Optimize appending to tables in core.serialize and dump

builtin/common/misc_helpers.lua

@@ -122,8 +122,16 @@ function dump(value, indent)
 	local newline = indent == "" and "" or "\n"
 
 	local rope = {}
-	local function write(str)
-		table.insert(rope, str)
+	local write
+	do
+		-- Keeping the length of the table as a local variable is *much*
+		-- faster than invoking the length operator.
+		-- See https://gitspartv.github.io/LuaJIT-Benchmarks/#test12.
+		local i = 0
+		function write(str)
+			i = i + 1
+			rope[i] = str
+		end
 	end
 
 	local n_refs = {}

builtin/common/serialize.lua

@@ -218,9 +218,13 @@ function core.serialize(value)
 		core.log("deprecated", "Support for dumping functions in `core.serialize` is deprecated.")
 	end
 	local rope = {}
+	-- Keeping the length of the table as a local variable is *much*
+	-- faster than invoking the length operator.
+	-- See https://gitspartv.github.io/LuaJIT-Benchmarks/#test12.
+	local i = 0
 	serialize(value, function(text)
-		 -- Faster than table.insert(rope, text) on PUC Lua 5.1
-		rope[#rope + 1] = text
+		i = i + 1
+		rope[i] = text
 	end)
 	return table_concat(rope)
 end