v18-0002-Use-fasthash-for-guc_name_hash.patch

From aa8d1f727b20f65b0506380c318ee823c0657849 Mon Sep 17 00:00:00 2001
From: John Naylor <john.naylor@postgresql.org>
Date: Sun, 21 Jan 2024 17:49:22 +0700
Subject: [PATCH v18 2/3] Use fasthash for guc_name_hash

---
 src/backend/utils/misc/guc.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 8f65ef3d89..67859baf69 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -33,6 +33,7 @@
 #include "catalog/objectaccess.h"
 #include "catalog/pg_authid.h"
 #include "catalog/pg_parameter_acl.h"
+#include "common/hashfn_unstable.h"
 #include "guc_internal.h"
 #include "libpq/pqformat.h"
 #include "parser/scansup.h"
@@ -1324,22 +1325,39 @@ guc_name_compare(const char *namea, const char *nameb)
 static uint32
 guc_name_hash(const void *key, Size keysize)
 {
-	uint32		result = 0;
 	const char *name = *(const char *const *) key;
+	const char *const start = name;
+	fasthash_state hs;
+
+	fasthash_init(&hs, 0);
 
 	while (*name)
 	{
-		char		ch = *name++;
+		int			chunk_len = 0;
 
-		/* Case-fold in the same way as guc_name_compare */
-		if (ch >= 'A' && ch <= 'Z')
-			ch += 'a' - 'A';
+		while (chunk_len < FH_SIZEOF_ACCUM && name[chunk_len] != '\0')
+		{
+			chunk_len++;
+			hs.accum <<= BITS_PER_BYTE;
+			hs.accum |= name[chunk_len];
+		}
 
-		/* Merge into hash ... not very bright, but it needn't be */
-		result = pg_rotate_left32(result, 5);
-		result ^= (uint32) ch;
+		/*
+		 * Quick ASCII-only downcasing. Note: This effectively pads spaces
+		 * when the input is not a multiple of 8. This would be okay even if
+		 * space were a valid name character, since the actual length acts as
+		 * a tiebreaker for the finalizer.
+		 */
+		hs.accum |= 0x2020202020202020;
+
+		/* merge into hash and reset for next iteration */
+		fasthash_combine(&hs);
+		hs.accum = 0;
+
+		name += chunk_len;
 	}
-	return result;
+
+	return fasthash_final32(&hs, name - start);
 }
 
 /*
-- 
2.43.0