Drill 5 Solutions

1. Does the current compiler ever put the address of a memory location on the heap onto the stack?

• Yes. For example, (let ((x (pair 1 2))) x) will store 1 and 2 on the heap, and put the memory address of 1 on the stack.

2. Does the current compiler ever put the address of a memory location on the heap into a register?

• Yes. For example, (pair 1 2) will store 1 and 2 on the heap, and put the memory address of 1 into rax.

3. How many 8-byte heap "cells" will be used when each of the following programs are compiled and executed? (Remember that each pair takes up two cells).

   • (+ (left (pair 1 2)) (right (pair 3 4)))
     

     • 4

   • (let ((x false)) 
         (pair 1 (if x (pair 2 (pair 3 (pair 4 false))) 
                         (pair 2 (pair 3 false))
                 )
         )
     )
     

     • 6. Remember that only one branch of the conditional will actually be executed: in this case, the else branch! We can't count heap usage just by counting occurences of pair.

4. For each expression below, will it result in an error or a valid result when run in the interpreter we have built in class?

   program	error	valid result
   (+ 1 3 4)	x
   (+ 1 (let ((x 2)) x))		x
   (+ 1 (let ((x 2)) false))	x
   (left (pair 1 false))		x
   (left (left (pair 1 false)))	x
   (** 2 3)	x
   (if true (* 2 3) (** 2 3))	x

5. For each expression below, will it result in a compile-time error, a run-time error, or a valid result when run in the compiler we have built in class? (Assume that we have added error handling for all supported operations).

   program	compile error	runtime error	valid result
   (+ 1 3 4)	x
   (+ 1 (let ((x 2)) x))			x
   (+ 1 (let ((x 2)) false))		x
   (left (pair 1 false))			x
   (left (left (pair 1 false)))		x
   (** 2 3)	x
   (if true (* 2 3) (** 2 3))	x