From 01b053b473d897c71725a7bb09a3127fc78140dc Mon Sep 17 00:00:00 2001
From: John Naylor <john.naylor@postgresql.org>
Date: Wed, 22 Nov 2023 18:45:48 +0700
Subject: [PATCH v4 3/3] Optimize GUC functions for simple hash

Only downcase a character when an equality check fails,
since we expect most names to be lower case. Also simplify
downcasing in the hash function.
---
 src/backend/utils/misc/guc.c | 40 ++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index bf05b022c3..7896deb63b 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -229,6 +229,7 @@ static int	GUCNestLevel = 0;	/* 1 when in main transaction */
 
 
 static int	guc_var_compare(const void *a, const void *b);
+static bool guc_name_eq(const char *namea, const char *nameb);
 static uint32 guc_name_hash(const char *name);
 static void InitializeGUCOptionsFromEnvironment(void);
 static void InitializeOneGUCOption(struct config_generic *gconf);
@@ -271,7 +272,7 @@ static bool call_enum_check_hook(struct config_enum *conf, int *newval,
 #define SH_KEY_TYPE		const char *
 #define	SH_KEY			gucname
 #define SH_HASH_KEY(tb, key)   	guc_name_hash(key)
-#define SH_EQUAL(tb, a, b)		(guc_name_compare(a, b) == 0)
+#define SH_EQUAL(tb, a, b)		(guc_name_eq(a, b))
 #define	SH_SCOPE		static inline
 #define SH_DECLARE
 #define SH_DEFINE
@@ -1308,6 +1309,38 @@ guc_name_compare(const char *namea, const char *nameb)
 	return 0;
 }
 
+static bool
+guc_name_eq(const char *namea, const char *nameb)
+{
+	char		cha;
+	char		chb;
+
+	while (*namea && *nameb)
+	{
+		cha = *namea++;
+		chb = *nameb++;
+
+		if (cha != chb)
+		{
+			/* Casefold lazily since we expect lower case */
+			if (cha >= 'A' && cha <= 'Z')
+				cha += 'a' - 'A';
+			if (chb >= 'A' && chb <= 'Z')
+				chb += 'a' - 'A';
+
+			if (cha != chb)
+				return false;
+		}
+	}
+
+	if (*namea == *nameb)
+		return true;
+	else
+		return false;
+
+	//Assert(guc_name_compare(namea, nameb) == 0);
+}
+
 /*
  * Hash function that's compatible with guc_name_compare
  */
@@ -1320,9 +1353,8 @@ guc_name_hash(const char *name)
 	{
 		char		ch = *name++;
 
-		/* Case-fold in the same way as guc_name_compare */
-		if (ch >= 'A' && ch <= 'Z')
-			ch += 'a' - 'A';
+		/* quick and dirty casefolding suitable for hashing */
+		ch |= 0x20;
 
 		/* Merge into hash ... not very bright, but it needn't be */
 		result = pg_rotate_left32(result, 5);
-- 
2.42.0