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
Let’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 “a & false” beforehand and substitutes “false” 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 a 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
Frequently Asked Questions
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.
Check out these useful blogs on -
Refer to our guided paths on Coding Ninjas Studio to learn more about DSA, Competitive Programming, JavaScript, System Design, etc. Enroll in our courses and refer to the mock test and problems available, interview puzzles, take a look at the interview experiences, and interview bundle for placement preparations.
Do upvote our blog to help other ninjas grow
Happy Reading!!
311+ registered 
