LuaJIT: How many times a script accessed global variables?

Question

I'm trying to optimise my LuaJIT code, and I'm wondering, if there is a debug tool, or if I can write one, to check how many times my script accessed global variables/tables/functions?

Answer 1

You can use a proxy table to store globals and divert any access to the global table to this proxy, embellished with tracing functionality.

local globals = {}
setmetatable(_G, {
    __newindex = function (_, k, v)
        print(debug.traceback("Setting global variable " .. k, 2))
        rawset(globals, k, v)
    end,
    __index = function (_, k)
        print(debug.traceback("Getting global variable " .. k, 2))
        return rawget(globals, k)
    end,
})

a = 1
a = 2

print(a)

Sample output:

Setting global variable a
stack traceback:
    prog.lua:15: in main chunk
    [C]: at 0x00404960
Setting global variable a
stack traceback:
    prog.lua:16: in main chunk
    [C]: at 0x00404960
Getting global variable a
stack traceback:
    prog.lua:18: in main chunk
    [C]: at 0x00404960
2

Live example on Wandbox