[OpenMP] libomp: add check of task function pointer for NULL

[AndreyChurbanov]

Summary

This patch allows to simplify compiler implementation on "taskwait nowait" construct.
The "taskwait nowait" is semantically equivalent to the empty task.
Instead of creating an empty routine as a task entry, compiler can just send NULL pointer to the runtime.
Then the runtime will make all the work with dependences and return because of the absent task routine.

[jdoerfert]

LG

[protze.joachim]

I already know, that OMPD folks will not be happy about this implementation. Nevertheless, it is a valid and better implementation than before.

Regarding OMPT implementation: since you are not calling into the application anymore, this case should now really use ompt_taskwait_complete rather than switching to the taskwait task and back.

openmp/runtime/src/kmp_tasking.cpp

if (kmp_itt_count_task) {
       if (thread->th.th_bar_arrive_time)
         cur_time = __itt_get_timestamp();
       else
         kmp_itt_count_task = 0; // thread is not on a barrier - skip timing
     }
     KMP_FSYNC_ACQUIRED(taskdata); // acquired self (new task)
 #endif
 
+    if (task->routine != NULL) {
 #ifdef KMP_GOMP_COMPAT
       if (taskdata->td_flags.native) {
         ((void (*)(void *))(*(task->routine)))(task->shareds);
       } else
 #endif /* KMP_GOMP_COMPAT */
       {
         (*(task->routine))(gtid, task);
       }
+    }
     KMP_POP_PARTITIONED_TIMER();
 
 #if USE_ITT_BUILD && USE_ITT_NOTIFY
     if (kmp_itt_count_task) {
       // Barrier imbalance - adjust arrive time with the task duration
       thread->th.th_bar_arrive_time += (__itt_get_timestamp() - cur_time);
     }
     KMP_FSYNC_CANCEL(taskdata); // destroy self (just executed)

openmp/runtime/test/tasking/kmp_taskwait_nowait.c

+// RUN: %libomp-compile-and-run
+
+// test checks IN dep kind in depend clause on taskwait nowait
+// uses codegen emulation
+// Note: no outlined task routine used
+#include <stdio.h>
+#include <omp.h>
+// ---------------------------------------------------------------------------
+// internal data to emulate compiler codegen
+#define TIED 1
+typedef struct DEP {
+  size_t addr;
+  size_t len;
+  int flags;
+} _dep;
+typedef struct ID {
+  int reserved_1;
+  int flags;
+  int reserved_2;
+  int reserved_3;
+  char *psource;
+} _id;
+typedef struct task {
+  void** shareds;
+  void* entry;
+  int part_id;
+  void* destr_thunk;
+  int priority;
+  long long device_id;
+  int f_priv;
+} task_t;
+typedef int(*entry_t)(int, task_t*);
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+extern int __kmpc_global_thread_num(_id*);
+task_t *__kmpc_omp_task_alloc(_id *loc, int gtid, int flags,
+                              size_t sz, size_t shar, entry_t rtn);
+int __kmpc_omp_task_with_deps(_id *loc, int gtid, task_t *task, int ndeps,
+                              _dep *dep_lst, int nd_noalias, _dep *noalias_l);
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+int main()
+{
+  int i1,i2,i3;
+  omp_set_num_threads(2);
+  printf("addresses: %p %p %p\n", &i1, &i2, &i3);
+  #pragma omp parallel
+  {
+    int t = omp_get_thread_num();
+    printf("thread %d enters parallel\n", t);
+    #pragma omp single
+    {
+      #pragma omp task depend(in: i3)
+      {
+        int th = omp_get_thread_num();
+        printf("task 0 created by th %d, executed by th %d\n", t, th);
+      }
+      #pragma omp task depend(in: i2)
+      {
+        int th = omp_get_thread_num();
+        printf("task 1 created by th %d, executed by th %d\n", t, th);
+      }
+//      #pragma omp taskwait depend(in: i1, i2) nowait
+      {
+        _dep sdep[2];
+        static _id loc = {0, 2, 0, 0, ";test.c;func;67;0;;"};
+        int gtid = __kmpc_global_thread_num(&loc);
+// instead of creating an empty task function we can now send NULL to runtime
+        task_t *ptr = __kmpc_omp_task_alloc(&loc, gtid, TIED,
+                                            sizeof(task_t), 0, NULL);
+        sdep[0].addr = (size_t)&i2;
+        sdep[0].flags = 1; // 1-in, 2-out, 3-inout, 4-mtx, 8-inoutset
+        sdep[1].addr = (size_t)&i1;
+        sdep[1].flags = 1; // in
+        __kmpc_omp_task_with_deps(&loc, gtid, ptr, 2, sdep, 0, NULL);
+      }
+      printf("single done\n");
+    }
+  }
+  printf("passed\n");
+  return 0;
+}