Interview: A question from when I did CS 101…

Look at the following code:

static int OpPerf_0(int x, int y, int z)
        {
            return x + y + z;
        }
        static int OpPerf_1(int x, int y, int z)
        {
            return sum(x,sum(y,z));
        }
        static int sum(int x, int y)
        {
            return x + y;
        }

First estimate the difference of performance between OpPerf_0 and OpPerf_1?

Write out pseudo-machine code on what will be executed with OpPerf_0 and OpPerf_1.

 

This is a C#, Java, C++, neutral question that tests the person’s understanding of what actually happens when code is executed.

 

OpPerf_0

LOADADDR OpPerf_O

PUSH ReturnAddress

PUSH X

PUSH Y

PUSH Z

CALL  LoadAddress

POP X

POP Y

POP Z

MOVE X, ACC-1

MOVE Y, ACC-0

ADD

MOVE Z, ACC-1

ADD

PUSH ACC-1

CALL ReturnAddress

 

I will leave OpPerf_1 to the reader….

 

OpPerf_1

LOADADDR OpPerf_1

PUSH ReturnAddress

PUSH X

PUSH Y

PUSH Z

CALL LoadAddress

POP X

POP Y

POP Z

LOADADDR sum

PUSH X

PUSH Y

PUSH This.Address_1

CALL LoadAddress

POP X

POP Y

MOVE X, ACC-1

MOVE Y, ACC-0

ADD

PUSH ACC-0

CALL This.Address_1

POP TEMP

PUSH This.Address_2

LOADADDR sum

PUSH TEMP

PUSH Z

CALL LoadAddress

POP X

POP Y

MOVE X, ACC-1

MOVE Y, ACC-0

ADD

PUSH ACC-0

CALL This.Address_2

POP TEMP

PUSH TEMP

CALL ReturnAddress

 

So one is ~ 35 steps and the other is 14 steps, or 250% more steps. Or, a style that instead of needing 10 servers, you need 25 servers….