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@berkes
Created January 27, 2024 19:16
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"Benchmarking" some micro optimizations: strange behaviour.
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);
}
@pictographer
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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

@berkes
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berkes commented Feb 9, 2024

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!

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