Re: Does pulling out locals from loops improve performance?
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- Subject: Re: Does pulling out locals from loops improve performance?
- From: Gavin Wraith <gavin@...>
- Date: 2011年9月18日 15:21:43 +0100
In message <4E75EDC0.10400@interfree.it> you wrote:
> I still don't understand what's wrong in my reasoning, since the
> implementation clearly proves me wrong as I showed in my first post.
>
> -- Lorenzo
Consider these three snippets:
A is
do
local e
for i = 1,10 do
e = i*i
print(e)
end -- for
end -- do
This compiles (5.2beta) to:
1 [3] LOADNIL 0 0
2 [4] LOADK 1 -1 ; 1
3 [4] LOADK 2 -2 ; 10
4 [4] LOADK 3 -1 ; 1
5 [4] FORPREP 1 4 ; to 10
6 [5] MUL 0 4 4
7 [6] GETTABUP 5 0 -3 ; _ENV "print"
8 [6] MOVE 6 0
9 [6] CALL 5 2 1
10 [4] FORLOOP 1 -5 ; to 6
11 [8] RETURN 0 1
B is
do
for i = 1,10 do
local e = i*i
print(e)
end -- for
end -- do
This compiles (5.2beta) to:
1 [3] LOADK 0 -1 ; 1
2 [3] LOADK 1 -2 ; 10
3 [3] LOADK 2 -1 ; 1
4 [3] FORPREP 0 4 ; to 9
5 [4] MUL 4 3 3
6 [5] GETTABUP 5 0 -3 ; _ENV "print"
7 [5] MOVE 6 4
8 [5] CALL 5 2 1
9 [3] FORLOOP 0 -5 ; to 5
10 [7] RETURN 0 1
C is
do
for i = 1,10 do
local e
e = i*i
print(e)
end -- for
end -- do
This compiles (5.2beta) to:
1 [3] LOADK 0 -1 ; 1
2 [3] LOADK 1 -2 ; 10
3 [3] LOADK 2 -1 ; 1
4 [3] FORPREP 0 5 ; to 10
5 [4] LOADNIL 4 0
6 [5] MUL 4 3 3
7 [6] GETTABUP 5 0 -3 ; _ENV "print"
8 [6] MOVE 6 4
9 [6] CALL 5 2 1
10 [3] FORLOOP 0 -6 ; to 5
11 [8] RETURN 0 1
Evidently B uses one less instruction. In A the extra
instruction (LOADNIL) occurs outside the loop, in C within it.
So the ranking ( < means "better") appears to be B < A < C.
The key point, I think, is whether the local declaration is part
of an assignment.
--
Gavin Wraith (gavin@wra1th.plus.com)
Home page: http://www.wra1th.plus.com/