I'm writing some code for arduino using the avr-hal crate, and when I print the result of a division operation, I'm getting the result value of 1 for almost every combination of numbers. Exceptions that I've found so far include when the divisor is a power of two, which I'm assuming is being optimized to a bitshift operation (this is not happening in my minimum code example, I do not know why but it happened in another context so I'm including this detail here in case it comes up again).

I tried this code:

#![no_std] #![no_main] extern crate panic_halt; use arduino_uno::prelude::*; #[arduino_uno::entry] fn main() -> ! { let dp = arduino_uno::Peripherals::take().unwrap(); let mut pins = arduino_uno::Pins::new( dp.PORTB, dp.PORTC, dp.PORTD, ); let mut serial = arduino_uno::Serial::new( dp.USART0, pins.d0, pins.d1.into_output(&mut pins.ddr), 57600.into_baudrate(), ); for x in (5..100).step_by(5) { for y in 1..5 { let z = x / y; ufmt::uwriteln!(&mut serial, "{} / {} = {}\r", x, y, z).void_unwrap(); } } loop {} }

Output:

5 / 1 = 1 5 / 2 = 1 5 / 3 = 1 5 / 4 = 1 10 / 1 = 1 10 / 2 = 1 10 / 3 = 1 10 / 4 = 1 -- snip -- 90 / 1 = 1 90 / 2 = 1 90 / 3 = 1 90 / 4 = 1 95 / 1 = 1 95 / 2 = 1 95 / 3 = 1 95 / 4 = 1 

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (c2de47a9a 2021-01-06) binary: rustc commit-hash: c2de47a9aa4c9812884f341f1852e9c9610f5f7a commit-date: 2021-01-06 host: x86_64-unknown-linux-gnu release: 1.51.0-nightly 

I'm using nightly-2021-01-07 since AVR support has been broken since then. The code has to be built with the --release flag due to this issue: rust-lang/compiler-builtins#400

Here's the repo with the code and configs for AVR: https://github.com/crclark96/avr-division-repro. I'm using an Arduino Uno board for this.