"Benchmarking" some micro optimizations: strange behaviour.

main.rs

use std::time::Instant;

fn main() {
    let iterations = 1_000_000;
    let mut odd_count = 0;

    // Using modulus
    let start_modulus = Instant::now();
    for i in 0..iterations {
        if i % 2 != 0 {
            odd_count += 1;
        }
    }
    let duration_modulus = start_modulus.elapsed();
    println!("Modulus: {:?} to count {} odd numbers", duration_modulus, odd_count);

    // Reset odd_count
    odd_count = 0;

    // Using bitwise AND
    let start_bitwise = Instant::now();
    for i in 0..iterations {
        if i & 1 == 1 {
            odd_count += 1;
        }
    }
    let duration_bitwise = start_bitwise.elapsed();
    println!("Bitwise AND: {:?} to count {} odd numbers", duration_bitwise, odd_count);
}

Try it again with optimization enabled.

# Compilation provided by Compiler Explorer at https://godbolt.org/

Standard error:
warning: function `main` is never used
 --> <source>:3:4
  |
3 | fn main() {
  |    ^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted


# Execution result with exit code 0
# Standard out:
Modulus: 39.295µs to count 500000 odd numbers
Bitwise AND: 207.765µs to count 500000 odd numbers

Modulus: 39.295µs to count 500000 odd numbers
Bitwise AND: 207.765µs to count 500000 odd numbers

Wow. that's even worse performing for bitwise and. Something weird is going on!