Reset OpenMP thread number if num_threads <= 0

[hzy46]

#4607 (comment)

[hzy46]

Use omp_get_num_threads + pragma omp parallel with a global variable will cause infinite waiting in Windows. I don't know the reason. Use omp_get_max_threads instead.

[hzy46]

The problem of infinite waiting in Windows is caused by ctypes.cdll.LoadLibrary in Python. To reproduce it, one can build the following C++ code to a DLL file in Windows.

#include <iostream>
#include <time.h>
#include <omp.h>

int OMP_NUM_THREADS() {
    int ret = 1;
#pragma omp parallel
#pragma omp master
    { ret = omp_get_num_threads(); }
    return ret;
}

int default_omp_num_threads = OMP_NUM_THREADS();

Then use the following python code to load it:

import ctypes
dll_file = '''<path to the dll>'''
ctypes.cdll.LoadLibrary(dll_file)

Then the code will be blocked forever. I have changed the implementation to use a static variable in function to resolve the problem caused by global variable.

[shiyu1994]

The changes LGTM. Thanks!

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