[PATCH] D114611: [AVR] Expand STDWSPQRr & STDSPQRr, approach #2

[Patryk Wychowaniec via Phabricator via llvm-commits]

Patryk27 added a comment.

So I think //something// this patch does is not right - I'm not sure how to prepare a minimized, llvm-only example, but if you try to run this code:

  #![no_std]
  #![no_main]
  #![feature(bench_black_box)]
  
  use atmega_hal::{pins, usart::Usart0, Peripherals};
  use core::hint::black_box;
  use panic_halt as _;
  use ufmt::*;
  use void::{ResultVoidExt, Void};
  
  pub type Clock = atmega_hal::clock::MHz20;
  
  #[atmega_hal::entry]
  fn main() -> ! {
      let dp = Peripherals::take().unwrap();
      let pins = pins!(dp);
  
      let mut serial =
          Usart0::<Clock>::new(dp.USART0, pins.pd0, pins.pd1.into_output(), 115200.into());
  
      fun(&mut serial, 23, 23);
      fun(&mut serial, 24, 24);
      fun(&mut serial, 25, 25); // commenting-out this line makes the code work
  
      loop {}
  }
  
  #[inline(never)]
  fn fun(serial: &mut dyn uWrite<Error = Void>, x: u32, y: u32) {
      let _ = black_box(serial);
      let _ = black_box(x / y);
  }

  [dependencies]
  atmega-hal = { git = "https://github.com/Rahix/avr-hal", features = ["atmega328p", "rt"] }
  embedded-hal = "0.2"
  panic-halt = "0.2"
  ufmt = "0.1"
  void = { version = "1", default-features = false }
  
  [profile.release]
  panic = "abort"
  codegen-units = 1
  opt-level = 1
  lto = false

... you'll see that the `25 / 25` division triggers what seems to be a CPU reset, and in reality is caused by a `ret` on a corrupted stack frame.

I've managed to nail the problem down to a miscompilation in `__udivmodsi4` (https://github.com/rust-lang/rust/issues/82242#issuecomment-996805546), as if the `ldd` instructions before `st` / `std` were removed (?), but I'm not sure how to investigate it further 😅


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