The Compiler Environment

In the LLAMA, parameters passed to a function and local variables are accessed via the dup instruction on the stack. For that reason, the stack position of parameters and variables has to be known at compile time. This task is performed by the compiler environment: it contains pairs associating parameters and local variables with stack positions.

Let us consider the following LL function:

(defun f5 (x y) (+ x 5 y))

The code produced by <> is:

.proc f5/2
  dup 2              % variable x
  push-constant 5
  dup 3              % variable y
  ll-builtin + 3
  return 2
.end

The arguments for the two dup instructions were determined with the help of the environment. As shown in the following diagram, the compiler has to simulate the effects of the instructions on the stack in order to keep track of the parameter positions:

In the transformation rules given in section 6.2, these changes of the environment are not made explicit. Only if variables are added to the environment (as in defun and let), the environment extension is shown.