@parameter

if statement

This will cause the if statement to run at compile time, there is no runtime performance cost because the path that doesn't run will be excluded from the final binary:

from TargetInfo import os_is_linux

@parameter
if os_is_linux():
    print("this will be included in the binary")
else:
    print("this will be eliminated from compilation process")

this will be included in the binary

function

This will run at compile time, so that you pay no runtime price for anything inside the function:

fn add_print[a: Int, b: Int](): 
    @parameter
    fn add[a: Int, b: Int]() -> Int:
        return a + b

    let x = add[a, b]()
    print(x)

add_print[5, 10]()

15

What this essentially translates to is:

fn add_print(): 
    let x = 15
    print(x)

add_print()

15

The add calculation ran at compile time, so those extra instructions don't happen at runtime