Add inline incremental hash functions for in-memory use

It can be useful for a hash function to expose separate initialization,
accumulation, and finalization steps.  In particular, this is useful
for building inline hash functions for simplehash.  Instead of trying
to whack around hash_bytes while maintaining its current behavior on
all platforms, we base this work on fasthash (MIT licensed) which
is simple, faster than hash_bytes for inputs over 12 bytes long,
and also passes the hash function testing suite SMHasher.

The fasthash functions have been reimplemented using our added-on
incremental interface to validate that this method will still give
the same answer, provided we have the input length ahead of time.

This functionality lives in a new header hashfn_unstable.h. The name
implies we have the freedom to change things across versions that
would be unacceptable for our other hash functions that are used for
e.g. hash indexes and hash partitioning. As such, these should only
be used for in-memory data structures like hash tables. There is also
no guarantee of being independent of endianness or pointer size.

As demonstration, use fasthash for pgstat_hash_hash_key.  Previously
this called the 32-bit murmur finalizer on the three elements,
then joined them with hash_combine(). The new function is simpler,
faster and takes up less binary space. While the collision and bias
behavior were almost certainly fine with the previous coding, now we
have objective confidence of that.

There are other places that could benefit from this, but that is left
for future work.

Reviewed by Jeff Davis, Heikki Linnakangas, Jian He, Junwang Zhao
Credit to Andres Freund for the idea

Discussion: https://postgr.es/m/20231122223432.lywt4yz2bn7tlp27%40awork3.anarazel.de

