Passing arguments to functions with const parameters: is it faster?

Question

Consider, for example:

int sum(int a, int b)
{
    return a + b;
}

vs.

int sum(const int a, const int b)
{
    return a + b;
}
         


        

Answer 1

Short answer: No

Long answer, no, with proof.

I ran this test, a couple of times, and saw no real time difference, on my MacBook pro compiled with clang:

int add(int a, int b)
{
    return a + b;
}

const int cadd(const int a, const int b)
{
    return a + b;
}

int main (int argc, char * argv[])
{
#define ITERS 1000000000

    clock_t start = clock();
    int j = 0;
    for (int i = 0; i < ITERS; i++)
    {
        j += add(i, i + 1);
    }

    printf("add took %li ticks\n", clock() - start);

    start = clock();
    j = 0;
    for (int i = 0; i < ITERS; i++)
    {
        j += cadd(i, i + 1);
    }

    printf("cadd took %li ticks\n", clock() - start);

    return 0;
}

Output

add took 4875711 ticks
cadd took 4885519 ticks

These times really should be taken with a grain of salt, however, as clock isn't the most accurate of timing functions, and can be influenced by other running programs.

So, here is the compared assembly generated:

_add:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register %rbp
    movl    %edi, -4(%rbp)
    movl    %esi, -8(%rbp)
    movl    -4(%rbp), %esi
    addl    -8(%rbp), %esi
    movl    %esi, %eax
    popq    %rbp
    ret

_cadd:                                 
    .cfi_startproc    
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register %rbp
    movl    %edi, -4(%rbp)
    movl    %esi, -8(%rbp)
    movl    -4(%rbp), %esi
    addl    -8(%rbp), %esi
    movl    %esi, %eax
    popq    %rb

So, as you can see, there is No difference between the two. Passing an argument as const is only a hint to the caller the the argument will not be changed, and in a simple scenario like the one described above, will not result in any different assembly compiled.