libgomp: disable barriers in nested teams

Both GCN and NVPTX allow nested parallel regions, but the barrier
implementation did not allow the nested teams to run independently of each
other (due to hardware limitations).  This patch fixes that, under the
assumption that each thread will create a new subteam of one thread, by
simply not using barriers when there's no other thread to synchronise.

libgomp/ChangeLog:

	* config/gcn/bar.c (gomp_barrier_wait_end): Skip the barrier if the
	total number of threads is one.
	(gomp_team_barrier_wake): Likewise.
	(gomp_team_barrier_wait_end): Likewise.
	(gomp_team_barrier_wait_cancel_end): Likewise.
	* config/nvptx/bar.c (gomp_barrier_wait_end): Likewise.
	(gomp_team_barrier_wake): Likewise.
	(gomp_team_barrier_wait_end): Likewise.
	(gomp_team_barrier_wait_cancel_end): Likewise.
	* testsuite/libgomp.c-c++-common/nested-parallel-unbalanced.c: New test.

3 files changed, 53 insertions, 11 deletions

diff --git a/libgomp/config/gcn/bar.c b/libgomp/config/gcn/bar.c
index 02fd197..a21529a 100644
--- a/libgomp/config/gcn/bar.c
+++ b/libgomp/config/gcn/bar.c
@@ -43,7 +43,8 @@ gomp_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state)
       __atomic_store_n (&bar->generation, bar->generation + BAR_INCR,
 			MEMMODEL_RELAXED);
     }
-  asm ("s_barrier" ::: "memory");
+  if (bar->total > 1)
+    asm ("s_barrier" ::: "memory");
 }
 
 void
@@ -71,7 +72,8 @@ gomp_barrier_wait_last (gomp_barrier_t *bar)
 void
 gomp_team_barrier_wake (gomp_barrier_t *bar, int count)
 {
-  asm ("s_barrier" ::: "memory");
+  if (bar->total > 1)
+    asm ("s_barrier" ::: "memory");
 }
 
 void
@@ -97,7 +99,8 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state)
 	  state &= ~BAR_CANCELLED;
 	  state += BAR_INCR - BAR_WAS_LAST;
 	  __atomic_store_n (&bar->generation, state, MEMMODEL_RELAXED);
-	  asm ("s_barrier" ::: "memory");
+	  if (bar->total > 1)
+	    asm ("s_barrier" ::: "memory");
 	  return;
 	}
     }
@@ -172,7 +175,8 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar,
 	{
 	  state += BAR_INCR - BAR_WAS_LAST;
 	  __atomic_store_n (&bar->generation, state, MEMMODEL_RELAXED);
-	  asm ("s_barrier" ::: "memory");
+	  if (bar->total > 1)
+	    asm ("s_barrier" ::: "memory");
 	  return false;
 	}
     }
@@ -195,7 +199,8 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar,
 	  abort();
 	}
 
-      asm ("s_barrier" ::: "memory");
+      if (bar->total > 1)
+	asm ("s_barrier" ::: "memory");
       gen = __atomic_load_n (&bar->generation, MEMMODEL_RELAXED);
       if (__builtin_expect (gen & BAR_CANCELLED, 0))
 	return true;
diff --git a/libgomp/config/nvptx/bar.c b/libgomp/config/nvptx/bar.c
index 125ca3e..1116561 100644
--- a/libgomp/config/nvptx/bar.c
+++ b/libgomp/config/nvptx/bar.c
@@ -41,7 +41,8 @@ gomp_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state)
       __atomic_store_n (&bar->generation, bar->generation + BAR_INCR,
 			MEMMODEL_RELEASE);
     }
-  asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+  if (bar->total > 1)
+    asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
 }
 
 void
@@ -69,7 +70,8 @@ gomp_barrier_wait_last (gomp_barrier_t *bar)
 void
 gomp_team_barrier_wake (gomp_barrier_t *bar, int count)
 {
-  asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+  if (bar->total > 1)
+    asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
 }
 
 void
@@ -95,7 +97,8 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state)
 	  state &= ~BAR_CANCELLED;
 	  state += BAR_INCR - BAR_WAS_LAST;
 	  __atomic_store_n (&bar->generation, state, MEMMODEL_RELEASE);
-	  asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+	  if (bar->total > 1)
+	    asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
 	  return;
 	}
     }
@@ -104,7 +107,8 @@ gomp_team_barrier_wait_end (gomp_barrier_t *bar, gomp_barrier_state_t state)
   state &= ~BAR_CANCELLED;
   do
     {
-      asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+      if (bar->total > 1)
+	asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
       gen = __atomic_load_n (&bar->generation, MEMMODEL_ACQUIRE);
       if (__builtin_expect (gen & BAR_TASK_PENDING, 0))
 	{
@@ -158,7 +162,8 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar,
 	{
 	  state += BAR_INCR - BAR_WAS_LAST;
 	  __atomic_store_n (&bar->generation, state, MEMMODEL_RELEASE);
-	  asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+	  if (bar->total > 1)
+	    asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
 	  return false;
 	}
     }
@@ -169,7 +174,8 @@ gomp_team_barrier_wait_cancel_end (gomp_barrier_t *bar,
   generation = state;
   do
     {
-      asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
+      if (bar->total > 1)
+	asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
       gen = __atomic_load_n (&bar->generation, MEMMODEL_ACQUIRE);
       if (__builtin_expect (gen & BAR_CANCELLED, 0))
 	return true;
diff --git a/libgomp/testsuite/libgomp.c-c++-common/nested-parallel-unbalanced.c b/libgomp/testsuite/libgomp.c-c++-common/nested-parallel-unbalanced.c
new file mode 100644
index 0000000..e777271
--- /dev/null
+++ b/libgomp/testsuite/libgomp.c-c++-common/nested-parallel-unbalanced.c
@@ -0,0 +1,31 @@
+/* Ensure that nested parallel regions work even when the number of loop
+   iterations is not divisible by the number of threads.  */
+
+#include <stdlib.h>
+
+int main() {
+  int A[30][40], B[30][40];
+  size_t n = 30;
+
+  for (size_t i = 0; i < 30; ++i)
+    for (size_t j = 0; j < 40; ++j)
+    A[i][j] = 42;
+
+#pragma omp target map(A[0:30][0:40], B[0:30][0:40])
+  {
+#pragma omp parallel for num_threads(8)
+    for (size_t i = 0; i < n; ++i)
+      {
+#pragma omp parallel for
+	for (size_t j = 0; j < n; ++j)
+	  {
+	    B[i][j] = A[i][j];
+	  }
+      }
+  }
+
+for (size_t i = 0; i < n; ++i)
+  for (size_t j = 0; j < n; ++j)
+    if (B[i][j] != 42)
+      abort ();
+}