Finding weird/wrong targets

[boschmitt]

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Describe the bug

A combination of assumptions in our build system and our code allows CUDA-Q to find nonsensical targets:

[LinkedLibraryHolder.cpp:185] Found Target nvidia-fp64 with config file nvidia-fp64.yml
[LinkedLibraryHolder.cpp:129] CUDA-Q Library Path is /workspace/dev/cuda-quantum/build/lib.
[LinkedLibraryHolder.cpp:140] Skip cusvsim-fp64 simulator for target nvidia-fp64 since it is not available
[LinkedLibraryHolder.cpp:129] CUDA-Q Library Path is /workspace/dev/cuda-quantum/build/lib.
[LinkedLibraryHolder.cpp:140] Skip custatevec-fp64 simulator for target nvidia-fp64 since it is not available
[LinkedLibraryHolder.cpp:195] Found Target: nvidia-fp64 -> (sim=qpp, platform=default)

The above is showing the nvidia-fp64 target being backed by the qpp simulator!

The assumptions:

• The build systems assumes that a user will always build all the targets his system is capable of supporting and will install the target configuration files independently on whether the targets were actually built. For example:
  • It assumes that nvqir-qpp and nvqir-stim are always built, since the user is quite likely to have a CPU (:
  • It assumes that gpu-backed targets, nvidia-*, are built, if a user has GPUs, CUDA and cuQuantum installed
• Our code seems to assume that the correct default simulator for all targets is qpp---even if the target is a different simulator and the runtime just happens to fail finding the shared library for it.

Implications:

Not all users build CUDA-Q in the same way. Developers might not want to build the whole world just to test (or benchmark!) specific parts of the system. In this cases, setting a target cudaq.set_target('nvidia') will silently load qpp.

Steps to reproduce the bug

• In a freshly configured build directory, build the python modules: ninja CUDAQuantumPythonModules
• You will also need to build the default platform ninja cudaq-default-platform and the qpp backend(?) ninja nvqir-qpp
• CUDAQ_LOG_LEVEL=info PYTHONPATH=$(pwd)/python python3 -c "import cudaq"

This will output logs showing all targets being "successfully" found and using the qpp simulator.

Expected behavior

• We should only install targets configuration files is the respective library is built
• The runtime should not assume all targets default to using the qpp simulator

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

Not a regression

Environment

• CUDA-Q version: CUDA-Q Version proto-0.8.0 (https://github.com/NVIDIA/cuda-quantum 7903c8b)
• Python version: Python 3.12.2
• C++ compiler: g++ (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
• Operating system: Ubuntu 24.04.1 LTS (noble)

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