ISRO | ISRO CS 2011 | Question 3

In compiler terminology reduction in strength means
(A) Replacing run time computation by compile time computation
(B) Removing loop invariant computation
(C) Removing common subexpressions
(D) replacing a costly operation by a relatively cheaper one


Answer: (D)

Explanation: Strength Reduction is a compiler optimization in which costly operations are replaced by cheaper ones. Example: Exponentiation is replaced by multiplication and multiplication is in return replaced by addition.
The following code is having multiplication operator:

a = 10;
for (i = 0; i < X; i++)
{
    Z[i] = a * i;
}

This code can be replaced by the following code by replacing the multiplication with addition.

a = 10;
k = 0;
for (i = 0; i < X; i++)
{
    Z[i] = k;
    k = k + a;
}

So, option (D) is correct.

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