Re: Why OP_CLOSE has been removed?

[Patrick Donnelly <batrick@...>]

On Fri, Jan 20, 2017 at 3:29 AM, Marco Bambini <marco@sqlabs.net> wrote:
> Hi all,
> I was studying the "Closures in Lua" PDF document published some time ago.
> The document explains the usage of the CLOSE opcode (OP_CLOSE) which has been removed in recents Lua versions.
>
> I wondering why this opcode has been removed.
> It is now always used implicitly? (I guess in RETURN and JUMP but what about when a var goes out scope in a block?)
> I think that this choice could add a lot of overhead, am I wrong?
>
> Thanks a lot for the explanation.

There was an unused argument to the JMP instruction (A) which is now
used (since Lua 5.2.0) to signal that closing upvalues is necessary.
(In RETURN, it's always necessary to close any open upvalues.)

See this example:

$ luac -l -p -
do
  local a
  local function b() return a end
end

main <stdin:0,0> (4 instructions at 0x12b5b60)
0+ params, 2 slots, 1 upvalue, 2 locals, 0 constants, 1 function
        1       [2]     LOADNIL         0 0
        2       [3]     CLOSURE         1 0     ; 0x12b5ee0
        3       [3]     JMP             1 0     ; to 4
        4       [4]     RETURN          0 1

function <stdin:3,3> (3 instructions at 0x12b5ee0)
0 params, 2 slots, 1 upvalue, 0 locals, 0 constants, 0 functions
        1       [3]     GETUPVAL        0 0     ; a
        2       [3]     RETURN          0 2
        3       [3]     RETURN          0 1


compare to:

$ luac -l -p -
do
  local a
  local b
end

main <stdin:0,0> (2 instructions at 0x13c3b60)
0+ params, 2 slots, 1 upvalue, 2 locals, 0 constants, 0 functions
        1       [2]     LOADNIL         0 1
        2       [4]     RETURN          0 1


No (seemingly redundant) JMP.

-- 
Patrick Donnelly