openmp: ignore nowait if async execution is unsupported [PR93481]

An OpenMP "nowait" clause on a target construct currently leads to
a call to GOMP_OFFLOAD_async_run in the plugin that is used for
offloading at execution time. The nvptx plugin contains only a stub
of this function that always produces a fatal error if called.

This commit changes the "nowait" implementation to ignore the clause
if the executing device's plugin does not implement GOMP_OFFLOAD_async_run.
The stub in the nvptx plugin is removed which effectively means that
programs containing "nowait" can now be executed with nvptx offloading
as if the clause had not been used.
This behavior is consistent with the OpenMP specification which says that
"[...] execution of the target task *may* be deferred" (emphasis added),
cf. OpenMP 5.0, page 172.

libgomp/

	* plugin/plugin-nvptx.c: Remove GOMP_OFFLOAD_async_run stub.
	* target.c (gomp_load_plugin_for_device): Make "async_run" loading
	optional.
	(gomp_target_task_fn): Assert "devicep->async_run_func".
	(clear_unsupported_flags): New function to remove unsupported flags
	(right now only GOMP_TARGET_FLAG_NOWAIT) that can be be ignored.
	(GOMP_target_ext): Apply clear_unsupported_flags to flags.
	* testsuite/libgomp.c/target-33.c:
	Remove xfail for offload_target_nvptx.
	* testsuite/libgomp.c/target-34.c: Likewise.

4 files changed, 15 insertions, 13 deletions

diff --git a/libgomp/plugin/plugin-nvptx.c b/libgomp/plugin/plugin-nvptx.c
index 6033c71a9db..ec103a2f40b 100644
--- a/libgomp/plugin/plugin-nvptx.c
+++ b/libgomp/plugin/plugin-nvptx.c
@@ -1931,9 +1931,4 @@ GOMP_OFFLOAD_run (int ord, void *tgt_fn, void *tgt_vars, void **args)
   nvptx_stacks_free (stacks, teams * threads);
 }
 
-void
-GOMP_OFFLOAD_async_run (int ord, void *tgt_fn, void *tgt_vars, void **args,
-			void *async_data)
-{
-  GOMP_PLUGIN_fatal ("GOMP_OFFLOAD_async_run unimplemented");
-}
+/* TODO: Implement GOMP_OFFLOAD_async_run. */
diff --git a/libgomp/target.c b/libgomp/target.c
index 3df007283f4..0ff727de47d 100644
--- a/libgomp/target.c
+++ b/libgomp/target.c
@@ -2022,6 +2022,16 @@ GOMP_target (int device, void (*fn) (void *), const void *unused,
   gomp_unmap_vars (tgt_vars, true);
 }
 
+static inline unsigned int
+clear_unsupported_flags (struct gomp_device_descr *devicep, unsigned int flags)
+{
+  /* If we cannot run asynchronously, simply ignore nowait.  */
+  if (devicep != NULL && devicep->async_run_func == NULL)
+    flags &= ~GOMP_TARGET_FLAG_NOWAIT;
+
+  return flags;
+}
+
 /* Like GOMP_target, but KINDS is 16-bit, UNUSED is no longer present,
    and several arguments have been added:
    FLAGS is a bitmask, see GOMP_TARGET_FLAG_* in gomp-constants.h.
@@ -2054,6 +2064,8 @@ GOMP_target_ext (int device, void (*fn) (void *), size_t mapnum,
   size_t tgt_align = 0, tgt_size = 0;
   bool fpc_done = false;
 
+  flags = clear_unsupported_flags (devicep, flags);
+
   if (flags & GOMP_TARGET_FLAG_NOWAIT)
     {
       struct gomp_thread *thr = gomp_thread ();
@@ -2524,6 +2536,7 @@ gomp_target_task_fn (void *data)
 	}
       ttask->state = GOMP_TARGET_TASK_READY_TO_RUN;
 
+      assert (devicep->async_run_func);
       devicep->async_run_func (devicep->target_id, fn_addr, actual_arguments,
 			       ttask->args, (void *) ttask);
       return true;
@@ -3040,7 +3053,7 @@ gomp_load_plugin_for_device (struct gomp_device_descr *device,
   if (device->capabilities & GOMP_OFFLOAD_CAP_OPENMP_400)
     {
       DLSYM (run);
-      DLSYM (async_run);
+      DLSYM_OPT (async_run, async_run);
       DLSYM_OPT (can_run, can_run);
       DLSYM (dev2dev);
     }
diff --git a/libgomp/testsuite/libgomp.c/target-33.c b/libgomp/testsuite/libgomp.c/target-33.c
index 15d2d7e38ab..1bed4b6bc67 100644
--- a/libgomp/testsuite/libgomp.c/target-33.c
+++ b/libgomp/testsuite/libgomp.c/target-33.c
@@ -1,6 +1,3 @@
-/* { dg-xfail-run-if "GOMP_OFFLOAD_async_run not implemented" { offload_target_nvptx } }
-   Cf. https://gcc.gnu.org/PR81688.  */
-
 extern void abort (void);
 
 int
diff --git a/libgomp/testsuite/libgomp.c/target-34.c b/libgomp/testsuite/libgomp.c/target-34.c
index 5a3596424d8..66d9f54202b 100644
--- a/libgomp/testsuite/libgomp.c/target-34.c
+++ b/libgomp/testsuite/libgomp.c/target-34.c
@@ -1,6 +1,3 @@
-/* { dg-xfail-run-if "GOMP_OFFLOAD_async_run not implemented" { offload_target_nvptx } }
-   Cf. https://gcc.gnu.org/PR81688.  */
-
 extern void abort (void);
 
 int