This is question 3.60 (pg 312) from Book: Computer Systems: A Programmers Perspective (3rd Edition):

Consider the following assembly code:

long loop(long x, int n)

x in %rdi, n in %esi

1 loop:

2 movl %esi, %ecx

3 movl $1, %edx

4 movl $0, %eax

5 jmp .L2

6 .L3:

7 movq %rdi, %r8

8 andq %rdx, %r8

9 orq %r8, %rax

10 salq %cl, %rdx

11 .L2:

12 testq %rdx, %rdx

13 jne .L3

14 rep; ret

The preceding code was generated by compiling C code that had the following overall form:

1 long loop(long x, long n)

2 {

3 long result = _____;

4 long mask;

5 for (mask = ______; mask ______; mask = _____) {

6 result I= ______;

7 }

8 return result;

9 }

Your task is to fill in the missing parts of the C code to get a program equivalent to the generated assembly code. Recall that the result of the function is returned in register %rax. You will find it helpful to examine the assembly code before, during, and after the loop to form a consistent mapping between the registers and the program variables.

A. Which registers hold program values x, n, result, and mask?

B. What are the initial values of result and mask?

C. What is the test condition for mask?

D. How does mask get updated?

E. How does result get updated?

F. Fill in all the missing parts of the C code.