Table of contents

Introduction

Loop Unrolling in Compiler Design

Benefits and Tradeoffs of Loop Unrolling

Techniques for Loop Unrolling

Other Loop Optimization Techniques

Frequently Asked Questions

6.1.

How does loop unrolling improve program performance?

6.2.

How do compilers decide when to use loop unrolling?

6.3.

Are there any alternatives to loop unrolling for loop optimisation?

Conclusion

Last Updated: Mar 27, 2024

Easy

Loop Unrolling in Compiler Design

Author Shiva

Do you think IIT Guwahati certified course can help you in your career?

Yes

Introduction

As an engineer, we can’t get satisfied with our code working. If a piece of code is working the way it's supposed to be, well, it’s not enough; it has to be optimized. And for that optimized code, there’s always room for more optimization. Today we’ll learn about loop unrolling in compiler design, an optimization technique. Loop unrolling is a technique used in compiler design to optimize the performance of loops in a program.

In this article, we'll understand how to loop unrolling in compiler design works, the benefits and tradeoffs, and some techniques compilers use to perform loop unrolling.

Loop Unrolling in Compiler Design

Let's start by defining what a loop means in computer programming. It's a sequence of code that runs repeatedly, only ending when a specific condition is met. As a basic component of algorithms, loops are widely utilized in programming.

The fundamental idea of loop unrolling is to decrease the number of loop iterations.

Let’s understand by an example.

for (int i = 0; i < 9; i++) {
    cout<<”Coding Ninjas\n”;
}

Output:

The body of the loop will run 9 times during this loop. We can, however, cut down on the number of iterations if we unroll the loop. For instance, the code might seem as follows if the loop were unrolled by a factor of 3:

for (int i = 0; i < 9; i += 3) {
    cout<<”Coding Ninjas\n”;
    cout<<”Coding Ninjas\n”;
    cout<<”Coding Ninjas\n”;
}

Output:

In this case, each iteration of the loop involves running 3 times. This reduces the number of iterations to 3, which may lead to a noticeable boost in performance. This is because compiler don’t have to check conditions of “for loops” for every single iteration. There is no difference in result whatsoever as we can see from the output above.

Now that we have a basic idea of loop unrolling, let’s get into the benefits and tradeoffs of loop unrolling in compiler design.

Benefits and Tradeoffs of Loop Unrolling

Loop unrolling in compiler design comes with its own set of tradeoffs. Unrolling a loop can increase the code size, impacting performance due to increased cache misses and instruction cache pressure.

Additionally, unrolling a loop can make it harder for the compiler to optimise the code, as it reduces the amount of common code that the compiler can recognise and optimise.

However, loop unrolling is most effective when the loop body is small, and the loop condition is simple.

It is also important to choose the unroll factor carefully, as too small a factor may not provide significant performance gains.

At the same time, too large a factor may result in diminishing returns or even performance degradation.

Techniques for Loop Unrolling

There are several techniques that compilers use to perform loop unrolling in compiler design.

One technique is called full unrolling, where the entire loop is unrolled into a series of instructions. This technique is only practical for very small loops, as it can result in a significant increase in code size.
Another technique is partial unrolling, where only a portion of the loop is unrolled. This technique is more practical for larger loops, as it can provide some benefits of loop unrolling without significantly increasing code size.

To decide whether to unroll a loop or not, compilers use a variety of heuristics and analysis techniques.

One common technique is profile-guided optimisation, where the compiler generates a profile of the program's execution and uses this information to make optimisation decisions.
Another technique is called interprocedural analysis, where the compiler analyses the entire program to identify loops that can be unrolled.

Other Loop Optimization Techniques

in addition to loop unrolling, Compilers use various loop optimisation techniques, including loop fusion and loop tiling. Loop tiling breaks the loop into smaller loops that can be conducted in parallel, and loop fusion combines many loops into a single loop to reduce the number of loop iterations.

Also see, cousins of compiler

Frequently Asked Questions

How does loop unrolling improve program performance?

Loop unrolling reduces the overhead associated with loop control mechanisms, such as loop condition checking and loop variable incrementing. This can result in faster loop execution times, especially for loops with small loop bodies and simple loop conditions.

How do compilers decide when to use loop unrolling?

Compilers use various heuristics to determine whether loop unrolling will result in improved performance. These heuristics include loop body size, loop condition complexity, and available hardware resources. Compilers may also use feedback-directed optimisation techniques to determine the optimal unroll factor for a given loop.

Are there any alternatives to loop unrolling for loop optimisation?

Yes, there are a variety of alternatives to traditional loop optimisation methods, such as software pipelining, strength reduction, and loop-invariant code motion. Compilers may combine different strategies to obtain the best performance because each methodology has advantages and disadvantages.