1.
Introduction
2.
Constant Folding in Compiler Design
3.
Example of Constant Folding in Compiler Design
3.1.
Example 1
3.2.
Example 2
3.3.
Example 3
4.
When is Constant Folding Applied in Compiler Design?
5.
Constant Propagation in Compiler Design
6.
Benefits of Constant Folding in Compiler Design
7.
7.1.
Are constant folding and constant propagation the same?
7.2.
What are the benefits of constant folding?
7.3.
What is a compiler optimization technique used to reduce?
8.
Conclusion
Last Updated: Mar 27, 2024
Easy

# Constant Folding in Compiler Design

Yukti Kumari
1 upvote
Master Python: Predicting weather forecasts
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Ashwin Goyal
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## Introduction

Have you ever wondered what happens behind the scenes when you run your code?

A compiler translates your source code in a high-level language, like C++, Java, etc., into machine code to create an executable program.

Once you have written a code, you must have felt the need to optimize it. Similarly, the compiler uses several optimization techniques to make the code more efficient.

Many optimization techniques include dead code elimination, code movement, constant propagation, constant folding in compiler design, etc.

In this article, we will discuss constant folding in compiler design and understand it's working through examples. We will also discuss its advantages.

Let's get started without any further ado.

## Constant Folding in Compiler Design

Constant folding in compiler design is a compiler optimization technique that eliminates expressions of the code whose value can be computed before executing the code.

It implies that if we can determine the value of an expression at compile time itself instead of computing it at run time, then this technique will eliminate it and make the code efficient.

You might wonder how can you determine the value of an expression at compile time itself?

If the expression contains any constant value or contains variables with constant values, then we can compute the value of the expression beforehand.

Basically, multiple constants are folded together and evaluated at the compile time.

Constant folding can be applied for the following data types:

• Boolean values

• Integers, except for division by zero exception.

• Floating point values, with caution while rounding.

The compiler applies these optimizations in the intermediate representation phase of the compilation, which is the 4th phase.

To learn more, check out Phases of a Compiler and,Lexical Analysis in Compiler Design

Letâ€™s understand it with the help of an example.

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## Example of Constant Folding in Compiler Design

In this section, we will walk you through several examples of constant folding in compiler design.

### Example 1

Letâ€™s say we have a statement in a code like:

``a = 500*900+30 ``

Then, the compiler will not generate two instructions, i.e., one multiply and one addition instruction. Instead, it will directly substitute 450030.

So, it replaces the above code by:

``a=450030 ``

### Example 2

If we have a code statement like this:

``b = (10+5)*c ``

Then, the compiler will replace the above code by:

``b = 15*c ``

So, here the compiler does not generate the addition instruction.

### Example 3

If we have a code statement like:

``bool flag = a & false;``

Then it will be optimized as:

``bool flag = false; ``

Constant folding computes the value of the expression â€śa & falseâ€ť beforehand and substitutes â€śfalseâ€ť in its place.

## When is Constant Folding Applied in Compiler Design?

Constant folding is applied:

• During the Intermediate Code Generation phase of the compiler, which generates an intermediate representation of source code.

• After other optimizations that generate constant expressions, which can be eliminated by constant folding.

## Constant Propagation in Compiler Design

Constant propagation is a local optimization technique that substitutes the values of variables and expressions whose values are known beforehand.

Let's see an example.

Consider the following statement of code:

``````int a = 4;
int b = (a+1)*2;``````

The compiler will propagate the constant value of variable in the next statement and replace it by:

``int b=(4+1)*2;``

Now, combining constant propagation and constant folding in compiler design, the compiler replaces the above statements by:

``int b = 5*10;``

It can be further optimized as:

``int b=50;``

## Benefits of Constant Folding in Compiler Design

The following are the benefits of applying constant folding in compiler design:

• It improves the execution time of the program.

• It reduces the overall memory requirement.

• It helps to avoid redundant computations in the code, hence making it more efficient.

• It also reduces power consumption.

• It makes hardware usage more efficient.

• It makes the code shorter overall.

Also see,  cousins of compiler

### Are constant folding and constant propagation the same?

No, constant folding and constant propagation are not the same, but they are related compiler optimization techniques. Constant propagation replaces the bound variable with a constant expression it is bound to. On the other hand, constant folding evaluates the expression with all compile-time inputs.

### What are the benefits of constant folding?

Constant folding is a compiler optimization technique that eliminates run-time computations with compile-time computations. It finds and eliminates the constant expressions at compile time rather than computing them at run-time.

### What is a compiler optimization technique used to reduce?

A compiler optimization technique minimizes or maximizes some of the attributes of an executable computer program. It aims to achieve efficiency by reducing execution time, memory requirements, power consumption, etc.

## Conclusion

In this article, we learned about constant folding in compiler design and understood it's working through examples. We will also discussed its advantages and how it helps in optimizing the code to achieve efficiency.

We hope this blog has helped you enhance your knowledge of constant folding in compiler design.

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