openmp: Fix up handling of target constructs in offloaded routines [PR100573]

OpenMP Nesting of Regions restrictions say:
- If a target update, target data, target enter data, or target exit data
construct is encountered during execution of a target region, the behavior is unspecified.
- If a target construct is encountered during execution of a target region and a device
clause in which the ancestor device-modifier appears is not present on the construct, the
behavior is unspecified.
That wording is about the dynamic (runtime) behavior, not about lexical nesting,
so while it is UB if omp target * is encountered in the target region, we need to make
it compile and link (for lexical nesting of target * inside of target we actually
emit a warning).

To make this work, I had to do multiple changes.
One was to mark .omp_data_{sizes,kinds}.* variables when static as "omp declare target".
Another one was to add stub GOMP_target* entrypoints to nvptx and gcn libgomp.a.
The entrypoint functions shouldn't be called or passed in the offload regions,
otherwise
libgomp: cuLaunchKernel error: too many resources requested for launch
was reported; fixed by changing those arguments of calls to GOMP_target_ext
to NULL.
And we didn't mark the entrypoints "omp target entrypoint" when the caller
has been "omp declare target".

2021-05-26  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/100573
gcc/
	* omp-low.c: Include omp-offload.h.
	(create_omp_child_function): If current_function_decl has
	"omp declare target" attribute and is_gimple_omp_offloaded,
	remove that attribute from the copy of attribute list and
	add "omp target entrypoint" attribute instead.
	(lower_omp_target): Mark .omp_data_sizes.* and .omp_data_kinds.*
	variables for offloading if in omp_maybe_offloaded_ctx.
	* omp-offload.c (pass_omp_target_link::execute): Nullify second
	argument to GOMP_target_data_ext in offloaded code.
libgomp/
	* config/nvptx/target.c (GOMP_target_ext, GOMP_target_data_ext,
	GOMP_target_end_data, GOMP_target_update_ext,
	GOMP_target_enter_exit_data): New dummy entrypoints.
	* config/gcn/target.c (GOMP_target_ext, GOMP_target_data_ext,
	GOMP_target_end_data, GOMP_target_update_ext,
	GOMP_target_enter_exit_data): Likewise.
	* testsuite/libgomp.c-c++-common/for-3.c (DO_PRAGMA, OMPTEAMS,
	OMPFROM, OMPTO): Define.
	(main): Remove #pragma omp target teams around all the tests.
	* testsuite/libgomp.c-c++-common/target-41.c: New test.
	* testsuite/libgomp.c-c++-common/target-42.c: New test.

1 files changed, 33 insertions, 6 deletions

diff --git a/gcc/omp-low.c b/gcc/omp-low.c
index e00051b..2d5cdf6 100644
--- a/gcc/omp-low.c
+++ b/gcc/omp-low.c
@@ -59,6 +59,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "gimple-pretty-print.h"
 #include "stringpool.h"
 #include "attribs.h"
+#include "omp-offload.h"
 
 /* Lowering of OMP parallel and workshare constructs proceeds in two
    phases.  The first phase scans the function looking for OMP statements
@@ -1944,16 +1945,25 @@ create_omp_child_function (omp_context *ctx, bool task_copy)
 	g->have_offload = true;
     }
 
-  if (cgraph_node::get_create (decl)->offloadable
-      && !lookup_attribute ("omp declare target",
-                           DECL_ATTRIBUTES (current_function_decl)))
+  if (cgraph_node::get_create (decl)->offloadable)
     {
       const char *target_attr = (is_gimple_omp_offloaded (ctx->stmt)
 				 ? "omp target entrypoint"
 				 : "omp declare target");
-      DECL_ATTRIBUTES (decl)
-	= tree_cons (get_identifier (target_attr),
-		     NULL_TREE, DECL_ATTRIBUTES (decl));
+      if (lookup_attribute ("omp declare target",
+			    DECL_ATTRIBUTES (current_function_decl)))
+	{
+	  if (is_gimple_omp_offloaded (ctx->stmt))
+	    DECL_ATTRIBUTES (decl)
+	      = remove_attribute ("omp declare target",
+				  copy_list (DECL_ATTRIBUTES (decl)));
+	  else
+	    target_attr = NULL;
+	}
+      if (target_attr)
+	DECL_ATTRIBUTES (decl)
+	  = tree_cons (get_identifier (target_attr),
+		       NULL_TREE, DECL_ATTRIBUTES (decl));
     }
 
   t = build_decl (DECL_SOURCE_LOCATION (decl),
@@ -12960,6 +12970,23 @@ lower_omp_target (gimple_stmt_iterator *gsi_p, omp_context *ctx)
 				 gimple_build_assign (TREE_VEC_ELT (t, i),
 						      clobber));
 	  }
+	else if (omp_maybe_offloaded_ctx (ctx->outer))
+	  {
+	    tree id = get_identifier ("omp declare target");
+	    tree decl = TREE_VEC_ELT (t, i);
+	    DECL_ATTRIBUTES (decl)
+	      = tree_cons (id, NULL_TREE, DECL_ATTRIBUTES (decl));
+	    varpool_node *node = varpool_node::get (decl);
+	    if (node)
+	      {
+		node->offloadable = 1;
+		if (ENABLE_OFFLOADING)
+		  {
+		    g->have_offload = true;
+		    vec_safe_push (offload_vars, t);
+		  }
+	      }
+	  }
 
       tree clobber = build_clobber (ctx->record_type);
       gimple_seq_add_stmt (&olist, gimple_build_assign (ctx->sender_decl,