Re: Temporary local variables

[JP Florijn <jpflorijn@...>]

This'll work more or less the same as it does in C, except for the role of the GC and dynamic typing.

Every distinct variable you allocate needs to be de-allocated by the garbage collector once it goes out of scope. So if you declare a dozen variables, that is a dozen objects the GC needs to mark and collect. If you declare two variables  and re-use them with objects of the same type, thats only two variables to mark and collect.

Will there be any performance gain? Yes; fewer allocations and fewer GC collections, if the objects get collected at all (at the default Lua pause of 200, the GC often won start a cycle once the program drops into a state where memory usage stabilizes).

Note, however, that dynamic typing clouds the issue. If you were to define 'a' first as a number, then later as a string, the variable could not be reused, and the only difference is that the GC might be able to collect the first number at some earlier time - though it is unlikely that it will, the GC working as it does.

For an overview of the GC, see Roberto's Lua Performance Tips: www.lua.org/gems/sample.pdf

On Wed, Jan 2, 2013 at 4:59 AM, Marc Lepage <mlepage@antimeta.com> wrote:

Say I have an object with about a dozen properties A-L.

I want to process them in small groups, something like:

local a, b = o.a, o.b

if a or b then

    doSomethingWithAB(a, b)

end

local c, d = o.c, o.d

doSomethingWithCD1(c, d)

doSomethingWithCD2(c, d)

It happens that I don't need to use AB after I go on to CD. Yet, I get them into separate local variables.

I am just wondering, will it be any kind of performance gain if I were to reuse the same local variables for CD as for AB?

I mean, what would be going on under the hood, and could a gain be reasonably expected (assuming these functions are called often on lots of objects)? Would it reduce the amount of memory churn?

I think it's more readable as it is, but if it were good practice to reduce the amount of locals in this function (say from 12  to 2) for a gain I'd be willing to do that. Mostly I'm just curious what the difference would be.