Kernel is translated into an invalid QASM

[srulre]

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Describe the bug

This is a continuation of #380.

Two major issues prevent generating a valid QASM:

• The gate name starts with an underscore (_). This is illegal in OpenQASM 2.0
• OpenQASM does not support forward references. Gates must be defined before they are used.
  According to the OpenQASM specification:

gate name(params) qargs {body}
...
Only built-in gate statements, calls to previously defined gates, and barrier statements can appear in body

Steps to reproduce the bug

The following example uses two recursive calls:

#include <cudaq.h>

#include <numbers>
#include <iostream>

int main() {
        auto [kernel1, qubit1] = cudaq::make_kernel<cudaq::qubit>();
        kernel1.x(qubit1);

        auto [kernel2, qubit2] = cudaq::make_kernel<cudaq::qubit>();
        kernel2.h(qubit2);
        kernel2.call(kernel1, qubit2);
        kernel2.h(qubit2);
        kernel2.mz(qubit2);

        auto kernel3 = cudaq::make_kernel();
        auto qubit3 = kernel3.qalloc();
        kernel3.call(kernel2, qubit3);

        std::cout << kernel3.to_quake() << std::endl;
        return 0;
}

The code generates the following MLIR:

module {
  func.func @__nvqpp__mlirgen____nvqppBuilderKernel_202375922897() attributes {"cudaq-entrypoint"} {
    %0 = quake.alloca !quake.ref
    call @__nvqpp__mlirgen____nvqppBuilderKernel_093606261879(%0) : (!quake.ref) -> ()
    return
  }
  func.func @__nvqpp__mlirgen____nvqppBuilderKernel_093606261879(%arg0: !quake.ref) {
    quake.h %arg0 : (!quake.ref) -> ()
    call @__nvqpp__mlirgen____nvqppBuilderKernel_367535629127(%arg0) : (!quake.ref) -> ()
    quake.h %arg0 : (!quake.ref) -> ()
    %0 = quake.mz %arg0 : (!quake.ref) -> i1 {registerName = ""}
    return
  }
  func.func @__nvqpp__mlirgen____nvqppBuilderKernel_367535629127(%arg0: !quake.ref) {
    quake.x %arg0 : (!quake.ref) -> ()
    return
  }
}

After running cudaq-translate --convert-to=openqasm example.qke, the resulting QASM output is given by:

// Code generated by NVIDIA's nvq++ compiler
OPENQASM 2.0;

include "qelib1.inc";

gate __nvqpp__mlirgen____nvqppBuilderKernel_093606261879 q0 {
  h q0;
  __nvqpp__mlirgen____nvqppBuilderKernel_367535629127 q0;
  h q0;
  creg var1[1];
  measure q0 -> var1[0];
}

gate __nvqpp__mlirgen____nvqppBuilderKernel_367535629127 q0 {
  x q0;
}

qreg var0[1];
__nvqpp__mlirgen____nvqppBuilderKernel_093606261879 var0[0];

Expected behavior

The result should be the QASM below or an equivalent. Note the removal of the leading underscores and the moving of the nvqpp__mlirgen____nvqppBuilderKernel_367535629127 gate definition to the beginning of the file:

// Code generated by NVIDIA's nvq++ compiler
OPENQASM 2.0;

include "qelib1.inc";

gate nvqpp__mlirgen____nvqppBuilderKernel_367535629127 q0 {
  x q0;
}

gate nvqpp__mlirgen____nvqppBuilderKernel_093606261879 q0 {
  h q0;
  nvqpp__mlirgen____nvqppBuilderKernel_367535629127 q0;
  h q0;
  creg var1[1];
  measure q0 -> var1[0];
}

qreg var0[1];
nvqpp__mlirgen____nvqppBuilderKernel_093606261879 var0[0];

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• cuda-quantum docker image version:sha256:9ec8ff5e307ccb6dd5c71a2b45cc486cd42221c10d7f98c6147a807f69029f1b
  More details:

                "org.opencontainers.image.created": "2023-09-01T21:03:00.664Z",
                "org.opencontainers.image.description": "CUDA Quantum toolkit for heterogeneous quantum-classical workflows",
                "org.opencontainers.image.licenses": "NOASSERTION",
                "org.opencontainers.image.ref.name": "ubuntu",
                "org.opencontainers.image.revision": "3fe9658a48ad6f2cca85253751367c2433a53d21",
                "org.opencontainers.image.source": "https://github.com/NVIDIA/cuda-quantum",
                "org.opencontainers.image.title": "cuda-quantum",
                "org.opencontainers.image.url": "https://github.com/NVIDIA/cuda-quantum",
                "org.opencontainers.image.version": "latest"

Suggestions

No response

[boschmitt]

Fixed by #604