Restrict keyword in C

The restrict keyword in C is a type qualifier that provides a hint to the compiler about the absence of aliasing between pointers. It’s used to optimize code by indicating that a particular pointer is the only one that will access a specific region of memory during its scope. The restrict keyword can improve the performance of certain programs by allowing the compiler to make more aggressive optimizations.

The primary purpose of restrict is to help the compiler generate more efficient code, especially in situations where it might otherwise need to reload data from memory multiple times or avoid certain optimizations due to pointer aliasing.

Here’s the basic syntax for using the restrict keyword:

void some_function(int* restrict ptr1, int* restrict ptr2) {
    // The compiler can assume that ptr1 and ptr2 do not alias each other
    // and can perform optimizations accordingly.
}

In this example, ptr1 and ptr2 are declared as restrict pointers. This means that within the scope of the some_function function, the compiler can assume that ptr1 and ptr2 do not point to overlapping memory regions. As a result, it can perform optimizations that would otherwise be unsafe in the presence of aliasing.

It’s important to use the restrict keyword with caution and only when you are absolutely certain that there is no aliasing between pointers. Violating the restrict keyword’s contract (i.e., having aliasing pointers) can lead to undefined behavior.

Keep in mind that the restrict keyword is primarily an optimization hint, and its use may not always lead to significant performance improvements. Its effectiveness depends on the specific code, compiler, and optimization settings.

Here’s an example of using the restrict keyword in a simple function:

#include <stdio.h>

void copy_data(int* restrict dest, const int* restrict src, size_t n) {
    for (size_t i = 0; i < n; i++) {
        dest[i] = src[i];
    }
}

int main() {
    int source[] = {1, 2, 3, 4, 5};
    int destination[5];

    copy_data(destination, source, 5);

    for (size_t i = 0; i < 5; i++) {
        printf("%d ", destination[i]);
    }
    printf("\n");

    return 0;
}

In this example, the copy_data function uses restrict to indicate that dest and src pointers do not alias each other. This allows the compiler to optimize the memory copy operation more effectively.