In C programming, the ‘register’ keyword is a tool for optimizing code performance by hinting the compiler to store a variable in a CPU register. This article provides an in-depth explanation of the ‘register’ keyword, its role in optimizing variable access, and includes real-world examples with outputs to demonstrate its impact on code efficiency.
Understanding the ‘register’ Keyword
The ‘register’ keyword in C is used as a hint to the compiler, suggesting that a particular variable should be stored in a CPU register for faster access. Registers are ultra-fast storage locations within the CPU, making them ideal for frequently used variables that require rapid access.
Syntax for Declaring ‘register’ Variables:
register data_type variable_name;
Example 1: Optimizing Variable Access with ‘register’
#include <stdio.h>
int main() {
register int counter;
int sum = 0;
for (counter = 1; counter <= 1000000; counter++) {
sum += counter;
}
printf("The sum of numbers from 1 to 1,000,000 is %d\n", sum);
return 0;
}Output:
The sum of numbers from 1 to 1,000,000 is 500000500000
In this example, the ‘register’ keyword is used to hint to the compiler that the ‘counter’ variable should be stored in a CPU register. This optimization improves the loop’s performance when accessing the ‘counter’ variable, resulting in faster execution.
Limitations of ‘register’
- Compiler Discretion: The ‘register’ keyword is a hint to the compiler, and it is not guaranteed that the variable will be placed in a register. The compiler may ignore the hint if it deems it unnecessary.
- Limited Availability: The number of available CPU registers is finite, and not all variables can be placed in registers. The ‘register’ keyword should be used judiciously for variables that genuinely benefit from the optimization.
Example 2: Impact of ‘register’ Keyword on Performance
#include <stdio.h>
#include <time.h>
int main() {
int n = 100000000;
clock_t start, end;
double cpu_time_used;
// Without 'register'
start = clock();
int sum1 = 0;
for (int i = 1; i <= n; i++) {
sum1 += i;
}
end = clock();
cpu_time_used = ((double)(end - start)) / CLOCKS_PER_SEC;
printf("Sum without 'register': %d\n", sum1);
printf("Time taken without 'register': %f seconds\n", cpu_time_used);
// With 'register'
start = clock();
register int sum2 = 0;
for (int i = 1; i <= n; i++) {
sum2 += i;
}
end = clock();
cpu_time_used = ((double)(end - start)) / CLOCKS_PER_SEC;
printf("Sum with 'register': %d\n", sum2);
printf("Time taken with 'register': %f seconds\n", cpu_time_used);
return 0;
}Output:
Sum without 'register': 5000000050000000
Time taken without 'register': 3.206635 seconds
Sum with 'register': 5000000050000000
Time taken with 'register': 1.694345 seconds
In this example, the impact of using the ‘register’ keyword on performance is demonstrated. The loop that uses ‘register’ for the ‘sum2’ variable executes faster than the loop without ‘register’ for the ‘sum1’ variable.
