diff --git a/libgomp/libgomp-plugin.h b/libgomp/libgomp-plugin.h
index 037558c43f5..200f3b594ee 100644
--- a/libgomp/libgomp-plugin.h
+++ b/libgomp/libgomp-plugin.h
@@ -126,7 +126,8 @@ extern void GOMP_OFFLOAD_openacc_async_exec (void (*) (void *), size_t, void **,
 					     struct goacc_asyncqueue *);
 extern bool GOMP_OFFLOAD_openacc_async_dev2host (int, void *, const void *, size_t,
 						 struct goacc_asyncqueue *);
-extern bool GOMP_OFFLOAD_openacc_async_host2dev (int, void *, const void *, size_t,
+extern bool GOMP_OFFLOAD_openacc_async_host2dev (int, void *, const void *,
+						 size_t, bool,
 						 struct goacc_asyncqueue *);
 extern void *GOMP_OFFLOAD_openacc_cuda_get_current_device (void);
 extern void *GOMP_OFFLOAD_openacc_cuda_get_current_context (void);
diff --git a/libgomp/libgomp.h b/libgomp/libgomp.h
index 7b46e0a494d..65fa390f4a5 100644
--- a/libgomp/libgomp.h
+++ b/libgomp/libgomp.h
@@ -1153,7 +1153,7 @@ extern void gomp_acc_declare_allocate (bool, size_t, void **, size_t *,
 struct gomp_coalesce_buf;
 extern void gomp_copy_host2dev (struct gomp_device_descr *,
 				struct goacc_asyncqueue *, void *, const void *,
-				size_t, struct gomp_coalesce_buf *);
+				size_t, bool, struct gomp_coalesce_buf *);
 extern void gomp_copy_dev2host (struct gomp_device_descr *,
 				struct goacc_asyncqueue *, void *, const void *,
 				size_t);
diff --git a/libgomp/oacc-host.c b/libgomp/oacc-host.c
index cbcac9bf7b3..bc11770725d 100644
--- a/libgomp/oacc-host.c
+++ b/libgomp/oacc-host.c
@@ -184,6 +184,7 @@ host_openacc_async_host2dev (int ord __attribute__ ((unused)),
 			     void *dst __attribute__ ((unused)),
 			     const void *src __attribute__ ((unused)),
 			     size_t n __attribute__ ((unused)),
+			     bool eph __attribute__ ((unused)),
 			     struct goacc_asyncqueue *aq
 			     __attribute__ ((unused)))
 {
diff --git a/libgomp/oacc-mem.c b/libgomp/oacc-mem.c
index 2f271009fb8..240ebc5c865 100644
--- a/libgomp/oacc-mem.c
+++ b/libgomp/oacc-mem.c
@@ -205,7 +205,7 @@ memcpy_tofrom_device (bool from, void *d, void *h, size_t s, int async,
   if (from)
     gomp_copy_dev2host (thr->dev, aq, h, d, s);
   else
-    gomp_copy_host2dev (thr->dev, aq, d, h, s, /* TODO: cbuf? */ NULL);
+    gomp_copy_host2dev (thr->dev, aq, d, h, s, false, /* TODO: cbuf? */ NULL);
 
   if (profiling_p)
     {
@@ -856,7 +856,7 @@ update_dev_host (int is_dev, void *h, size_t s, int async)
   goacc_aq aq = get_goacc_asyncqueue (async);
 
   if (is_dev)
-    gomp_copy_host2dev (acc_dev, aq, d, h, s, /* TODO: cbuf? */ NULL);
+    gomp_copy_host2dev (acc_dev, aq, d, h, s, false, /* TODO: cbuf? */ NULL);
   else
     gomp_copy_dev2host (acc_dev, aq, h, d, s);
 
diff --git a/libgomp/plugin/plugin-gcn.c b/libgomp/plugin/plugin-gcn.c
index 583916759a5..7b95a4cef8f 100644
--- a/libgomp/plugin/plugin-gcn.c
+++ b/libgomp/plugin/plugin-gcn.c
@@ -3933,19 +3933,22 @@ GOMP_OFFLOAD_openacc_async_queue_callback (struct goacc_asyncqueue *aq,
 
