Does the Lua compiler optimize local vars?

Question

Is the current Lua compiler smart enough to optimize away local variables that are used for clarity?

local top = x - y
local bottom = x + y
someCall(top, bottom)

Or does inlining things by hand run faster?

someCall(x - y, x + y)

Answer 1

Since Lua often compiles source code into byte code on the fly, it is designed to be a fast single-pass compiler. It does do some constant folding, but other than that there are not many optimizations. You can usually check what the compiler does by executing luac -l -l -p file.lua and looking at the generated (disassembled) byte code.

In your case the Lua code

function a( x, y )
  local top = x - y
  local bottom = x + y
  someCall(top, bottom)
end

function b( x, y )
  someCall(x - y, x + y)
end

results int the following byte code listing when run through luac5.3 -l -l -p file.lua (some irrelevant parts skipped):

function <file.lua:1,5> (7 instructions at 0xcd7d30)
2 params, 7 slots, 1 upvalue, 4 locals, 1 constant, 0 functions
    1   [2] SUB         2 0 1
    2   [3] ADD         3 0 1
    3   [4] GETTABUP    4 0 -1  ; _ENV "someCall"
    4   [4] MOVE        5 2
    5   [4] MOVE        6 3
    6   [4] CALL        4 3 1
    7   [5] RETURN      0 1
constants (1) for 0xcd7d30:
    1   "someCall"
locals (4) for 0xcd7d30:
    0   x   1   8
    1   y   1   8
    2   top 2   8
    3   bottom  3   8
upvalues (1) for 0xcd7d30:
    0   _ENV    0   0

function <file.lua:7,9> (5 instructions at 0xcd7f10)
2 params, 5 slots, 1 upvalue, 2 locals, 1 constant, 0 functions
    1   [8] GETTABUP    2 0 -1  ; _ENV "someCall"
    2   [8] SUB         3 0 1
    3   [8] ADD         4 0 1
    4   [8] CALL        2 3 1
    5   [9] RETURN      0 1
constants (1) for 0xcd7f10:
    1   "someCall"
locals (2) for 0xcd7f10:
    0   x   1   6
    1   y   1   6
upvalues (1) for 0xcd7f10:
    0   _ENV    0   0

As you can see, the first variant (the a function) has two additional MOVE instructions, and two additional locals.

If you are interested in the details of the opcodes, you can check the comments for the OpCode enum in lopcodes.h . Eg the opcode format for OP_ADD is:

OP_ADD,/*       A B C   R(A) := RK(B) + RK(C)                           */

So the 2 [3] ADD 3 0 1 from above takes the values from registers 0 and 1 (the locals x and y in this case), adds them together, and stores the result in register 3. It is the second opcode in this function and the corresponding source code is on line 3.