Debugging Tips

Run-time Dependencies

There are some environment variables that will affect the execution of Catalyst. For example, Catalyst relies on the PATH environment variable to be set, just like most other UNIX programs. Most of the time, Catalyst should work without special configuration of the environment, however here are some errors and warnings you might see and how to fix them.

  • Compiler clang failed during the execution of command ...: During the linking process, Catalyst uses some well-known compilers that may be installed in your computer. If for whatever reason the compiler fails to execute, this warning will be issued. Please note that more attempts to link will be issued with different compilers. You may be able to fix this warning by re-installing or uninstalling the faulty compiler.

  • Unable to link... All available compiler options exhausted ...: Even though Catalyst has a list of compilers that may be available in your system, it is possible that no compiler successfully linked the program to the libraries shipped with Catalyst. This should be a rare case, but if you ever find yourself seeing this error, you should check that one of the following compilers or interfaces are available in your PATH: clang, gcc, c99, c89, cc. Correctly installing any of these should be sufficient. Alternatively, you can also specify a preferred compiler to use during linking using the environment variable CATALYST_CC. For example CATALYST_CC=clang-14 specifies that clang-14 will be the preferred compiler to be used during the linking stage.

  • User defined compiler is not in path: Following the example above, you may have installed clang-14 in a different directory that is not currently on the PATH. Please make sure that the executable specified in CATALYST_CC is available in the PATH and is flag compatible with gcc or clang.

Running without Python runtime

Sometimes, it might be useful to run the JIT compiled function without Python, for example in case of debugging. If the program succeeds without Python, then it is likely that the error is found in the interface between Python and the JIT compiled function, or within Catalyst’s Python internals. As a quick sanity check you might want to run the JIT compiled function without Python. Below is an example of how to obtain a C program that can be linked against the generated function:

def identity(x):
    return x


Using the QJIT.get_cmain function, the following string is returned to the user:

#include <complex.h>
#include <stddef.h>
#include <stdint.h>

typedef int64_t int64;
typedef double float64;
typedef float float32;
typedef double complex complex128;
typedef float complex complex64;

struct memref_float64x0_t
    float64* allocated;
    float64* aligned;
    size_t offset;

struct result_t {
    struct memref_float64x0_t f_0;

extern void setup(int, char**);
extern void _catalyst_ciface_jit_identity(struct result_t*, struct memref_float64x0_t*);
extern void teardown();

main(int argc, char** argv)

    struct result_t result_val;
    float64 buff_0 = 1.0;
    struct memref_float64x0_t arg_0 = { &buff_0, &buff_0, 0 };

    setup(1, &argv[0]);
    _catalyst_ciface_jit_identity(&result_val, &arg_0);

The user can now compile and link this program and run it without Python.

Verbose Mode

Catalyst uses a number of command line tools under the hood. In order to see how these tools are used, one can use the verbose mode. The verbose mode prints out the tools and flags used by Catalyst.

In order to enable verbose mode, the user must use the keyword argument verbose and set it to True for the @qjit wrapper. For example:

def circuit():

Will print out something close to the following:

[RUNNING] mlir-hlo-opt --allow-unregistered-dialect --canonicalize --chlo-legalize-to-hlo --stablehlo-legalize-to-hlo --mhlo-legalize-control-flow --hlo-legalize-to-linalg --mhlo-legalize-to-std --convert-to-signless --canonicalize /tmp/tmpwsoh3acq/circuit.mlir -o /tmp/tmpwsoh3acq/circuit.nohlo.mlir
[RUNNING] quantum-opt --lower-gradients --convert-arraylist-to-memref /tmp/tmpwsoh3acq/circuit.nohlo.mlir -o /tmp/tmpwsoh3acq/circuit.nohlo.opt.mlir
[RUNNING] quantum-opt --inline --gradient-bufferize --scf-bufferize --convert-tensor-to-linalg --convert-elementwise-to-linalg --arith-bufferize --empty-tensor-to-alloc-tensor --bufferization-bufferize --tensor-bufferize --linalg-bufferize --tensor-bufferize --quantum-bufferize --func-bufferize --finalizing-bufferize --buffer-loop-hoisting --convert-bufferization-to-memref --canonicalize --cp-global-memref /tmp/tmpwsoh3acq/circuit.nohlo.opt.mlir -o /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.mlir
[RUNNING] quantum-opt --convert-linalg-to-loops --convert-scf-to-cf --expand-strided-metadata --lower-affine --arith-expand --convert-complex-to-standard --convert-complex-to-llvm --convert-math-to-llvm --convert-math-to-libm --convert-arith-to-llvm --finalize-memref-to-llvm=use-generic-functions --convert-index-to-llvm --convert-gradient-to-llvm --convert-quantum-to-llvm --emit-catalyst-py-interface --canonicalize --reconcile-unrealized-casts /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.mlir -o /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.mlir
[RUNNING] mlir-translate --mlir-to-llvmir /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.mlir -o /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.ll
[RUNNING] llc --filetype=obj --relocation-model=pic /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.ll -o /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.o
[RUNNING] clang -shared -rdynamic -Wl,-no-as-needed -Wl,-rpath,runtime/build/lib/capi:runtime/build/lib/backend:mlir/llvm-project/build/lib -Lmlir/llvm-project/build/lib -Lruntime/build/lib/capi -Lruntime/build/lib/backend -lrt_backend -lrt_capi -lpthread -lmlir_c_runner_utils /tmp/tmpwsoh3acq/circuit.nohlo.opt.buff.llvm.o -o /tmp/tmpwsoh3acq/

