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…ting a gate cx a, b over two registers unrolled to cx a[0], a[1]; cx b[0], b[1] because every operand was flattened into one list and chunked by the gate's arity. When a call has one operand per gate qubit, register operands now pair by index and single qubits repeat, as the spec defines. Slices, aliases, subroutine register arguments and ctrl @ take the same path. Calls with fewer operands than gate qubits, such as cx q2, keep filling operands from the register as before. Closes #384
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Argus · 9/10 — Empty broadcast operands raise IndexError
Check 1 warning before you merge. Argus reviewed 3 files.
- 🟡 Warning · Empty broadcast operands raise IndexError ·
src/pyqasm/visitor.py:L1654
What this PR does · 9 author checks
- Fix gate broadcasting so register operands pair element by element and single-qubit operands repeat, rather than silently unrolling to a different circuit.
- Excludes: Rejecting the existing register-fills-several-operands extension or the four-single-qubit cx calling form.
- Excludes: Changing existing behavior for opaque gates or non-constant modifier arguments.
Author's checks
- _visit_generic_gate_operation broadcasts first through _broadcast_operands, resolving operands separately and preserving operand boundaries.
- Inside subroutines, operand resolution preserves formal names so existing argument mapping still applies.
- Calls with one operand per gate qubit, including controls, pair register operands by index and repeat single-qubit operands.
- Registers of different lengths raise ValidationError; for cx with lengths 2 and 3, report Cannot broadcast 'cx' over registers of different lengths [2, 3].
- Each broadcast application uses the normal path, retaining support for modifiers, custom gates, external gates and subroutines.
- Calls with fewer operands than gate qubits and the cx q[0], q[1], q[2], q[3] form retain existing behavior.
- Opaque gates and non-constant modifier arguments retain existing behavior.
- tests/qasm3/test_broadcast.py covers registers, slices, aliases, subroutine arguments, swap, ctrl @, mixed operands, g4 and length mismatch.
- The spec's mixed-operand g4 example emits three applications, pairing qr1 and qr3 by index while repeating qr0[0] and qr2[0].
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Cross-Repo PR Coverage
No conflict found with qBraid/qBraid#1468.
Updated at 10:26 UTC on 2026-10-24
| span=operation.span, | ||
| ) | ||
| size = lengths.pop() | ||
| return [[bits[i] if len(bits) > 1 else bits[0] for bits in resolved] for i in range(size)] |
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🟡 Warning · Bug: Empty broadcast operands raise IndexError
Invalid empty operands bypass QASM error handling and crash unroll with an uncaught IndexError.
Same issue at src/pyqasm/visitor.py:L1643.
| return [[bits[i] if len(bits) > 1 else bits[0] for bits in resolved] for i in range(size)] | |
| if any(not bits for bits in resolved): | |
| raise_qasm3_error( | |
| f"Cannot broadcast '{operation.name.name}' over an empty operand", | |
| error_node=operation, | |
| span=operation.span, | |
| ) | |
| return [[bits[i] if len(bits) > 1 else bits[0] for bits in resolved] for i in range(size)] |
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Closes #384
Problem
unroll()flattened every operand of a gate call into one list and chunked it by the gate's arity. With register operands this silently produced a different circuit:cx a, b;(twoqubit[2])cx a[0], a[1]; cx b[0], b[1];cx a[0], b[0]; cx a[1], b[1];cx q[0:1], q[2:3];cx q[0], q[1]; cx q[2], q[3];cx q[0], q[2]; cx q[1], q[3];let a = q[0:1]; let b = q[2:3]; cx a, b;def f(qubit[2] a, qubit[2] b) { cx a, b; }f(r, s);cx r[0], r[1]; cx s[0], s[1];cx r[0], s[0]; cx r[1], s[1];ctrl @ x a, b;cx a[0], a[1]; cx a[0], b[0]; cx a[0], b[1];cx a[0], b[0]; cx a[1], b[1];cx q, r[0];(qubit[3] q)ValidationErrorcx q[0], r[0]; cx q[1], r[0]; cx q[2], r[0];g4 qr0[0], qr1, qr2[0], qr3;ValidationErrorg4calls, one per index ofqr1/qr3Change
_visit_generic_gate_operationnow broadcasts first, in a new_broadcast_operands:ValidationError: Cannot broadcast 'cx' over registers of different lengths [2, 3].cx q2;,inv @ ccx q[0], q2;,ctrl (2) @ x a, b[0];, which existing tests rely on), thecx q[0], q[1], q[2], q[3]form, opaque gates, and non-constant modifier arguments. The issue left it to the maintainers whether to keep that register-fills-several-operands extension, so this PR leaves it alone.Testing
tests/qasm3/test_broadcast.py: two registers, slices, aliases, subroutine register arguments,swap,ctrl @, the mixed register/qubit form, the spec'sg4example, and the length mismatch. 9 of its 10 tests fail onmain.black,isort,pylint(10.00),headerspass.mypyreports only thetyper/richimport errors that come from the tox environment lacking the CLI extra.negctrl @, a parametrised custom two-qubit gate on registers, andcxkept withunroll(external_gates=["cx"])all pair by index.Found while testing qBraid/qBraid#1468, whose
qasm3 -> qasm2conversion currently detects register broadcasts and falls back to another path to avoid this bug.