The register keyword in C is a hint to the compiler that a particular variable should be stored in a CPU register instead of in memory. This is used to optimize performance, as accessing a CPU register is typically faster than accessing memory. However, it's important to note that the use of register is just a suggestion to the compiler; modern compilers may ignore this hint if they determine that it won't improve performance.

When to Use register:

1. Frequently Accessed Variables: Use register for variables that are frequently accessed, such as loop counters.

2. Small and Fast Operations: Ideal for variables involved in small, fast operations where the overhead of memory access can be a bottleneck.

When Not to Use register:

1. Large Variables: Avoid using register for large data structures or arrays, as they can't be fit into a register.

2. Pointer Variables: Generally, it's not advisable for pointers, since modern compilers are typically better at optimization.

3. Address Required: If you need the address of a variable, don’t use register, as you can’t take the address of a register.

#include <stdio.h>

int main() {
    // Using register for a loop counter
    register int i;

    for (i = 0; i < 10; i++) {
        printf("%d\n", i);
    }

    return 0;
}

Explanation:

• Here, the variable i is suggested to be stored in a register for faster access, which can be beneficial in the loop for incrementing and checking the loop counter.

• This is a typical scenario where register might be useful, especially in older or less sophisticated compilers.

• However, it's worth noting that modern compilers are quite good at optimizing such uses even without the register keyword.