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GO Classes DPP | COMPILER|CODE OPTIMIZATION

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2 2 votes

Consider the following C code segment:
 

int a = 10, b = 20;
for (int i = 0; i < n; i++)
{
    int constant_val = a * b + 5;
    for (int j = 0; j < n; j++)
    {
        int temp = i * 8;
        arr[i][j] = temp + j;
        
        if (0) {
            printf("Debugging: %d", arr[i][j]);
        }
    }
}


Which one of the following is FALSE?

  1. THE CODE CONTAINS LOOP INVARIANT COMPUTATION
     
  2. THERE IS SCOPE FOR STRENGTH REDUCTION IN THIS CODE
     
  3. THERE IS SCOPE FOR DEAD CODE ELIMINATION IN THIS CODE
     
  4. THERE IS SCOPE FOR COMMON SUB-EXPRESSION ELIMINATION IN THIS CODE

2 Answers

0 0 votes

Loop Invariant Computation (Option A is TRUE): The expression $\verb|int constant_val = a * b + 5;|$ inside the outer loop does not depend on the loop variable $\verb|i|$. It can be moved outside the entire loop structure.

Strength Reduction (Option B is TRUE): The expression $\verb|temp = i * 8|$ involves a multiplication. A compiler can replace this with a bitwise shift $(i \ll 3)$ or by using addition in each iteration of the loop, which are "cheaper" operations.

Dead Code Elimination (Option C is TRUE): The block $\verb|if (0)| ~\{ ~\ldots ~\}$ will never execute because the condition is always false. The compiler can remove this entire block of code during the optimization phase.

Common Sub-expression Elimination (Option D is FALSE): In this specific snippet, there are no identical expressions calculated multiple times that could be replaced by a single variable. Each calculation $\verb|(a * b + 5|$, $\verb|i * 8|$, and $\verb|temp + j|)$ is unique within its scope.

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