-O0 miscompiles unused struct-returning call inside while loop

[g-r-a-n-t]

fe test -O0 appears to miscompile a while loop that stores the result of one struct-returning function call, then performs a second unused struct-returning function call in the same loop body.

The same source passes at -O1 and -O2.

Version

fe 26.1.0

Minimal Reproduction

  pub struct Point { pub x: u256, pub y: u256 }

  pub fn point_add(_ p: Point, _ q: Point) -> Point {
      Point { x: p.x + q.x, y: p.y + q.y }
  }

  pub fn repro() -> Point {
      let mut rx: u256 = 0
      let mut ry: u256 = 1
      let mut s: u256 = 1

      while s > 0 {
          let r = point_add(Point { x: rx, y: ry }, Point { x: 5, y: 7 })
          rx = r.x
          ry = r.y

          let _unused = point_add(Point { x: 5, y: 7 }, Point { x: 5, y: 7 })
          s = 0
      }

      Point { x: rx, y: ry }
  }

  #[test]
  fn returns_saved_result_not_unused_result() {
      let result = repro()
      assert(result.x == 5)
      assert(result.y == 8)
  }

Commands

  fe check repro.fe
  fe test -O0 repro.fe --call-trace
  fe test -O1 repro.fe
  fe test -O2 repro.fe

Observed

fe check passes.

-O0 fails:

  FAIL  [0.0004s] returns_saved_result_not_unused_result
      Test reverted: 0x4e487b710000000000000000000000000000000000000000000000000000000000000001
  --- call trace ---
  CALL $0 data= -> revert ret=4e487b710000000000000000000000000000000000000000000000000000000000000001
  --- end trace ---

  test result: FAILED. 0 passed; 1 failed

-O1 and -O2 both pass:

  PASS  [0.0002s] returns_saved_result_not_unused_result

  test result: ok. 1 passed; 0 failed

Expected

-O0, -O1, and -O2 should all return Point { x: 5, y: 8 } and pass the test.

Extra Note

If the test is changed to assert the unused call’s result, Point { x: 10, y: 14 }, it passes at -O0. That suggests the second unused struct-returning call may be overwriting, aliasing, or being read in place of the saved result.

[sbillig]

Fixed in 26.2

I think in one of these commits, or related changes:
b918a9f Avoid aggregate object copies in runtime lowering
1ae5e5d Avoid copying read-only owned aggregates