Speed up lexing of long JSON strings

Use optimized linear search when looking ahead for end quotes,
backslashes, and non-printable characters. This results in nearly 40%
faster JSON parsing on x86-64 when most values are long strings, and
all platforms should see some improvement.

Reviewed by Andres Freund and Nathan Bossart
Discussion: https://www.postgresql.org/message-id/CAFBsxsGhaR2KQ5eisaK%3D6Vm60t%3DaxhD8Ckj1qFoCH1pktZi%2B2w%40mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAFBsxsESLUyJ5spfOSyPrOvKUEYYNqsBosue9SV1j8ecgNXSKA%40mail.gmail.com

src/common/jsonapi.c

@@ -19,6 +19,7 @@
 
 #include "common/jsonapi.h"
 #include "mb/pg_wchar.h"
+#include "port/pg_lfind.h"
 
 #ifndef FRONTEND
 #include "miscadmin.h"
@@ -844,7 +845,7 @@ json_lex_string(JsonLexContext *lex)
 		}
 		else
 		{
-			char	   *p;
+			char	   *p = s;
 
 			if (hi_surrogate != -1)
 				return JSON_UNICODE_LOW_SURROGATE;
@@ -853,11 +854,17 @@ json_lex_string(JsonLexContext *lex)
 			 * Skip to the first byte that requires special handling, so we
 			 * can batch calls to appendBinaryStringInfo.
 			 */
-			for (p = s; p < end; p++)
+			while (p < end - sizeof(Vector8) &&
+				   !pg_lfind8('\\', (uint8 *) p, sizeof(Vector8)) &&
+				   !pg_lfind8('"', (uint8 *) p, sizeof(Vector8)) &&
+				   !pg_lfind8_le(31, (uint8 *) p, sizeof(Vector8)))
+				p += sizeof(Vector8);
+
+			for (; p < end; p++)
 			{
 				if (*p == '\\' || *p == '"')
 					break;
-				else if ((unsigned char) *p < 32)
+				else if ((unsigned char) *p <= 31)
 				{
 					/* Per RFC4627, these characters MUST be escaped. */
 					/*

src/test/regress/expected/json.out

@@ -42,6 +42,19 @@ LINE 1: SELECT '"\v"'::json;
                ^
 DETAIL:  Escape sequence "\v" is invalid.
 CONTEXT:  JSON data, line 1: "\v...
+-- Check fast path for longer strings (at least 16 bytes long)
+SELECT ('"'||repeat('.', 12)||'abc"')::json; -- OK
+       json        
+-------------------
+ "............abc"
+(1 row)
+
+SELECT ('"'||repeat('.', 12)||'abc\n"')::json; -- OK, legal escapes
+        json         
+---------------------
+ "............abc\n"
+(1 row)
+
 -- see json_encoding test for input with unicode escapes
 -- Numbers.
 SELECT '1'::json;				-- OK

src/test/regress/sql/json.sql

@@ -7,6 +7,11 @@ SELECT '"abc
 def"'::json;					-- ERROR, unescaped newline in string constant
 SELECT '"\n\"\\"'::json;		-- OK, legal escapes
 SELECT '"\v"'::json;			-- ERROR, not a valid JSON escape
+
+-- Check fast path for longer strings (at least 16 bytes long)
+SELECT ('"'||repeat('.', 12)||'abc"')::json; -- OK
+SELECT ('"'||repeat('.', 12)||'abc\n"')::json; -- OK, legal escapes
+
 -- see json_encoding test for input with unicode escapes
 
 -- Numbers.