Question

Consider the following C code:

long fact(long x) { if (x <= 1) {

return 1; }

  long px = x;
  long fx = fact(x - 1);
  return px * fx;

}

Copy the above C code into a C file. For each line of C code responsible for a push or pop operation (either directly or as a result of a call or return), add a comment describing the stack operation and its purpose.

Hint: try compiling the C code to x86-64 using Arch gcc with the -S, -Og flags.

Answer 1

long fact(long x) {        // BEFORE ENTERING fact: push the value of x onto the stack
                           // BEFORE ENTERING fact: push the return address of next statement (from wherever fact is called) on stack

   if (x <= 1) {
       return 1;            // WHILE RETURNING from fact: pop local variable px off the stack      
                           // WHILE RETURNING from fact: pop return address and save this so that jmp can be done to this address
                           // WHILE RETURNING FROM fact: pop parameter (value of x) from stack
   }

long px = x;               // push local variable px onto the stack
long fx = fact(x - 1);    // BEFORE ENTERING fact: push the value of (x - 1) onto the stack
                           // BEFORE ENTERING fact: push the return address of next statement (i.e. return px * fx) on stack
                           // WHILE RETURNING: pop local variable px off the stack
                           // WHILE RETURNING from fact: pop return address and save this so that jmp can be done to this address
                           // WHILE RETURNING FROM fact: pop parameter (value of x - 1) from stack
                          
return px * fx;           // WHILE RETURNING from fact: pop local variable px off the stack
                           // WHILE RETURNING from fact: pop return address and save this so that jmp can be done to this address
                           // WHILE RETURNING FROM fact: pop parameter (value of x) from stack

}