How can I optimize my metatables?

["Soni \"They/Them\" L." <fakedme@...>]

Hi!

I have some code that looks like this:

debug.setmetatable("", {__index=function(o,k)
  local mt = metatables[coroutine.running()].string
  if mt then
    local __index = rawget(mt, "__index")
    if type(__index) == "function" then
      return __index(o,k)
    else
      return __index[k]
    end
  end
  error()
end})

It takes the metatables for the basic Lua types (string, number, nil, 
etc) and replaces them with a proxy metatable. This proxy metatable 
forwards to a different table based on the currently running coroutine. 
This gives me virtualization of those metatables.

It's really slow (3-4x slower[1] than the default string metatable) and 
I'd like to make it faster. Is that possible?

[1] - I haven't actually benchmarked it, but default string metatable 
gives about 2 table accesses per operation; this thing does at least 8 
when using globals, and that doesn't take into account interpreter 
overhead and all the function calls!

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