Show awareness of what a compiler has to do to translate recursive programming code

19.2 Recursion – Compiler Translation Overview

When a programmer writes a recursive function, the compiler must generate code that can correctly manage repeated self‑calls at run‑time. This involves several distinct tasks, from analysing the source to producing machine instructions that maintain the call stack, allocate activation records, and, where possible, optimise tail‑calls.

Key Compiler Tasks for Recursive Code

1. Parse the function definition and build an abstract syntax tree (AST).
2. Perform semantic analysis:

   • Check that the function is declared before it is called (allowing forward references).
   • Validate type consistency for parameters and return values.
   • Detect potential infinite recursion (optional static analysis).

3. Generate activation‑record (stack‑frame) layout for each call.
4. Insert code to:

   • Push arguments onto the stack or into registers.
   • Save the return address.
   • Allocate space for local variables.
   • Restore the previous frame on return.

5. Apply optimisation passes, e.g., tail‑call optimisation, inlining, or recursion elimination.
6. Emit target machine code or intermediate representation (IR) that the runtime can execute.

Activation Record (Stack Frame) Structure

An activation record holds all information needed for a single invocation of a recursive function. A typical layout is shown below.

Managing the Call Stack

Each recursive call pushes a new activation record onto the call stack. The stack grows downward (or upward, depending on architecture) and is unwound as calls return. The compiler must ensure:

• Correct ordering of pushes and pops.
• Preservation of caller‑saved registers.
• Proper handling of stack overflow conditions (e.g., recursion depth limits).

Tail‑Call Optimisation (TCO)

If a recursive call is the final action of a function, the compiler can replace the standard call/return sequence with a jump, reusing the current activation record. This transforms a potentially unbounded recursion into an iterative loop at the machine level.

Formally, a function f is tail‑recursive if it can be expressed as:

\$\$f(x) = \begin{cases}

g(x) & \text{if base case} \\

f(h(x)) & \text{otherwise (tail call)}

\end{cases}\$\$

After TCO, the generated code eliminates the need for additional stack frames, reducing memory usage from \$O(n)\$ to \$O(1)\$ for \$n\$ recursive calls.

Example: Factorial Function

int factorial(int n) {
if (n == 0) return 1;
return n * factorial(n - 1);
}

Compiler actions for the above code:

1. Parse and build AST for the conditional and multiplication.
2. Generate code to evaluate n == 0 and branch to the base case.
3. For the recursive case:

   • Push n-1 as argument.
   • Call factorial (push return address, save frame pointer).
   • Upon return, multiply the result by n.
   • Return the product.

4. If the compiler supports TCO, it could transform the function into an iterative loop, but because the multiplication occurs after the recursive call, TCO is not applicable here.

Summary Table of Compiler Responsibilities