GCN: Honor OpenMP 5.1 'num_teams' lower bound

Corresponding to commit 9fa72756d9
"libgomp, nvptx: Honor OpenMP 5.1 num_teams lower bound", these are the
GCN offloading changes to fix:

    PASS: libgomp.c/../libgomp.c-c++-common/teams-2.c (test for excess errors)
    [-FAIL:-]{+PASS:+} libgomp.c/../libgomp.c-c++-common/teams-2.c execution test

    PASS: libgomp.c++/../libgomp.c-c++-common/teams-2.c (test for excess errors)
    [-FAIL:-]{+PASS:+} libgomp.c++/../libgomp.c-c++-common/teams-2.c execution test

..., and omptests' 't-critical' test case.  I've cross checked that those test
cases are the ones that regress for nvptx offloading, if I locally revert the
"libgomp, nvptx: Honor OpenMP 5.1 num_teams lower bound" changes.

	libgomp/
	* config/gcn/libgomp-gcn.h (GOMP_TEAM_NUM): Inject.
	* config/gcn/target.c (GOMP_teams4): Handle.
	* config/gcn/team.c (gomp_gcn_enter_kernel): Initialize.
	* config/gcn/teams.c (omp_get_team_num): Adjust.

libgomp/config/gcn/libgomp-gcn.h

@@ -34,10 +34,11 @@
 #define DEFAULT_TEAM_ARENA_SIZE (64*1024)
 
 /* These define the LDS location of data needed by OpenMP.  */
-#define TEAM_ARENA_START 16  /* LDS offset of free pointer.  */
-#define TEAM_ARENA_FREE  24  /* LDS offset of free pointer.  */
-#define TEAM_ARENA_END   32  /* LDS offset of end pointer.  */
-#define GCN_LOWLAT_HEAP  40  /* LDS offset of the OpenMP low-latency heap.  */
+#define GOMP_TEAM_NUM    16
+#define TEAM_ARENA_START 24  /* LDS offset of free pointer.  */
+#define TEAM_ARENA_FREE  32  /* LDS offset of free pointer.  */
+#define TEAM_ARENA_END   40  /* LDS offset of end pointer.  */
+#define GCN_LOWLAT_HEAP  48  /* LDS offset of the OpenMP low-latency heap.  */
 
 struct heap
 {

libgomp/config/gcn/target.c

@@ -33,26 +33,37 @@ bool
 GOMP_teams4 (unsigned int num_teams_lower, unsigned int num_teams_upper,
 	     unsigned int thread_limit, bool first)
 {
+  int __lds *gomp_team_num = (int __lds *) GOMP_TEAM_NUM;
+  unsigned int num_workgroups = __builtin_gcn_dim_size (0);
   if (!first)
-    return false;
+    {
+      unsigned int team_num;
+      if (num_workgroups > gomp_num_teams_var)
+	return false;
+      team_num = *gomp_team_num;
+      if (team_num > gomp_num_teams_var - num_workgroups)
+	return false;
+      *gomp_team_num = team_num + num_workgroups;
+      return true;
+    }
   if (thread_limit)
     {
       struct gomp_task_icv *icv = gomp_icv (true);
       icv->thread_limit_var
 	= thread_limit > INT_MAX ? UINT_MAX : thread_limit;
     }
-  unsigned int num_workgroups, workgroup_id;
-  num_workgroups = __builtin_gcn_dim_size (0);
-  workgroup_id = __builtin_gcn_dim_pos (0);
-  /* FIXME: If num_teams_lower > num_workgroups, we want to loop
-     multiple times at least for some workgroups.  */
-  (void) num_teams_lower;
-  if (!num_teams_upper || num_teams_upper >= num_workgroups)
+  if (!num_teams_upper)
     num_teams_upper = ((GOMP_ADDITIONAL_ICVS.nteams > 0
 			&& num_workgroups > GOMP_ADDITIONAL_ICVS.nteams)
 		       ? GOMP_ADDITIONAL_ICVS.nteams : num_workgroups);
-  else if (workgroup_id >= num_teams_upper)
+  else if (num_workgroups < num_teams_lower)
+    num_teams_upper = num_teams_lower;
+  else if (num_workgroups < num_teams_upper)
+    num_teams_upper = num_workgroups;
+  unsigned int workgroup_id = __builtin_gcn_dim_pos (0);
+  if (workgroup_id >= num_teams_upper)
     return false;
+  *gomp_team_num = workgroup_id;
   gomp_num_teams_var = num_teams_upper - 1;
   return true;
 }

libgomp/config/gcn/team.c

@@ -68,6 +68,9 @@ gomp_gcn_enter_kernel (void)
       /* Starting additional threads is not supported.  */
       gomp_global_icv.dyn_var = true;
 
+      int __lds *gomp_team_num = (int __lds *) GOMP_TEAM_NUM;
+      *gomp_team_num = 0;
+
       /* Initialize the team arena for optimized memory allocation.
          The arena has been allocated on the host side, and the address
          passed in via the kernargs.  Each team takes a small slice of it.  */

libgomp/config/gcn/teams.c

@@ -44,10 +44,11 @@ omp_get_num_teams (void)
   return gomp_num_teams_var + 1;
 }
 
-int __attribute__ ((__optimize__ ("O2")))
+int
 omp_get_team_num (void)
 {
-  return __builtin_gcn_dim_pos (0);
+  int __lds *gomp_team_num = (int __lds *) GOMP_TEAM_NUM;
+  return *gomp_team_num;
 }
 
 ialias (omp_get_num_teams)