[PATCH, OpenACC] Enable 0-length array data mappings for implicit data clauses

[Chung-Lin Tang]

Hi Thomas, this patch allows the gimplifier to create 0-length array mappings
for certain pointer and reference typed variables. Without this, array usage
of certain pointer variables do not work as usually intended, this is the
original description by Cesar when applied to og7:
https://gcc.gnu.org/ml/gcc-patches/2017-10/msg00673.html

Note that this patch requires this to cleanly apply (still awaiting approval):
https://gcc.gnu.org/ml/gcc-patches/2018-06/msg01911.html

Thomas, since this only touches OpenACC, I suppose you have the powers to 
approve.

Thanks,
Chung-Lin

2018-11-06  Cesar Philippidis  <ce...@codesourcery.com>

        gcc/
        * gimplify.c (oacc_default_clause): Create implicit 0-length
        array data clauses for pointers and reference types.

        libgomp/
        * testsuite/libgomp.oacc-c-c++-common/fp-dyn-arrays.c: New test.

diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index 9e2b0aa..58ef3de 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -7081,7 +7081,12 @@ oacc_default_clause (struct gimplify_omp_ctx *ctx, tree 
decl, unsigned flags)
     case ORT_ACC_PARALLEL:
       rkind = "parallel";
 
-      if (is_private)
+      if (TREE_CODE (type) == REFERENCE_TYPE
+         && TREE_CODE (TREE_TYPE (type)) == POINTER_TYPE)
+       flags |= GOVD_MAP | GOVD_MAP_0LEN_ARRAY;
+      else if (!lang_GNU_Fortran () && TREE_CODE (type) == POINTER_TYPE)
+       flags |= GOVD_MAP | GOVD_MAP_0LEN_ARRAY;
+      else if (is_private)
        flags |= GOVD_FIRSTPRIVATE;
       else if (on_device || declared)
        flags |= GOVD_MAP;
diff --git a/libgomp/testsuite/libgomp.oacc-c-c++-common/fp-dyn-arrays.c 
b/libgomp/testsuite/libgomp.oacc-c-c++-common/fp-dyn-arrays.c
new file mode 100644
index 0000000..c57261f
--- /dev/null
+++ b/libgomp/testsuite/libgomp.oacc-c-c++-common/fp-dyn-arrays.c
@@ -0,0 +1,42 @@
+/* Expect dynamic arrays to be mapped on the accelerator via
+   GOMP_MAP_0LEN_ARRAY.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+
+int
+main ()
+{
+  const int n = 1000;
+  int *a, *b, *c, *d, i;
+
+  d = (int *) 12345;
+  a = (int *) malloc (sizeof (int) * n);
+  b = (int *) malloc (sizeof (int) * n);
+  c = (int *) malloc (sizeof (int) * n);
+
+  for (i = 0; i < n; i++)
+    {
+      a[i] = -1;
+      b[i] = i+1;
+      c[i] = 2*(i+1);
+    }
+
+#pragma acc enter data create(a[0:n]) copyin(b[:n], c[:n])
+#pragma acc parallel loop
+  for (i = 0; i < n; ++i)
+    {
+      a[i] = b[i] + c[i] + *((int *)&d);
+    }
+#pragma acc exit data copyout(a[0:n]) delete(b[0:n], c[0:n])
+
+  for (i = 0; i < n; i++)
+    assert (a[i] == 3*(i+1) + 12345);
+
+  free (a);
+  free (b);
+  free (c);
+
+  return 0;
+}