This patch fixes a problem in which nested OpenMP parallel regions cause
errors if the number of inner teams is not balanced (i.e. the number of
loop iterations is not divisible by the number of physical threads). A
testcase is included.
On NVPTX the symptom was a fatal error:
libgomp: cuCtxSynchronize error: an illegal instruction was encountered
This was caused by mismatched "bar.sync" instructions (one waiting for
32 threads while another is waiting for 256). The source of the mismatch
being that some threads were still busy while others had run out of work
to do.
On GCN there was no such error (GCN barriers always wait for all
threads), but it worked only by chance: the idle threads were "matching"
different barriers to the busy threads, but it was harmless because the
thread function pointer remained NULL.
This patch simply skips barriers when they would "wait" for only one
thread (the current thread). This means that teams nested inside other
teams now run independently, instead of strictly in lock-step, and is
only valid as long as inner teams are limited to one thread each
(currently the case). When the inner regions exit then the barriers for
the outer region will sync everything up again.
OK to commit?
Andrew
P.S. I can approve the amdgcn portion myself; I'm seeking approval for
the nvptx portion.
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.
diff --git a/libgomp/config/gcn/bar.c b/libgomp/config/gcn/bar.c
index 02fd19710d4..a21529a624b 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 125ca3e49ec..0a723087b9e 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 +70,9 @@ 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));
+ asm ("bar.sync 1, %0;" : : "r" (32 * 8/*bar->total*/));
+ if (bar->total > 1)
+ asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
}
void
@@ -95,7 +98,8 @@ 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 +108,8 @@ 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 +163,8 @@ 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 +175,8 @@ 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 00000000000..e777271dde1
--- /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 ();
+}
