[OpenMP] Fix the issue where `num_threads` still takes effect incorrectly

[tianshilei1992]

Summary

This patch fixes the issue that, if we have a compile-time serialized parallel
region (such as if (0)) with num_threads, followed by a regular parallel
region, the regular parallel region will pick up the value set in the serialized
parallel region incorrectly. The reason is, in the front end, if we can prove a
parallel region has to serialized, instead of emitting __kmpc_fork_call, the
front end directly emits __kmpc_serialized_parallel, body, and __kmpc_end_serialized_parallel.
However, this "optimization" doesn't consider the case where num_threads is
used such that __kmpc_push_num_threads is still emitted. Since we don't reset
the value in __kmpc_serialized_parallel, it will affect the next parallel region
followed by it.

Fix #63197.

[tlwilmar]

LGTM

openmp/runtime/src/kmp_runtime.cpp

void __kmp_serialized_parallel(ident_t *loc, kmp_int32 global_tid) {
   } else if (proc_bind == proc_bind_default) {
     // No proc_bind clause was specified, so use the current value
     // of proc-bind-var for this parallel region.
     proc_bind = this_thr->th.th_current_task->td_icvs.proc_bind;
   }
   // Reset for next parallel region
   this_thr->th.th_set_proc_bind = proc_bind_default;
 
+  // Reset num_threads for next parallel region
+  this_thr->th.th_set_nproc = 0;
+
 #if OMPT_SUPPORT
   ompt_data_t ompt_parallel_data = ompt_data_none;
   void *codeptr = OMPT_LOAD_RETURN_ADDRESS(global_tid);
   if (ompt_enabled.enabled &&
       this_thr->th.ompt_thread_info.state != ompt_state_overhead) {
 
     ompt_task_info_t *parent_task_info;
     parent_task_info = OMPT_CUR_TASK_INFO(this_thr);

openmp/runtime/test/parallel/bug63197.c

+// RUN: %libomp-compile-and-run | FileCheck %s
+
+#include <omp.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+#pragma omp parallel num_threads(3) if (false)
+#pragma omp single
+  { printf("BBB %2d\n", omp_get_num_threads()); }
+
+#pragma omp parallel
+#pragma omp single
+  { printf("CCC %2d\n", omp_get_num_threads()); }
+  return 0;
+}
+
+// CHECK-NOT: CCC  3