Re: Local Variables
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- Subject: Re: Local Variables
- From: Michael Welsh Duggan <mwd@...>
- Date: 2014年8月08日 15:53:30 -0400
Michael Welsh Duggan <mwd@cert.org> writes:
> 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):
I take that back. There was a trivial bug in my code. The following
results seem closer to what I would expect. Code attached.
1 global = 3 local = 3 upvalue = 3
2 global = 5 local = 4 upvalue = 4
3 global = 6 local = 6 upvalue = 5
4 global = 8 local = 6 upvalue = 6
5 global = 10 local = 8 upvalue = 7
6 global = 11 local = 8 upvalue = 9
7 global = 12 local = 10 upvalue = 9
8 global = 14 local = 11 upvalue = 10
9 global = 16 local = 12 upvalue = 11
10 global = 17 local = 14 upvalue = 12
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 = 10000000
collectgarbage("stop")
for times = 1, 10 do
local out = {tostring(times)}
for i, v in ipairs(tests) do
local stime, etime, diff
local fn = v[2]
stime = os.time()
for i = 1, repeats do
fn(times)
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)