Re: question on <const>
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- Subject: Re: question on <const>
- From: pocomane <pocomane_7a@...>
- Date: 2020年5月20日 17:32:54 +0200
On Wed, May 20, 2020 at 4:51 PM Andrea <andrea.l.vitali@gmail.com> wrote:
> what are the reasons why <const> is necessary in Lua?
> does this enable optimized and faster bytecode? (I hope so)
As far as I understand, it was introduced just because <close> need
it, and it was for free. However, there is a "Constant elimination
mechanism":
local debug <const> = false
local x = debug or 'x'
print(x)
is translated to (output of luac -l)
0+ params, 3 slots, 1 upvalue, 1 local, 2 constants, 0 functions
1 [1] VARARGPREP 0
2 [2] LOADK 0 0 ; "x"
3 [3] GETTABUP 1 0 1 ; _ENV "print"
4 [3] MOVE 2 0
5 [3] CALL 1 2 1 ; 1 in 0 out
6 [3] RETURN 1 1 1 ; 0 out
while without <const>:
0+ params, 4 slots, 1 upvalue, 2 locals, 2 constants, 0 functions
1 [1] VARARGPREP 0
2 [1] LOADFALSE 0
3 [2] TESTSET 1 0 1
4 [2] JMP 1 ; to 6
5 [2] LOADK 1 0 ; "x"
6 [3] GETTABUP 2 0 1 ; _ENV "print"
7 [3] MOVE 3 1
8 [3] CALL 2 2 1 ; 1 in 0 out
9 [3] RETURN 2 1 1 ; 0 out
However last time I checked this kind of optimization are not always
made. I think that for now is just a promise of future optimization.