Common subexpression elimination in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Common Subexpression Elimination (CSE) in compiler design?

CSE is an optimization technique that finds expressions which are computed more than once and reuses the result instead of recomputing it.

beginner

Why is Common Subexpression Elimination useful?

It reduces the number of repeated calculations, which makes the program run faster and use less resources.

beginner

Give a simple example of a common subexpression.

In the code a = b + c; d = b + c + e;, the expression b + c is a common subexpression.

intermediate

What is a key challenge when applying CSE?

Ensuring that the values used in the expression have not changed between uses, so the reused result is still correct.

intermediate

How does CSE improve program efficiency?

By avoiding repeated calculations, it saves CPU time and can reduce power consumption, especially in large or complex programs.

What does Common Subexpression Elimination do?

A Adds comments to explain code

B Removes unused variables from the code

C Changes variable names to shorter ones

D Reuses results of repeated expressions to avoid recalculation

CSE finds expressions that appear multiple times and reuses their computed values to save time.

Which of the following is an example of a common subexpression?

A x = y + z; a = y + z + 1;

B x = y + z; a = y - z;

C x = y * z; a = y / z;

D x = y + 1; a = z + 1;

The expression y + z appears in both assignments, so it is a common subexpression.

What must be true for CSE to safely reuse a computed value?

A The expression is at the start of the program

B The variables in the expression have not changed since last calculation

C The expression uses only constants

D The expression is inside a loop

If variables change, the old result is no longer valid, so CSE must ensure values are unchanged.

Which of these is NOT a benefit of CSE?

A Reduced memory usage by removing variables

B Less CPU work by avoiding repeated calculations

C Faster program execution

D Potentially lower power consumption

CSE focuses on avoiding repeated calculations, not on removing variables to save memory.

When is CSE typically applied in the compilation process?

A During the optimization phase after code generation

B Before parsing the source code

C During the optimization phase after intermediate code generation

D During linking of compiled files

CSE is usually done on intermediate code to optimize before final code generation.

Explain in your own words what Common Subexpression Elimination is and why it is important.

Think about how repeating the same math over and over can slow down a program.

Describe a simple example where CSE can be applied and what the compiler does to optimize it.

Use a small code snippet with repeated addition or multiplication.