Pass Pipelines

The compilation steps which take MLIR as an input and lower it to binary are broken into MLIR pass pipelines. The pipelines argument of the qjit function may be used to alter the steps used for compilation. The default set of pipelines is defined via the catalyst.compiler.DEFAULT_PIPELINES list. Its structure is shown below.


One could customize what compilation passes are executed. A good use case of this would be if you are debugging Catalyst itself or you want to enable or disable passes within a specific pipeline. It is recommended to copy the default pipelines and edit them to suit your goals and afterwards passing them to the @qjit decorator. E.g. if you want to disable inlining

my_pipelines = [
            # "inline",

 def circuit():

Here, each item represents a pipeline. Each pipeline has a name and a list of MLIR passes to perform. Most of the standard passes are described in the MLIR passes documentation. Quantum MLIR passes are implemented in Catalyst and can be found in the sources.

All pipelines are executed in sequence, the output MLIR of each non-empty pipeline is stored in memory and becomes available via the get_output_of method of the QJIT object.

Printing the IR generated by Pass Pipelines

We won’t get into too much detail here, but sometimes it is useful to look at the output of a specific pass pipeline. To do so, simply use the get_output_of method available in QJIT. For example, if one wishes to inspect the output of the BufferizationPass pipeline, simply run the following command.


Compilation Steps

The compilation process of a QJITed quantum function moves through various stages of the compilation pipeline including:

  • Quantum Tape: the quantum record of hybrid quantum programs in a single qml.QNode

  • JAXPR: the graph data structure maintained by JAX for the classical & quantum parts of the compiled program

  • MLIR: a novel compiler framework and intermediate representation

  • HLO (XLA) + Quantum Dialect: Lowering to HLO is the first stage inside MLIR after leaving JAXPR.

  • Builtin + Quantum Dialects: HLO is then converted to a variety of classical dialects in MLIR.

  • Bufferized MLIR: All tensors are converted to memory buffer allocations at this step.

  • LLVM Dialect: Lowering the code to the LLVM Dialect in MLIR simplifies the translation to LLVMIR by providing a one-to-one mapping.

  • QIR (LLVMIR): a specification for quantum programs in LLVMIR

To ensure that you have access to all the stages, the keep_intermediate=True flag must be specified in the qjit decorator. In the following example, we also compile ahead-of-time so that there is no requirements to pass actual parameters:

@qml.qnode(qml.device("lightning.qubit", wires=2))
def circuit(x: float, y: float):
    theta = jnp.sin(x) + y
    qml.RY(theta, wires=0)
    return qml.state()



{ lambda ; a:f64[] b:f64[]. let
    c:c128[4] = func[
    call_jaxpr={ lambda ; d:f64[] e:f64[]. let
        f:AbstractQreg() = qalloc 2
        g:f64[] = sin d
        h:f64[] = add g e
        i:AbstractQbit() = qextract f 0
        j:AbstractQbit() = qinst[op=RY qubits_len=1 runtime=lightning] i h
        k:AbstractQbit() = qextract f 1
        l:AbstractQbit() m:AbstractQbit() = qinst[
        ] j k
        _:AbstractObs(num_qubits=2,primitive=compbasis) = compbasis l m
        n:c128[4] = state l m
        = qdealloc f
        in (n,) }
    fn=<QNode: wires=2, device='lightning.qubit', interface='autograd', diff_method='best'>
    ] a b
in (c,) }

The next stage is the JAXPR equivalent in MLIR, expressed using the MHLO dialect for classical computation and the Quantum dialect for quantum computation. Note that the MHLO dialect is a representation of HLO in MLIR, where HLO is the input IR to the accelerated linear algebra (XLA) compiler used by TensorFlow.


Lowering out of the MHLO dialect leaves us with the classical computation represented by generic dialects such as arith, math, or linalg. This allows us to later generate machine code via standard LLVM-MLIR tooling.


The quantum compilation pipeline expands high-level quantum instructions like adjoint, and applies quantum differentiation methods and optimization techniques.


An important step in getting to machine code from a high-level representation is allocating memory for all the tensor/array objects in the program.


The LLVM dialect can be considered the “exit point” from MLIR when using LLVM for low-level compilation:


And finally some real LLVMIR adhering to the QIR specification:


The LLVMIR code is compiled to an object file using the LLVM static compiler and linked to the runtime libraries. The generated shared object is stored by the caching mechanism in Catalyst for future calls.