ISRO-DEC2017-67

Question

Consider the code segment

int i,j,x,y,m,n;
n = 20;
for(i=0;i<n;i++)
{
    for(j=0;j<n;j++)
    {
        if(i%2)
        {
            x += ((4*j)+5*i);
            y += (7+4*j)
        }
    }
}
m=x+y;

Which one of the following is false?

• A. The code contains loop invariant computation
• B. There is scope of common sub-expression elimination in this code
• C. There is scope of strength reduction in this code
• D. There is scope of dead code elimination in this code

Comments

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Answer 1

• loop invariant: A fragment of code that resides in the loop and computes the same value in all iterations. In given loop if(i%2) computes same value within loop of j. This code can be moved out of the loop.

 for(i=0;i<n;i++) {
if(i%2)
   {
    for(j=0;j<n;j++)
    {
       ......

This movement is called code hoisting

A is true 

• The sub expression  4*j is used at two places which can be eliminated.

B is true

• Strength reduction: Operations that consume more time and space can be replaced by simple operations that produce the same result.

  4*j can be replaced by j <<2

C is true

• There is no dead / Unreachable code.

D is false

Answer is D

Answer 2

option D

There is no scope of dead code elimination.