compiler construction - What is "Lambda Lifting"?

Question

I just ran into this while going through the Erlang compiler source.

I'm not really getting it. (go figure ;)), considering that I just realized that there is such a thing 5 minutes ago).

Forgive me for asking first without first trying to understand reasons for its existence.

There is a wikipedia article about it, but it is pretty cryptic.

Answer 1

Lambda lifting is used to turn a closure into a pure function. By passing extra arguments to the function, you reduce the number of its free variables. As you "lift" the lambda into higher and higher scopes, you add arguments to accommodate the local variables declared in that scope (which would be free variables otherwise). Once the lambda has no free variables it is a pure "top level" function.

Of course you can only do this if you know all the lambda's call sites; in other words, only if the lambda does not escape.

The benefit in a compiler optimizer is that closures (function environments) can be eliminated. This might make it possible to pass the arguments in registers rather than stack (or heap) allocating them as free variables.