Re: Local Variables
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- Subject: Re: Local Variables
- From: Michael Welsh Duggan <mwd@...>
- Date: 2014年8月08日 15:45:30 -0400
Mason Mackaman <masondeanm@aol.com> writes:
> So to make a program run as fast as possible should you localize
> everything in _ENV you’re going to use in your program?
I was going to say, "No; only if you're going to use it more than once."
But my empirical studies seem to demonstrate otherwise. The included
program has three functions, all which use a global sub-function "foo".
The first function uses it directly as a global reference. The second
stashes it in a local variable and uses it from there. The third has it
pre-stashed as an upvalue. In each run, the function is referenced one
more time than in the previous run. The results look like they speak
for themselves. I tried to be careful in my methodology, but I wouldn't
be surprised if I didn't account for something in my experiment. Feel
free to attempt to replicate my results (numbers are number of seconds):
1 global = 30 local = 23 upvalue = 22
2 global = 29 local = 23 upvalue = 22
3 global = 29 local = 23 upvalue = 22
4 global = 30 local = 23 upvalue = 22
5 global = 29 local = 23 upvalue = 22
6 global = 30 local = 23 upvalue = 22
7 global = 29 local = 23 upvalue = 22
8 global = 29 local = 23 upvalue = 22
9 global = 29 local = 23 upvalue = 22
10 global = 30 local = 23 upvalue = 22
function foo(a, b)
return a + b
end
-- uses global foo
local function bar1(n)
local total = 0
for i = 1, n do
total = total + foo(1, 1)
end
return total
end
-- uses local foo
local function bar2(n)
local total = 0
local foo = foo
for i = 1, n do
total = total + foo(1, 1)
end
return total
end
-- uses upvalue foo
local fooup = foo
local function bar3(n)
local total = 0
for i = 1, n do
total = total + fooup(1, 1)
end
return total
end
local tests = {{"global", bar1}, {"local", bar2}, {"upvalue", bar3}}
local repeats = 20000
collectgarbage("stop")
for i = 1, 10 do
local out = {tostring(i)}
for i, v in ipairs(tests) do
local stime, etime, diff
local fn = v[2]
stime = os.time()
for i = 1, repeats do
fn(i)
end
etime = os.time()
diff= os.difftime(etime, stime)
out[i + 1] = v[1] .. " = " .. diff
collectgarbage()
end
print (table.unpack(out))
end
--
Michael Welsh Duggan
(mwd@cert.org)