Author: j-naylor

src/include/common/hashfn_unstable.h

@@ -0,0 +1,221 @@
+/*
+ * hashfn_unstable.h
+ *
+ * Building blocks for creating fast inlineable hash functions. The
+ * functions in this file are not guaranteed to be stable between versions,
+ * and may differ by hardware platform. Hence they must not be used in
+ * indexes or other on-disk structures. See hashfn.h if you need stability.
+ *
+ *
+ * Portions Copyright (c) 2024, PostgreSQL Global Development Group
+ *
+ * src/include/common/hashfn_unstable.h
+ */
+#ifndef HASHFN_UNSTABLE_H
+#define HASHFN_UNSTABLE_H
+
+#include "port/pg_bitutils.h"
+#include "port/pg_bswap.h"
+
+/*
+ * fasthash is a modification of code taken from
+ * https://code.google.com/archive/p/fast-hash/source/default/source
+ * under the terms of the MIT license. The original copyright
+ * notice follows:
+ */
+
+/* The MIT License
+
+   Copyright (C) 2012 Zilong Tan (eric.zltan@gmail.com)
+
+   Permission is hereby granted, free of charge, to any person
+   obtaining a copy of this software and associated documentation
+   files (the "Software"), to deal in the Software without
+   restriction, including without limitation the rights to use, copy,
+   modify, merge, publish, distribute, sublicense, and/or sell copies
+   of the Software, and to permit persons to whom the Software is
+   furnished to do so, subject to the following conditions:
+
+   The above copyright notice and this permission notice shall be
+   included in all copies or substantial portions of the Software.
+
+   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+   BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+   ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+   SOFTWARE.
+*/
+
+/*
+ * fasthash as implemented here has two interfaces:
+ *
+ * 1) Standalone functions, e.g. fasthash32() for a single value with a
+ * known length.
+ *
+ * 2) Incremental interface. This can used for incorporating multiple
+ * inputs. The standalone functions use this internally, so see fasthash64()
+ * for an an example of how this works.
+ */
+
+
+typedef struct fasthash_state
+{
+	/* staging area for chunks of input */
+	uint64		accum;
+
+	uint64		hash;
+} fasthash_state;
+
+#define FH_SIZEOF_ACCUM sizeof(uint64)
+
+#define FH_UNKNOWN_LENGTH 1
+
+/*
+ * Initialize the hash state.
+ *
+ * 'len' is the length of the input, if known ahead of time.
+ * If that is not known, pass FH_UNKNOWN_LENGTH.
+ * 'seed' can be zero.
+ */
+static inline void
+fasthash_init(fasthash_state *hs, int len, uint64 seed)
+{
+	memset(hs, 0, sizeof(fasthash_state));
+	hs->hash = seed ^ (len * 0x880355f21e6d1965);
+}
+
+/* both the finalizer and part of the combining step */
+static inline uint64
+fasthash_mix(uint64 h, uint64 tweak)
+{
+	h ^= (h >> 23) + tweak;
+	h *= 0x2127599bf4325c37;
+	h ^= h >> 47;
+	return h;
+}
+
+/* combine one chunk of input into the hash */
+static inline void
+fasthash_combine(fasthash_state *hs)
+{
+	hs->hash ^= fasthash_mix(hs->accum, 0);
+	hs->hash *= 0x880355f21e6d1965;
+
+	/* reset hash state for next input */
+	hs->accum = 0;
+}
+
+/* accumulate up to 8 bytes of input and combine it into the hash */
+static inline void
+fasthash_accum(fasthash_state *hs, const char *k, int len)
+{
+	uint32		lower_four;
+
+	Assert(hs->accum == 0);
+	Assert(len <= FH_SIZEOF_ACCUM);
+
+	switch (len)
+	{
+		case 8:
+			memcpy(&hs->accum, k, 8);
+			break;
+		case 7:
+			hs->accum |= (uint64) k[6] << 48;
+			/* FALLTHROUGH */
+		case 6:
+			hs->accum |= (uint64) k[5] << 40;
+			/* FALLTHROUGH */
+		case 5:
+			hs->accum |= (uint64) k[4] << 32;
+			/* FALLTHROUGH */
+		case 4:
+			memcpy(&lower_four, k, sizeof(lower_four));
+			hs->accum |= lower_four;
+			break;
+		case 3:
+			hs->accum |= (uint64) k[2] << 16;
+			/* FALLTHROUGH */
+		case 2:
+			hs->accum |= (uint64) k[1] << 8;
+			/* FALLTHROUGH */
+		case 1:
+			hs->accum |= (uint64) k[0];
+			break;
+		case 0:
+			return;
+	}
+
+	fasthash_combine(hs);
+}
+
+/*
+ * The finalizer
+ *
+ * 'tweak' is intended to be the input length when the caller doesn't know
+ * the length ahead of time, such as for NUL-terminated strings, otherwise
+ * zero.
+ */
+static inline uint64
+fasthash_final64(fasthash_state *hs, uint64 tweak)
+{
+	return fasthash_mix(hs->hash, tweak);
+}
+
+/*
+ * Reduce a 64-bit hash to a 32-bit hash.
+ *
+ * This optional step provides a bit more additional mixing compared to
+ * just taking the lower 32-bits.
+ */
+static inline uint32
+fasthash_reduce32(uint64 h)
+{
+	/*
+	 * Convert the 64-bit hashcode to Fermat residue, which shall retain
+	 * information from both the higher and lower parts of hashcode.
+	 */
+	return h - (h >> 32);
+}
+
+/* finalize and reduce */
+static inline uint32
+fasthash_final32(fasthash_state *hs, uint64 tweak)
+{
+	return fasthash_reduce32(fasthash_final64(hs, tweak));
+}
+
+/*
+ * The original fasthash64 function, re-implemented using the incremental
+ * interface. Returns a 64-bit hashcode. 'len' controls not only how
+ * many bytes to hash, but also modifies the internal seed.
+ * 'seed' can be zero.
+ */
+static inline uint64
+fasthash64(const char *k, int len, uint64 seed)
+{
+	fasthash_state hs;
+
+	fasthash_init(&hs, len, seed);
+
+	while (len >= FH_SIZEOF_ACCUM)
+	{
+		fasthash_accum(&hs, k, FH_SIZEOF_ACCUM);
+		k += FH_SIZEOF_ACCUM;
+		len -= FH_SIZEOF_ACCUM;
+	}
+
+	fasthash_accum(&hs, k, len);
+	return fasthash_final64(&hs, 0);
+}
+
+/* like fasthash64, but returns a 32-bit hashcode */
+static inline uint64
+fasthash32(const char *k, int len, uint64 seed)
+{
+	return fasthash_reduce32(fasthash64(k, len, seed));
+}
+
+#endif							/* HASHFN_UNSTABLE_H */

src/include/utils/pgstat_internal.h

@@ -14,7 +14,7 @@
 #define PGSTAT_INTERNAL_H
 
 
-#include "common/hashfn.h"
+#include "common/hashfn_unstable.h"
 #include "lib/dshash.h"
 #include "lib/ilist.h"
 #include "pgstat.h"
@@ -776,16 +776,10 @@ pgstat_cmp_hash_key(const void *a, const void *b, size_t size, void *arg)
 static inline uint32
 pgstat_hash_hash_key(const void *d, size_t size, void *arg)
 {
-	const PgStat_HashKey *key = (PgStat_HashKey *) d;
-	uint32		hash;
+	const char *key = (const char *) d;
 
 	Assert(size == sizeof(PgStat_HashKey) && arg == NULL);
-
-	hash = murmurhash32(key->kind);
-	hash = hash_combine(hash, murmurhash32(key->dboid));
-	hash = hash_combine(hash, murmurhash32(key->objoid));
-
-	return hash;
+	return fasthash32(key, size, 0);
 }
 
 /*

src/tools/pgindent/typedefs.list

@@ -3328,6 +3328,7 @@ exec_thread_arg
 execution_state
 explain_get_index_name_hook_type
 f_smgr
+fasthash_state
 fd_set
 fe_scram_state
 fe_scram_state_enum