[FEATURE] External Function Calls

[christian512]

Feature Description

I have a custom gate definition in OpenQASM3 and a backend that supports this custom gate natively. When converting to QIR I want that qbraid_qir creates a call to an externally linked function for this native gate. Currently the visitor inserts the custom gate operation via _visit_custom_gate_operation().

An example: I have a backend that natively supports rxx gates. Since rxx is not part of the stdgates.inc, this creates a custom gate definition in OpenQASM3:

OPENQASM 3.0;
include "stdgates.inc";
gate rxx(p0) _gate_q_0, _gate_q_1 {
  h _gate_q_0;
  h _gate_q_1;
  cx _gate_q_0, _gate_q_1;
  rz(p0) _gate_q_1;
  cx _gate_q_0, _gate_q_1;
  h _gate_q_1;
  h _gate_q_0;
}
bit[2] c;
qubit[2] q;
rxx(0.5) q[0], q[1];

I could declare an externally linked function in pyqir for the rxx gate:

__quantum__qis__rxx__body (%Qubit* target1, %Qubit* target2, double theta)

The visitor should then add a call to this function instead of replacing the rxx with the custom gate definition.

This issue might occur more often, since the quantum instruction set is not (yet) fixed and backends have different native gate sets, which are not in stdgates.inc nor in the QIS.

Implementation (Optional)

I suggest to add a dictionary _external_gates to the BasicQasmVisitor class, that holds the external gate functions:

self._external_gates: dict[str, pyqir.Function] = {}

When visiting a generic gate operation, it should be first checked if an external function call is possible (probably insert here).

The initialization of the pyqir.Function objects might be a bit difficult, since qbraid_qir would have to add the external function declarations to the qir module using mod.add_external_function(). Hence, the user would have to pass the pyqir.FunctionType and the name of the function to the BasicQasmVisitor.

An example of externally linked functions in pyqir can be found here.