lecture-Oct-13.md

Compiling Functions

The R4 Language

Concrete Syntax:

type ::= ... | (type... -> type)
exp ::= ... | (exp exp...)
def ::= (define (var [var : type]...) : type exp)
R4 ::= def... exp

Abstract Syntax:

exp ::= ... | (Apply exp exp...)
def ::= (Def var ([var : type] ...) type '() exp)
R4 ::= (ProgramDefsExp '() (def ...) exp)

• Because of the function type (type... -> type), functions are first-class in that they can be passed as arguments to other functions and returned from them. They can also be stored inside tuples.

• Functions may be recursive and even mutually recursive. That is, each function name is in scope for the entire program.

Example program:

(define (map-vec [f : (Integer -> Integer)]
                 [v : (Vector Integer Integer)]) : (Vector Integer Integer)
   (vector (f (vector-ref v 0)) (f (vector-ref v 1))))
   
(define (add1 [x : Integer]) : Integer
   (+ x 1))
   
(vector-ref (map-vec add1 (vector 0 41)) 1)

Go over the interpreter (Fig. 6.4)

Functions in x86

Labels can be used to mark the beginning of a function

The address of a label can be obtained using the leaq instruction and PC-relative addressing:

leaq add1(%rip), %rbx

Calling a function whose address is in a register, i.e., indirect function call.

callq *%rbx

Abstract Syntax:

arg ::= ... | (FunRef label)
instr ::= ... | (IndirectCallq arg) | (TailJmp arg) 
       | (Instr 'leaq (list arg arg))
def ::= (Def label '() '() info ((label . block) ...))
x86_3 ::= (ProgramDefs info (def...))

Calling Conventions

The callq instruction

1. pushes the return address onto the stack
2. jumps to the target label or address (for indirect call)

But there is more to do to make a function call:

1. parameter passing
2. pushing and popping frames on the procedure call stack
3. coordinating the use of registers for local variables

Parameter Passing

The C calling convention uses the following six registers (in that order) for argument passing:

rdi, rsi, rdx, rcx, r8, r9

The calling convention says that the stack may be used for argument passing if there are more than six arguments, but we shall take an alternate approach that makes it easier to implement efficient tail calls. If there are more than six arguments, then r9 will store a tuple containing the sixth argument and the rest of the arguments.

Pushing and Popping Frames

The instructions for each function will have a prelude and conclusion similar to the one we've been generating for main.

The most important aspect of the prelude is moving the stack pointer down by the size needed the function's frame. Similarly, the conclusion needs to move the stack pointer back up.

Recall that we are storing variables of vector type on the root stack. So the prelude needs to move the root stack pointer r15 up and the conclusion needs to move the root stack pointer back down. Also, in the prelude, this frame's slots in the root stack must be initialized to 0 to signal to the garbage collector that those slots do not yet contain a pointer to a vector.

As we did for main, the prelude must also save the contents of the old base pointer rbp and set it to the top of the frame, so that we can use it for accessing local variables that have been spilled to the stack.

Caller View	Callee View	Contents	Frame
8(%rbp)		return address
0(%rbp)		old rbp
-8(%rbp)		callee-saved	Caller (e.g. map-vec)
...		...
-8(j+1)(%rbp)		spill
...		...
	8(%rbp)	return address
	0(%rbp)	old rbp
	-8(%rbp)	callee-saved	Callee (e.g. add1 as f)
	...	...
	-8(j+1)(%rbp)	spill
	...	...

Coordinating Registers

Recall that the registers are categorized as either caller-saved or callee-saved.

If the function uses any of the callee-saved registers, then the previous contents of those registers needs to be saved and restored in the prelude and conclusion of the function.

Regarding caller-saved registers, nothing new needs to be done. Recall that we make sure not to assign call-live variables to caller-saved registers.

Efficient Tail Calls

Normally the amount of stack space used by a program is O(d) where d is the depth of nested function calls.

This means that recursive functions almost always use at least O(n) space.

However, we can sometimes use much less space.

A tail call is a function call that is the last thing to happen inside another function.

Example: the recursive call to tail-sum is a tail call.

(define (tail-sum [n : Integer] [r : Integer]) : Integer
  (if (eq? n 0) 
      r
      (tail-sum (- n 1) (+ n r))))

(+ (tail-sum 5 0) 27)


(define (sum [n : Integer]) : Integer
  (if (eq? n 0) 
      0
      (+ n (sum (- n 1))))) ;; not a tail call

Because a tail call is the last thing to happen, we no longer need the caller's frame and can reuse that stack space for the callee's frame. So we can clean up the current frame and then jump to the callee. However, some care must be taken regarding argument passing.

The standard convention for passing more than 6 arguments is to use slots in the caller's frame. But we're deleting the caller's frame. We could use the callee's frame, but its difficult to move all the variables without stomping on eachother because the caller and callee frames overlap in memory. This could be solved by using auxilliary memory somewhere else, but that increases the amount of memory traffic.

We instead recommend using the heap to pass the arguments that don't fit in the 6 registers.

Instead of callq, use jmp for the tail call because the return address that is already on the stack is the correct one.

Use rax to hold the target address for an indirect jump.