[RFC][PATCH v1 2/4] C and C++ FE changes to support flexible array members in unions and alone in structures.

[Qing Zhao <qing.zhao@oracle.com>]

gcc/c/ChangeLog:

	* c-decl.cc (finish_struct): Change errors to pedwarns for the cases
	flexible array members in union or alone in structures.

gcc/cp/ChangeLog:

	* class.cc (diagnose_flexarrays): Change error to pdewarn for the case
	flexible array members alone in structures.
	* decl.cc (grokdeclarator): Change error to pdewarn for the case
	flexible array members in unions.

gcc/ChangeLog:

	* stor-layout.cc (place_union_field): Use zero sizes for flexible array
	member fields.
---
 gcc/c/c-decl.cc    | 16 +++++-----------
 gcc/cp/class.cc    | 11 ++++++++---
 gcc/cp/decl.cc     |  7 +++----
 gcc/stor-layout.cc |  9 +++++++--
 4 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 345090dae38b..947f3cd589eb 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9471,11 +9471,8 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       if (flexible_array_member_type_p (TREE_TYPE (x)))
 	{
 	  if (TREE_CODE (t) == UNION_TYPE)
-	    {
-	      error_at (DECL_SOURCE_LOCATION (x),
-			"flexible array member in union");
-	      TREE_TYPE (x) = error_mark_node;
-	    }
+	    pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
+		     "flexible array member in union is a GCC extension");
 	  else if (!is_last_field)
 	    {
 	      error_at (DECL_SOURCE_LOCATION (x),
@@ -9483,12 +9480,9 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 	      TREE_TYPE (x) = error_mark_node;
 	    }
 	  else if (!saw_named_field)
-	    {
-	      error_at (DECL_SOURCE_LOCATION (x),
-			"flexible array member in a struct with no named "
-			"members");
-	      TREE_TYPE (x) = error_mark_node;
-	    }
+	    pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
+		     "flexible array member in a struct with no named "
+		     "members is a GCC extension");
 	}
 
       if (pedantic && TREE_CODE (t) == RECORD_TYPE
diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 5f258729940b..0c8afb72550f 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -7624,6 +7624,7 @@ diagnose_flexarrays (tree t, const flexmems_t *fmem)
   bool diagd = false;
 
   const char *msg = 0;
+  const char *msg_fam = 0;
 
   if (TYPE_DOMAIN (TREE_TYPE (fmem->array)))
     {
@@ -7649,15 +7650,19 @@ diagnose_flexarrays (tree t, const flexmems_t *fmem)
       if (fmem->after[0])
 	msg = G_("flexible array member %qD not at end of %q#T");
       else if (!fmem->first)
-	msg = G_("flexible array member %qD in an otherwise empty %q#T");
+	msg_fam = G_("flexible array member %qD in an otherwise"
+		     " empty %q#T is a GCC extension");
 
-      if (msg)
+      if (msg || msg_fam)
 	{
 	  location_t loc = DECL_SOURCE_LOCATION (fmem->array);
 	  diagd = true;
 
 	  auto_diagnostic_group d;
-	  error_at (loc, msg, fmem->array, t);
+	  if (msg)
+	    error_at (loc, msg, fmem->array, t);
+	  else
+	    pedwarn (loc, OPT_Wpedantic, msg_fam, fmem->array, t);
 
 	  /* In the unlikely event that the member following the flexible
 	     array member is declared in a different class, or the member
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 65ab64885ff8..9a91c6f80da1 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -14566,10 +14566,9 @@ grokdeclarator (const cp_declarator *declarator,
 	    if (ctype
 		&& (TREE_CODE (ctype) == UNION_TYPE
 		    || TREE_CODE (ctype) == QUAL_UNION_TYPE))
-	      {
-		error_at (id_loc, "flexible array member in union");
-		type = error_mark_node;
-	      }
+	      pedwarn (id_loc, OPT_Wpedantic,
+		       "flexible array member in union is a GCC extension");
+
 	    else
 	      {
 		/* Array is a flexible member.  */
diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index e34be19689c0..10c0809914cd 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1245,13 +1245,18 @@ place_union_field (record_layout_info rli, tree field)
       && TYPE_TYPELESS_STORAGE (TREE_TYPE (field)))
     TYPE_TYPELESS_STORAGE (rli->t) = 1;
 
+  /* We might see a flexible array member field (with no DECL_SIZE_UNIT), use
+     zero size for such field.  */
+  tree field_size_unit = DECL_SIZE_UNIT (field)
+			 ? DECL_SIZE_UNIT (field)
+			 : build_int_cst (sizetype, 0);
   /* We assume the union's size will be a multiple of a byte so we don't
      bother with BITPOS.  */
   if (TREE_CODE (rli->t) == UNION_TYPE)
-    rli->offset = size_binop (MAX_EXPR, rli->offset, DECL_SIZE_UNIT (field));
+    rli->offset = size_binop (MAX_EXPR, rli->offset, field_size_unit);
   else if (TREE_CODE (rli->t) == QUAL_UNION_TYPE)
     rli->offset = fold_build3 (COND_EXPR, sizetype, DECL_QUALIFIER (field),
-			       DECL_SIZE_UNIT (field), rli->offset);
+			       field_size_unit, rli->offset);
 }
 
 /* A bitfield of SIZE with a required access alignment of ALIGN is allocated
-- 
2.31.1