Odd code generation in Lua

[Sean Conner <sean@...>]

I've been working on some projects and I've noticed that Lua (5.1.4, with
all the current patches) seems to produce some extraneous instructions.  For
instance, the following Lua code [1]:

local function Y(f)  
  local function g(...) return f(g, ...) end
  return g
end

generates the following Lua opcodes:

main <y.lua:0,0> (2 instructions, 8 bytes at 0x9333d10)
0+ params, 2 slots, 0 upvalues, 1 local, 0 constants, 1 function
        1       [4]     CLOSURE         0 0     ; 0x9333eb0
        2       [4]     RETURN          0 1

function <y.lua:1,4> (5 instructions, 20 bytes at 0x9333eb0)
1 param, 2 slots, 0 upvalues, 2 locals, 0 constants, 1 function
        1       [2]     CLOSURE         1 0     ; 0x9334008
        2       [2]     MOVE            0 0
        3       [2]     MOVE            0 1
        4       [3]     RETURN          1 2
        5       [4]     RETURN          0 1	*** WHAT?

function <y.lua:2,2> (6 instructions, 24 bytes at 0x9334008)
0+ params, 4 slots, 2 upvalues, 1 local, 0 constants, 0 functions
        1       [2]     GETUPVAL        1 0     ; f
        2       [2]     GETUPVAL        2 1     ; g
        3       [2]     VARARG          3 0
        4       [2]     TAILCALL        1 0 0
        5       [2]     RETURN          1 0	*** WHAT?
        6       [2]     RETURN          0 1	*** WHAT?

I've marked the three instructions that appear to be extraneous, in that,
how can the code ever reach this spot, and why are these instructions even
generated?

  -spc (It just seems odd to me to see redundant instructions being
	generated)

[1]	This is the famous Y combinator [2] in Lua.

[2]	Basically, this allows you to create recursive lambda functions.
	http://lua-users.org/lists/lua-l/2010-12/msg01219.html