[OpenMP] Fix the issue where num_threads still takes effect incorre…

…ctly

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.

Reviewed By: tlwilmar

Differential Revision: https://reviews.llvm.org/D152883

openmp/runtime/src/kmp_runtime.cpp

@@ -1153,6 +1153,9 @@ void __kmp_serialized_parallel(ident_t *loc, kmp_int32 global_tid) {
   // 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);

openmp/runtime/test/parallel/bug63197.c

@@ -0,0 +1,17 @@
+// 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