Add libomp/libiomp compatibility experiment scripts

[ogrisel]

Adds reproducible experiment scripts for follow-up investigation from PR joblib#222 about whether the Linux libiomp + libomp warning is still needed with recent library versions.

Findings (Linux, Jul 2026)

Tested with intel-openmp 2025.0.0 (libiomp perf lib 5.0.20240820) and LLVM libomp 18.1.8 (Clang 18.1 build), matching the pylatest_conda_mkl_clang_gcc CI setup.

The warning is still needed on Linux. Recent versions remain incompatible:

Test	Result
C reproducer: call omp_get_max_threads from both runtimes	Deadlock
ctypes calls into both loaded runtimes	OMP Error joblib#15 → SIGABRT
threadpool_limits() after nested prange+BLAS	Hangs on Python exit
KMP_DUPLICATE_LIB_OK=TRUE	Suppresses abort but still unreliable

Loading both libraries without calling into both simultaneously can appear fine; active use of both OpenMP APIs still crashes or deadlocks.

Scripts

• experiments/libomp_libiomp_repro.c — minimal C reproducer (LLVM bug #43565)
• experiments/libomp_libiomp_variants.c — load-only / intel-first / llvm-first variants
• experiments/run_experiments.py — Python integration tests mirroring threadpoolctl usage

How to run

# Setup: conda env with MKL numpy/scipy + LLVM 18 libomp (see CI install.sh)
export CC_OUTER_LOOP=clang-18 CC_INNER_LOOP=gcc
bash continuous_integration/build_test_ext.sh

# C reproducer
gcc -o repro experiments/libomp_libiomp_repro.c -ldl
LD_LIBRARY_PATH=$CONDA_PREFIX/lib:/usr/lib/llvm-18/lib:$LD_LIBRARY_PATH timeout 15 ./repro

# Python experiments
PYTHONPATH=tests/_openmp_test_helper:. python experiments/run_experiments.py

Recommendation

Keep the Linux-only warning from PR joblib#222. Do not remove the warning on any platform without new evidence that the runtimes have become compatible.