[OpenMP][libomp] Fix crash when LIBOMP_NUM_HIDDEN_HELPER_THREADS=1.

[AndreyChurbanov]

Summary

It looks logical to treat user request for number of helper threads as
the number of threads doing actual job, excluding main thread of hidden helper team
which does not participate in tasks execution.

The increment also fixes the division by 0 caused by % operation:
#define KMP_GTID_TO_SHADOW_GTID(gtid) ((gtid) % (kmp_hidden_helper_threads_num - 1) + 2)
when kmp_hidden_helper_threads_num == 1.

[RaviNarayanaswamy]

Thanks Andrey for the fix

openmp/runtime/src/kmp_settings.cpp

1249

once -> since

[AndreyChurbanov]

Addressed Ravi's comment.

[RaviNarayanaswamy]

LGTM

[tianshilei1992]

Thanks for the fix. Can you mention "Fixes #48656" in the commit message as well such that #48656 can be closed automatically.

openmp/runtime/src/kmp_settings.cpp

1249

openmp/runtime/src/kmp_settings.cpp

 static void __kmp_stg_parse_num_hidden_helper_threads(char const *name,
                                                       char const *value,
                                                       void *data) {
   __kmp_stg_parse_int(name, value, 0, 16, &__kmp_hidden_helper_threads_num);
   // If the number of hidden helper threads is zero, we disable hidden helper
   // task
   if (__kmp_hidden_helper_threads_num == 0) {
     __kmp_enable_hidden_helper = FALSE;
+  } else {
+    // Since the main thread of hidden helper team dooes not participate
+    // in tasks execution let's increment the number of threads by one
+    // so that requested number of threads do actual job.
+    __kmp_hidden_helper_threads_num++;
   }
 } // __kmp_stg_parse_num_hidden_helper_threads
 
 static void __kmp_stg_print_num_hidden_helper_threads(kmp_str_buf_t *buffer,
                                                       char const *name,
                                                       void *data) {
+  if (__kmp_hidden_helper_threads_num == 0) {
     __kmp_stg_print_int(buffer, name, __kmp_hidden_helper_threads_num);
+  } else {
+    KMP_DEBUG_ASSERT(__kmp_hidden_helper_threads_num > 1);
+    // Let's exclude the main thread of hidden helper team and print
+    // number of worker threads those do actual job.
+    __kmp_stg_print_int(buffer, name, __kmp_hidden_helper_threads_num - 1);
+  }
 } // __kmp_stg_print_num_hidden_helper_threads
 
 static void __kmp_stg_parse_use_hidden_helper(char const *name,
                                               char const *value, void *data) {
   __kmp_stg_parse_bool(name, value, &__kmp_enable_hidden_helper);
 #if !KMP_OS_LINUX
   __kmp_enable_hidden_helper = FALSE;
   K_DIAG(1,

openmp/runtime/test/tasking/hidden_helper_task/single_helper_thread.c

+// RUN: %libomp-compile && env LIBOMP_NUM_HIDDEN_HELPER_THREADS=1 %libomp-run
+
+// The test checks that "devide-by-0" bug fixed in runtime.
+// The fix is to increment number of threads by 1 if positive,
+// so that operation
+//   (gtid) % (__kmp_hidden_helper_threads_num - 1)
+// does not cause crash.
+
+#include <stdio.h>
+#include <omp.h>
+
+int main(){
+#pragma omp target nowait
+   {
+      printf("----- in  target region\n");
+   }
+  printf("------ before taskwait\n");
+#pragma omp taskwait
+  printf("passed\n");
+  return 0;
+}