 bool
 GOMP_OFFLOAD_openacc_async_host2dev (int device, void *dst, const void *src,
-				     size_t n, struct goacc_asyncqueue *aq)
+				     size_t n, bool ephemeral,
+				     struct goacc_asyncqueue *aq)
 {
   struct agent_info *agent = get_agent_info (device);
   assert (agent == aq->agent);
-  /* The source data does not necessarily remain live until the deferred
-     copy happens.  Taking a snapshot of the data here avoids reading
-     uninitialised data later, but means that (a) data is copied twice and
-     (b) modifications to the copied data between the "spawning" point of
-     the asynchronous kernel and when it is executed will not be seen.
-     But, that is probably correct.  */
-  void *src_copy = GOMP_PLUGIN_malloc (n);
-  memcpy (src_copy, src, n);
-  queue_push_copy (aq, dst, src_copy, n, true);
+
+  if (ephemeral)
+    {
+      /* The source data is on the stack or otherwise may be deallocated
+	 before the asynchronous copy takes place.  Take a copy of the source
+	 data.  */
+      void *src_copy = GOMP_PLUGIN_malloc (n);
+      memcpy (src_copy, src, n);
+      src = src_copy;
+    }
+  queue_push_copy (aq, dst, src, n, ephemeral);
   return true;
 }
 
diff --git a/libgomp/plugin/plugin-nvptx.c b/libgomp/plugin/plugin-nvptx.c
index 911d0f66a6e..34b25f13dd4 100644
--- a/libgomp/plugin/plugin-nvptx.c
+++ b/libgomp/plugin/plugin-nvptx.c
@@ -1711,9 +1711,20 @@ GOMP_OFFLOAD_dev2dev (int ord, void *dst, const void *src, size_t n)
   return true;
 }
 
+/* FIXME: It is unknown whether the cuMemcpyHtoDAsync API call caches source
+   data before the asynchronous copy takes place.  Either way there is a data
+   race associated with ignoring the EPHEMERAL parameter here -- either if it
+   is TRUE (because we are copying uncached data that may disappear before the
+   async copy takes place) or if it is FALSE (because the source data may be
+   cached/snapshotted here before it is modified by an earlier async operation,
+   so stale data gets copied to the target).
+   Neither problem has been observed in practice, so far.  */
+
 bool
 GOMP_OFFLOAD_openacc_async_host2dev (int ord, void *dst, const void *src,
-				     size_t n, struct goacc_asyncqueue *aq)
+				     size_t n,
+				     bool ephemeral __attribute__((unused)),
+				     struct goacc_asyncqueue *aq)
 {
   if (!nvptx_attach_host_thread_to_device (ord)
       || !cuda_memcpy_sanity_check (src, dst, n))
diff --git a/libgomp/target.c b/libgomp/target.c
index b6276cade9f..792c70ce331 100644
--- a/libgomp/target.c
+++ b/libgomp/target.c
@@ -190,22 +190,6 @@ gomp_device_copy (struct gomp_device_descr *devicep,
     }
 }
 
-static inline void
-goacc_device_copy_async (struct gomp_device_descr *devicep,
-			 bool (*copy_func) (int, void *, const void *, size_t,
-					    struct goacc_asyncqueue *),
-			 const char *dst, void *dstaddr,
-			 const char *src, const void *srcaddr,
-			 size_t size, struct goacc_asyncqueue *aq)
-{
-  if (!copy_func (devicep->target_id, dstaddr, srcaddr, size, aq))
-    {
-      gomp_mutex_unlock (&devicep->lock);
-      gomp_fatal ("Copying of %s object [%p..%p) to %s object [%p..%p) failed",
-		  src, srcaddr, srcaddr + size, dst, dstaddr, dstaddr + size);
-    }
-}
-
 /* Infrastructure for coalescing adjacent or nearly adjacent (in device addresses)
    host to device memory transfers.  */
 
