Loop optimization (invariant code motion) in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is loop invariant code motion?

Loop invariant code motion is a compiler optimization technique that moves calculations or expressions that do not change inside a loop to outside the loop. This reduces repeated work and improves performance.

beginner

Why is moving invariant code outside a loop beneficial?

Moving invariant code outside a loop avoids repeating the same calculation multiple times, which saves processing time and makes the program run faster.

beginner

Give an example of a loop invariant expression.

An example is calculating a constant value like 'x = 5 * 3' inside a loop. Since 5 * 3 is always 15, it can be moved outside the loop to avoid repeated calculation.

intermediate

What conditions must be met for code to be considered loop invariant?

The code must not change during the loop iterations, meaning it does not depend on variables modified inside the loop and produces the same result every time.

intermediate

Can loop invariant code motion affect program correctness?

If done correctly, it does not affect correctness. However, if the code depends on variables that change inside the loop, moving it outside can cause wrong results.

Which of the following is an example of loop invariant code?

A A constant value calculation

B An assignment to a loop counter

C A function call that changes global state

D A calculation using a variable updated inside the loop

A constant value calculation does not change during the loop and is loop invariant.

What is the main goal of loop invariant code motion?

A To reduce repeated calculations inside the loop

B To move code inside the loop for clarity

C To increase the number of loop iterations

D To change the loop's output

The goal is to reduce repeated calculations by moving invariant code outside the loop.

Which condition disqualifies code from being loop invariant?

A Code uses only constants

B Code depends on variables updated inside the loop

C Code is a simple arithmetic operation

D Code is outside the loop

If code depends on variables updated inside the loop, it is not invariant.

What could happen if loop invariant code motion is applied incorrectly?

A Program runs faster without issues

B Loop becomes infinite

C Loop runs fewer times

D Program output may become incorrect

Incorrectly moving code that depends on loop variables can cause wrong results.

Which of these is a benefit of loop invariant code motion?

A Makes code harder to read

B Increases memory usage

C Reduces CPU cycles spent in the loop

D Increases loop complexity

Moving invariant code outside the loop reduces CPU cycles spent repeating calculations.

Explain what loop invariant code motion is and why it is useful.

Think about what code inside a loop does not change and how moving it helps.

Describe the conditions that must be met for code to be safely moved outside a loop.

Consider what makes code 'invariant' in the context of a loop.