Incorrect code generation for nalgebra's Matrix::swap_rows() · Issue #54462 · rust-lang/rust (original) (raw)

The nalgebra linear algebra library has a swap_rows method which allows the user to swap two rows of a matrix. Unfortunately, I'm currently investigating a code generation heisenbug which causes this method to corrupt the matrix data in some circumstances.

Given the UB-like symptoms, and the fact that the implementation of swap_rows takes multiple (non-overlapping) &mut references to the target matrix, I wondered if this could be a violation of Rust's aliasing rules. However, @nagisa confirmed that this is not not the case, and that the compiler is probably the culprit here. He identified the recent upgrade from LLVM 7 to LLVM 8 as a cause of this issue (but that later turned out to be incorrect).

Here is a minimal reproducer of my problem:

extern crate nalgebra;

use nalgebra::Matrix3x4;

fn swappy() -> Matrix3x4 { let mut mat = Matrix3x4::new(1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12.);

// NOTE: This printf makes the bug go away, suggesting UB or a codegen issue
// println!("Result: {}", mat);

for i in 0..2 {
    for j in i+1..3 {
        if mat[(j, 3)] > mat[(i, 3)] { mat.swap_rows(i, j); }
    }
}

mat

}

fn main() { let mat = swappy(); println!("Result: {}", mat); }

To reproduce the issue, you must build in release mode. The issue is also sensitive to the amount of codegen units in flight, therefore I strongly recommend building with codegen-units=1 as well.

I expect the following output:

  ┌             ┐
  │  9 10 11 12 │
  │  5  6  7  8 │
  │  1  2  3  4 │
  └             ┘

Instead, on my systems (nalgebra 0.16.2, rust 1.29, Ivy Bridge & Haswell CPUs) I get the following output:

  ┌             ┐
  │  9 10 11 12 │
  │  5  6  7  8 │
  │  1  6  7  4 │
  └             ┘