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swernli opened this issue Apr 11, 2025 · 0 comments · Fixed by #2388
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QIR Generation panics when conditional branches use early return #2290

swernli opened this issue Apr 11, 2025 · 0 comments · Fixed by #2388
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swernli commented Apr 11, 2025 •
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The following code causes a panic in QIR generation:

function Branch(value : Bool) : Int {
    if (value) {
        return 3;
    } else {
        return 4;
    }
}
@EntryPoint()
operation Main() : Int {
    use qubits = Qubit[2];
    H(qubits[0]);
    CNOT(qubits[0], qubits[1]);
    let result = Branch(MResetZ(qubits[0]) == Zero);
    result
}

However, the same code with implicit returns in the function works as expected:

function Branch(value : Bool) : Int {
    if (value) {
        3
    } else {
        4
    }
}
@EntryPoint()
operation Main() : Int {
    use qubits = Qubit[2];
    H(qubits[0]);
    CNOT(qubits[0], qubits[1]);
    let result = Branch(MResetZ(qubits[0]) == Zero);
    result
}

This appears to be because of how the BranchControlFlow::Return is used to short circuit the handling of the rest of the block:

qsharp/compiler/qsc_partial_eval/src/lib.rs

Lines 1837 to 1839 in cd5c2e3

if body_control.is_return() {
return Ok(BranchControlFlow::Return(body_control.into_value()));
}

This causes the generated RIR program to be missing branch and jump terminator instructions at the end of blocks. The empty block (should have one terminating jump) is what triggers the specific panic, but other non-empty blocks also are missing terminators that mean the program is invalid.

Either the early return should be fully handled by refactoring the control flow handling for dynamic branches (which must correctly handling inlining for functions like the example above) or the RuntimeFeatureFlags::ReturnWithinDynamicScope should be updated to require more than just TargetCapabilityFlags::Adaptive so that RCA rejects this pattern at compile time.
@swernli swernli added bug Something isn't working needs triage labels Apr 11, 2025
@swernli swernli mentioned this issue May 8, 2025
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swernli added a commit that referenced this issue May 8, 2025
@swernli
Fix panic in QIR generation when conditional branches use early return
fac6ee5 
This change avoids the panic in early returns by blocking them in RCA and providing the user with compile-time feedback before failing during QIR generation. It also includes updates to partial evaluation to ensure that the panicking case is now treated as a unimplemented graceful failure, since we don't have proper support for it.

Since QIR generation for programs with early return have never properly been handled, this doesn't take any functionality away but rather ensures earlier indication of the unsupported patterns at compile time. I filed #2387 to cover the additional work needed to fully support dynamic explicit returns.

Fixes #2290
@swernli swernli mentioned this issue May 8, 2025
Merged
github-merge-queue bot pushed a commit that referenced this issue May 13, 2025
@swernli
Fix panic in QIR generation when conditional branches use early return (
466f059 
#2388)

This change avoids the panic in early returns by blocking them in RCA
and providing the user with compile-time feedback before failing during
QIR generation. It also includes updates to partial evaluation to ensure
that the panicking case is now treated as a unimplemented graceful
failure, since we don't have proper support for it.

Since QIR generation for programs with early return have never properly
been handled, this doesn't take any functionality away but rather
ensures earlier indication of the unsupported patterns at compile time.
I filed #2387 to cover the additional work needed to fully support
dynamic explicit returns.

Fixes #2290
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Fix panic in QIR generation when conditional branches use early return microsoft/qsharp
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