omp_thread_count(): use limits from OpenMP runtime

omp_get_max_threads() will consult the default number of threads for the
current nested parallelism layer, which is set either by the runtime or
from the OMP_NUM_THREADS environment variable.

omp_get_thread_limit() will consult the separate limit on the total
number of threads, set by the OMP_THREAD_LIMIT environment variable.

Author: aitap

src/utils.cpp

@@ -84,9 +84,9 @@ dMappedCSC extract_mapped_csc(Rcpp::S4 input) {
 int omp_thread_count() {
   int n = 0;
 #ifdef _OPENMP
-#pragma omp parallel reduction(+ : n)
+  int max_threads = omp_get_max_threads(), thread_limit = omp_get_thread_limit();
+  n = max_threads < thread_limit ? max_threads : thread_limit;
 #endif
-  n += 1;
   return n;
 }