Question

A function, fun_a, has the following overall structure: int fun_a(unsigned x) I int val 0 while retura Th e Gcc Ccompiler generates the following assembly code: %ebp+8 novi movl test1 x at 8 (%ebp), %eax 80, %eax %edx, %eax 2 L.7 5 .L10 %eax, %edx し10 xor1 %eax shr1 Shift right by1 ne 9.L7 10 andl $1, %eax Reverse engineer the operation of this code and then do the following: A. Use the assembly-code version to fill in the missing parts of the C code. B. Describe in English what this function computes.

Answer 1

   x at %ebp+8

   movl 8(%ebp) ,%edx     ; moving x in edx

   movl $0   ,%eax             ;moving 0 in eax

   testl   %edx,%edx           ;if edx is zero jump at .L7

   je     .L7

.L10

     xorl   %edx,%eax       ; take xor of edx and eax and put in to eax

     shrl %edx                    ; shift by 1 to right

     jne .L10                        ;jump to .L10 if not equal to zero

.L7

       andl $1,%eax             ;take and with 1 of eax as output

//Your C code

int fun_a(unsigned x) {
    int val = 0;
    while(x != 0) {
        val ^= x;  x>>=1;      
    }
    return val & 1;
 }

This function returns 1 if x has odd number of 1's in its binary representation as, 2,4,7 etc.

This function returns 0 if x has even number of 1's in its binary representation as, 0,3,5,6 etc.

//if you have any doubt.Please feel free to ask.

Thanks