Upvalue efficiency [was: Fast metamethods for Userdata types]

[David Favro <lua@...>]

On 04/07/2015 08:07 AM, Roberto Ierusalimschy wrote:
> Yes. The point is that, when a closure has no external variables, the
> difference between the closure and the "function" (or "prototype", as
> it is called in the source code) is very thin. (In fact, in the current
> implementation, there is a 1-1 correspondence between closures and
> functions when there are no external variables.)

This reminds me of a coding-style question that comes up often for me.  It 
amounts to, which is the "better" way to get data into a function, accessing 
an upvalue or passing in a parameter, especially when the use of the 
function is localized such as a function defined within another.  My 
definition of "better" is typically run-time efficiency when the function is 
called many times, but I'd be interested in opinions of style as well.  I 
sometimes superstitiously think that there may be some "advantage" in 
creating a function that has no external references rather than one which 
requires separate closures, but I honestly have no idea if this is true.

I suppose there are two cases, depending on how often the parameter data is 
changed, i.e. is the upvalue's value set prior to each function-call or does 
its value remain the same across many calls.  An example of the latter case:

-- Given:
local foo = {};

-- Is this faster:
local function bar1( x )
    foo[#foo+1] = x;
end
for i = 1, 1000000 do
    bar1( "boo" );
end

-- Or this?
local function bar2( tbl, x )
    tbl[#tbl+1] = x;
end
for i = 1, 1000000 do
    bar2( foo, "boo" );
end