Constant folding is the process of evaluating constant expressions at compile-time instead of at run-time. This optimization technique helps in reducing the amount of redundant computation performed during program execution. It involves identifying common sub-expressions and replacing them with their computed values at compile-time.

Constant folding is beneficial for improving the performance of a program as it eliminates the need for repeated calculations of the same expression. By performing these computations at compile-time, the program can avoid the overhead of performing the same calculations again and again during run-time.

Constant folding can be applied to arithmetic expressions, logical expressions, and conditional statements. It involves the following steps:

1. Identification of common sub-expressions:
- During the compilation process, the compiler analyzes the code to identify expressions that are computed multiple times.
- These expressions are referred to as common sub-expressions because they appear in more than one location in the code.

2. Replacement of run-time computations by compile-time computations:
- Once the common sub-expressions are identified, the compiler replaces their occurrences in the code with their computed values.
- This replacement is done at compile-time, which means that the expressions are evaluated and their results are directly substituted into the code.

3. Example:
- Consider the following code snippet:
```c
int a = 5;
int b = 10;
int c = a + b;
int d = a + b;
```
- In this code, the expressions `a + b` are common sub-expressions because they are computed twice.
- With constant folding, the compiler can evaluate `a + b` at compile-time and replace it with the computed value.
- After constant folding, the code becomes:
```c
int a = 5;
int b = 10;
int c = 15;
int d = 15;
```
- As a result, the addition operation `a + b` is only performed once during compile-time, reducing the run-time computations.

Constant folding is a local optimization technique that can be applied within a single block of code. It is a part of a broader optimization strategy called data flow analysis, which aims to identify and eliminate redundant computations in a program. By replacing run-time computations with compile-time computations, constant folding significantly improves the efficiency and performance of a program.