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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