Fix miscompile due to SSA aliasing by swernli · Pull Request #3775 · microsoft/qdk

Stefan J. Wernli (swernli) · 2026-10-01T17:43:27Z

This change updates variable mapping logic in RIR passes to differentiate between "deep" and "shallow" mappings, where the former indicates the mapping should be followed through successive links until the terminator operand is reached and the latter indicates no further mapping should be performed. Fixes #3769

Andrew Casey (amcasey) · 2026-10-01T22:50:34Z

operation Main() : Int {
    use q0 = Qubit();
    use q1 = Qubit();

    X(q0);
    X(q1);

    mutable n = 0;
    if MResetZ(q0) == One {
        n = 1;
    }

    let h = n;

    if MResetZ(q1) == One {
        n = 2;
    }

    h + n
}

Expected: 3
Actual: 4

Speculative explanation (copilot): At ssa_transform.rs:167, only Shallow(n) aliases are redirected when the earlier phi for n is created. An alias that is still Deep(n) remains tied to the mutable ID. Later, ssa_transform.rs:190 can overwrite the shallow mapping created by a branch-local mutation.

Stefan J. Wernli (swernli) · 2026-10-02T14:13:25Z

operation Main() : Int {

    use q0 = Qubit();

    use q1 = Qubit();



    X(q0);

    X(q1);



    mutable n = 0;

    if MResetZ(q0) == One {

        n = 1;

    }



    let h = n;



    if MResetZ(q1) == One {

        n = 2;

    }



    h + n

}
Expected: 3

Actual: 4

Speculative explanation (copilot): At ssa_transform.rs:167, only Shallow(n) aliases are redirected when the earlier phi for n is created. An alias that is still Deep(n) remains tied to the mutable ID. Later, ssa_transform.rs:190 can overwrite the shallow mapping created by a branch-local mutation.

This is the resolved in the latest changes.

This change updates variable mapping logic in RIR passes to differentiate between "strong" and "weak" mappings, where the former indicates the mapping should be followed through successive links until the terminator operand is reached and the latter indicates no further mapping should be performed. Fixes #3769

Andrew Casey (amcasey)

I'm not super familiar with this code, but the explanation made sense and I couldn't find any remaining issues in buddy testing.

Stefan J. Wernli (swernli) requested a review from Ian Davis (idavis) as a code owner October 1, 2026 17:43

Stefan J. Wernli (swernli) requested a review from Andrew Casey (amcasey) October 1, 2026 17:44