segment_ndx];
+	(void) hash_initial_lookup(hashp, hashvalue, &prevBucketPtr);
 	currBucket = *prevBucketPtr;
 
 	/*
@@ -1159,14 +1147,10 @@ hash_update_hash_key(HTAB *hashp,
 					 const void *newKeyPtr)
 {
 	HASHELEMENT *existingElement = ELEMENT_FROM_KEY(existingEntry);
-	HASHHDR    *hctl = hashp->hctl;
 	uint32		newhashvalue;
 	Size		keysize;
 	uint32		bucket;
 	uint32		newbucket;
-	long		segment_num;
-	long		segment_ndx;
-	HASHSEGMENT segp;
 	HASHBUCKET	currBucket;
 	HASHBUCKET *prevBucketPtr;
 	HASHBUCKET *oldPrevPtr;
@@ -1187,17 +1171,8 @@ hash_update_hash_key(HTAB *hashp,
 	 * this to be able to unlink it from its hash chain, but as a side benefit
 	 * we can verify the validity of the passed existingEntry pointer.
 	 */
-	bucket = calc_bucket(hctl, existingElement->hashvalue);
-
-	segment_num = bucket >> hashp->sshift;
-	segment_ndx = MOD(bucket, hashp->ssize);
-
-	segp = hashp->dir[segment_num];
-
-	if (segp == NULL)
-		hash_corrupted(hashp);
-
-	prevBucketPtr = &segp[segment_ndx];
+	bucket = hash_initial_lookup(hashp, existingElement->hashvalue,
+								 &prevBucketPtr);
 	currBucket = *prevBucketPtr;
 
 	while (currBucket != NULL)
@@ -1219,18 +1194,7 @@ hash_update_hash_key(HTAB *hashp,
 	 * chain we want to put the entry into.
 	 */
 	newhashvalue = hashp->hash(newKeyPtr, hashp->keysize);
-
-	newbucket = calc_bucket(hctl, newhashvalue);
-
-	segment_num = newbucket >> hashp->sshift;
-	segment_ndx = MOD(newbucket, hashp->ssize);
-
-	segp = hashp->dir[segment_num];
-
-	if (segp == NULL)
-		hash_corrupted(hashp);
-
-	prevBucketPtr = &segp[segment_ndx];
+	newbucket = hash_initial_lookup(hashp, newhashvalue, &prevBucketPtr);
 	currBucket = *prevBucketPtr;
 
 	/*
@@ -1741,6 +1705,33 @@ element_alloc(HTAB *hashp, int nelem, int freelist_idx)
 	return true;
 }
 
+/*
+ * Do initial lookup of a bucket for the given hash value, retrieving its
+ * bucket number and its hash bucket.
+ */
+static inline uint32
+hash_initial_lookup(HTAB *hashp, uint32 hashvalue, HASHBUCKET **bucketptr)
+{
+	HASHHDR    *hctl = hashp->hctl;
+	HASHSEGMENT segp;
+	long		segment_num;
+	long		segment_ndx;
+	uint32		bucket;
+
+	bucket = calc_bucket(hctl, hashvalue);
+
+	segment_num = bucket >> hashp->sshift;
+	segment_ndx = MOD(bucket, hashp->ssize);
+
+	segp = hashp->dir[segment_num];
+
+	if (segp == NULL)
+		hash_corrupted(hashp);
+
+	*bucketptr = &segp[segment_ndx];
+	return bucket;
+}
+
 /* complain when we have detected a corrupted hashtable */
 static void
 hash_corrupted(HTAB *hashp)