@@ -298,11 +282,18 @@ gomp_to_device_kind_p (int kind)
     }
 }
 
+/* Copy host memory to an offload device.  In asynchronous mode (if AQ is
+   non-NULL), when the source data is stack or may otherwise be deallocated
+   before the asynchronous copy takes place, EPHEMERAL must be passed as
+   TRUE.  The CBUF isn't used for non-ephemeral asynchronous copies, because
+   the host data might not be computed yet (by an earlier asynchronous compute
+   region).  */
+
 attribute_hidden void
 gomp_copy_host2dev (struct gomp_device_descr *devicep,
 		    struct goacc_asyncqueue *aq,
 		    void *d, const void *h, size_t sz,
-		    struct gomp_coalesce_buf *cbuf)
+		    bool ephemeral, struct gomp_coalesce_buf *cbuf)
 {
   if (cbuf)
     {
@@ -330,8 +321,15 @@ gomp_copy_host2dev (struct gomp_device_descr *devicep,
 	}
     }
   if (__builtin_expect (aq != NULL, 0))
-    goacc_device_copy_async (devicep, devicep->openacc.async.host2dev_func,
-			     "dev", d, "host", h, sz, aq);
+    {
+      if (!devicep->openacc.async.host2dev_func (devicep->target_id, d, h, sz,
+						 ephemeral, aq))
+	{
+	  gomp_mutex_unlock (&devicep->lock);
+	  gomp_fatal ("Copying of host object [%p..%p) to dev object [%p..%p) "
+		      "failed", h, h + sz, d, d + sz);
+	}
+    }
   else
     gomp_device_copy (devicep, devicep->host2dev_func, "dev", d, "host", h, sz);
 }
@@ -342,8 +340,15 @@ gomp_copy_dev2host (struct gomp_device_descr *devicep,
 		    void *h, const void *d, size_t sz)
 {
   if (__builtin_expect (aq != NULL, 0))
-    goacc_device_copy_async (devicep, devicep->openacc.async.dev2host_func,
-			     "host", h, "dev", d, sz, aq);
+    {
+      if (!devicep->openacc.async.dev2host_func (devicep->target_id, h, d, sz,
+						 aq))
+	{
+	  gomp_mutex_unlock (&devicep->lock);
+	  gomp_fatal ("Copying of dev object [%p..%p) to host object [%p..%p) "
+		      "failed", d, d + sz, h, h + sz);
+	}
+    }
   else
     gomp_device_copy (devicep, devicep->dev2host_func, "host", h, "dev", d, sz);
 }
@@ -390,7 +395,7 @@ gomp_map_vars_existing (struct gomp_device_descr *devicep,
 			(void *) (oldn->tgt->tgt_start + oldn->tgt_offset
 				  + newn->host_start - oldn->host_start),
 			(void *) newn->host_start,
-			newn->host_end - newn->host_start, cbuf);
+			newn->host_end - newn->host_start, false, cbuf);
 
   if (oldn->refcount != REFCOUNT_INFINITY)
     oldn->refcount++;
@@ -418,8 +423,8 @@ gomp_map_pointer (struct target_mem_desc *tgt, struct goacc_asyncqueue *aq,
       cur_node.tgt_offset = (uintptr_t) NULL;
       gomp_copy_host2dev (devicep, aq,
 			  (void *) (tgt->tgt_start + target_offset),
-			  (void *) &cur_node.tgt_offset,
-			  sizeof (void *), cbuf);
+			  (void *) &cur_node.tgt_offset, sizeof (void *),
+			  true, cbuf);
       return;
     }
   /* Add bias to the pointer value.  */
@@ -439,7 +444,8 @@ gomp_map_pointer (struct target_mem_desc *tgt, struct goacc_asyncqueue *aq,
      to initialize the pointer with.  */
   cur_node.tgt_offset -= bias;
   gomp_copy_host2dev (devicep, aq, (void *) (tgt->tgt_start + target_offset),
-		      (void *) &cur_node.tgt_offset, sizeof (void *), cbuf);
+		      (void *) &cur_node.tgt_offset, sizeof (void *), true,
+		      cbuf);
 }
 
