GATE | GATE-CS-2006 | Question 60

Consider the following C code segment. 

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

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

Answer: (D)

Explanation:

Question asks about false statement

4*j is common subexpression elimination so B is true.

5*i can be moved out of inner loop so can be i%2. 
Means, A is true as we have loop invariant computation.

Next, 4*j as well as 5*i can be replaced with a = - 4;
before j loop then a = a + 4; where 4*j is computed,
likewise for 5*i. C is true as there is scope of strength 
reduction. 

By choice elimination, we have D.

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