Does pulling out locals from loops improve performance?

[Lorenzo Donati <lorenzodonatibz@...>]

Hi all!

Till now I believed that pulling out local definitions from loops would 
improve performance of Lua code. Please compare this trivial example:

----------------------------------
local function f1( array )
   local e
   for i = 1, #array do
      e = array[ i ]
      print( e )
   end
end

local function f2( array )
   for i = 1, #array do
      local e = array[ i ]
      print( e )
   end
end
----------------------------------

I would have said that f1 was faster than f2 because in f2 the "local" 
statement was executed every time, since Lua doesn't do almost any kind 
of optimization when compiling.

But recently by chance I compared the "disassembled code" for those two 
functions, and they look almost the same to me (the case was more 
complex, with more locals - I stripped it down to reduce clutter here):


function <lua_scratchpad9.lua:1,7> (10 instructions, 40 bytes at 003D4510)
1 param, 8 slots, 0 upvalues, 6 locals, 2 constants, 0 functions
	1	[3]	LOADK    	2 -1	; 1
	2	[3]	LEN      	3 0
	3	[3]	LOADK    	4 -1	; 1
	4	[3]	FORPREP  	2 4	; to 9
	5	[4]	GETTABLE 	1 0 5
	6	[5]	GETGLOBAL	6 -2	; print
	7	[5]	MOVE     	7 1
	8	[5]	CALL     	6 2 1
	9	[3]	FORLOOP  	2 -5	; to 5
	10	[7]	RETURN   	0 1

function <lua_scratchpad9.lua:9,14> (10 instructions, 40 bytes at 003D4CB0)
1 param, 8 slots, 0 upvalues, 6 locals, 2 constants, 0 functions
	1	[10]	LOADK    	1 -1	; 1
	2	[10]	LEN      	2 0
	3	[10]	LOADK    	3 -1	; 1
	4	[10]	FORPREP  	1 4	; to 9
	5	[11]	GETTABLE 	5 0 4
	6	[12]	GETGLOBAL	6 -2	; print
	7	[12]	MOVE     	7 5
	8	[12]	CALL     	6 2 1
	9	[10]	FORLOOP  	1 -5	; to 5
	10	[14]	RETURN   	0 1


For readability and for scope minimization I would like to use the style 
of f2, but I learned to use the style of f1 (predeclaring all the 
temporary locals used in the loop) when performance could be at stake.

I cannot trace back where first I learned to do that, but I fear now I 
have interpreted wrongly some optimization tip. I reread Roberto's Lua 
Gem chapter on optimization and the optimization-related pages on the 
WIKI, but nowhere I could find support for that approach. Therefore I'm 
beginning to suspect I was acting under a false belief.

Am I missing something (I'm not an expert of Lua VM so I may have missed 
something related to the meaning of the instructions operands in the 
listings above)?

Any clarification or pointer appreciated!
TIA

-- Lorenzo