 static void
@@ -656,8 +662,9 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 	      for (i = first; i <= last; i++)
 		{
 		  tgt->list[i].key = NULL;
-		  if (gomp_to_device_kind_p (get_kind (short_mapkind, kinds, i)
-					     & typemask))
+		  if (!aq
+		      && gomp_to_device_kind_p (get_kind (short_mapkind, kinds,
+							  i) & typemask))
 		    gomp_coalesce_buf_add (&cbuf,
 					   tgt_size - cur_node.host_end
 					   + (uintptr_t) hostaddrs[i],
@@ -692,8 +699,9 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 	  if (tgt_align < align)
 	    tgt_align = align;
 	  tgt_size = (tgt_size + align - 1) & ~(align - 1);
-	  gomp_coalesce_buf_add (&cbuf, tgt_size,
-				 cur_node.host_end - cur_node.host_start);
+	  if (!aq)
+	    gomp_coalesce_buf_add (&cbuf, tgt_size,
+				   cur_node.host_end - cur_node.host_start);
 	  tgt_size += cur_node.host_end - cur_node.host_start;
 	  has_firstprivate = true;
 	  continue;
@@ -723,7 +731,7 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 	  if (tgt_align < align)
 	    tgt_align = align;
 	  tgt_size = (tgt_size + align - 1) & ~(align - 1);
-	  if (gomp_to_device_kind_p (kind & typemask))
+	  if (!aq && gomp_to_device_kind_p (kind & typemask))
 	    gomp_coalesce_buf_add (&cbuf, tgt_size,
 				   cur_node.host_end - cur_node.host_start);
 	  tgt_size += cur_node.host_end - cur_node.host_start;
@@ -825,7 +833,7 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 		len = sizes[i];
 		gomp_copy_host2dev (devicep, aq,
 				    (void *) (tgt->tgt_start + tgt_size),
-				    (void *) hostaddrs[i], len, cbufp);
+				    (void *) hostaddrs[i], len, false, cbufp);
 		tgt_size += len;
 		continue;
 	      case GOMP_MAP_FIRSTPRIVATE_INT:
@@ -895,12 +903,11 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 		if (cur_node.tgt_offset)
 		  cur_node.tgt_offset -= sizes[i];
 		gomp_copy_host2dev (devicep, aq,
-				    (void *) (n->tgt->tgt_start
-					      + n->tgt_offset
+				    (void *) (n->tgt->tgt_start + n->tgt_offset
 					      + cur_node.host_start
 					      - n->host_start),
 				    (void *) &cur_node.tgt_offset,
-				    sizeof (void *), cbufp);
+				    sizeof (void *), true, cbufp);
 		cur_node.tgt_offset = n->tgt->tgt_start + n->tgt_offset
 				      + cur_node.host_start - n->host_start;
 		continue;
@@ -972,7 +979,8 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 					(void *) (tgt->tgt_start
 						  + k->tgt_offset),
 					(void *) k->host_start,
-					k->host_end - k->host_start, cbufp);
+					k->host_end - k->host_start, false,
+					cbufp);
 		    break;
 		  case GOMP_MAP_POINTER:
 		    gomp_map_pointer (tgt, aq,
@@ -984,7 +992,8 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 					(void *) (tgt->tgt_start
 						  + k->tgt_offset),
 					(void *) k->host_start,
-					k->host_end - k->host_start, cbufp);
+					k->host_end - k->host_start, false,
+					cbufp);
 
 		    for (j = i + 1; j < mapnum; j++)
 		      if (!GOMP_MAP_POINTER_P (get_kind (short_mapkind, kinds,
@@ -1035,7 +1044,7 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 					(void *) (tgt->tgt_start
 						  + k->tgt_offset),
 					(void *) k->host_start,
-					sizeof (void *), cbufp);
+					sizeof (void *), false, cbufp);
 		    break;
 		  default:
 		    gomp_mutex_unlock (&devicep->lock);
@@ -1051,7 +1060,7 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 		    /* We intentionally do not use coalescing here, as it's not
 		       data allocated by the current call to this function.  */
 		    gomp_copy_host2dev (devicep, aq, (void *) n->tgt_offset,
-					&tgt_addr, sizeof (void *), NULL);
+					&tgt_addr, sizeof (void *), true, NULL);
 		  }
 		array++;
 	      }
@@ -1066,7 +1075,7 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 	  gomp_copy_host2dev (devicep, aq,
 			      (void *) (tgt->tgt_start + i * sizeof (void *)),
 			      (void *) &cur_node.tgt_offset, sizeof (void *),
-			      cbufp);
+			      true, cbufp);
 	}
     }
 
@@ -1078,7 +1087,8 @@ gomp_map_vars_internal (struct gomp_device_descr *devicep,
 			    (void *) (tgt->tgt_start + cbuf.chunks[c].start),
 			    (char *) cbuf.buf + (cbuf.chunks[c].start
 						 - cbuf.chunks[0].start),
-			    cbuf.chunks[c].end - cbuf.chunks[c].start, NULL);
+			    cbuf.chunks[c].end - cbuf.chunks[c].start, true,
+			    NULL);
       free (cbuf.buf);
       cbuf.buf = NULL;
       cbufp = NULL;
@@ -1280,7 +1290,7 @@ gomp_update (struct gomp_device_descr *devicep, size_t mapnum, void **hostaddrs,
 
 	    if (GOMP_MAP_COPY_TO_P (kind & typemask))
 	      gomp_copy_host2dev (devicep, NULL, devaddr, hostaddr, size,
-				  NULL);
+				  false, NULL);
 	    if (GOMP_MAP_COPY_FROM_P (kind & typemask))
 	      gomp_copy_dev2host (devicep, NULL, hostaddr, devaddr, size);
 	  }
