[GCC13][Patch][V4][PATCH 1/2] Add a new option -fstrict-flex-arrays[=n] and new attribute strict_flex_arrays

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

Add the following new option -fstrict-flex-arrays[=n] and a corresponding
attribute strict_flex_arrays to GCC:

'-fstrict-flex-arrays'
     Treat the trailing array of a structure as a flexible array member
     in a stricter way.  The positive form is equivalent to
     '-fstrict-flex-arrays=3', which is the strictest.  A trailing array
     is treated as a flexible array member only when it is declared as a
     flexible array member per C99 standard onwards.  The negative form
     is equivalent to '-fstrict-flex-arrays=0', which is the least
     strict.  All trailing arrays of structures are treated as flexible
     array members.

'-fstrict-flex-arrays=LEVEL'
     Treat the trailing array of a structure as a flexible array member
     in a stricter way.  The value of LEVEL controls the level of
     strictness.

     The possible values of LEVEL are the same as for the
     'strict_flex_arrays' attribute (*note Variable Attributes::).

     You can control this behavior for a specific trailing array field
     of a structure by using the variable attribute 'strict_flex_arrays'
     attribute (*note Variable Attributes::).

'strict_flex_arrays (LEVEL)'
     The 'strict_flex_arrays' attribute should be attached to the
     trailing array field of a structure.  It specifies the level of
     strictness of treating the trailing array field of a structure as a
     flexible array member.  LEVEL must be an integer betwen 0 to 3.

     LEVEL=0 is the least strict level, all trailing arrays of
     structures are treated as flexible array members.  LEVEL=3 is the
     strictest level, only when the trailing array is declared as a
     flexible array member per C99 standard onwards ('[]'), it is
     treated as a flexible array member.

     There are two more levels in between 0 and 3, which are provided to
     support older codes that use GCC zero-length array extension
     ('[0]') or one-size array as flexible array member ('[1]'): When
     LEVEL is 1, the trailing array is treated as a flexible array
     member when it is declared as either '[]', '[0]', or '[1]'; When
     LEVEL is 2, the trailing array is treated as a flexible array
     member when it is declared as either '[]', or '[0]'.

     This attribute can be used with or without the
     '-fstrict-flex-arrays'.  When both the attribute and the option
     present at the same time, the level of the strictness for the
     specific trailing array field is determined by the attribute.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_strict_flex_arrays_attribute): New function.
	(c_common_attribute_table): New item for strict_flex_arrays.
	* c.opt: (fstrict-flex-arrays): New option.
	(fstrict-flex-arrays=): New option.

gcc/c/ChangeLog:

	* c-decl.cc (flexible_array_member_type_p): New function.
	(one_element_array_type_p): Likewise.
	(zero_length_array_type_p): Likewise.
	(add_flexible_array_elts_to_size): Call new utility
	routine flexible_array_member_type_p.
	(is_flexible_array_member_p): New function.
	(finish_struct): Set the new DECL_NOT_FLEXARRAY flag.

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_bools): Stream out new bit
	decl_not_flexarray.
	(trees_in::core_bools): Stream in new bit decl_not_flexarray.

gcc/ChangeLog:

	* doc/extend.texi: Document strict_flex_arrays attribute.
	* doc/invoke.texi: Document -fstrict-flex-arrays[=n] option.
	* print-tree.cc (print_node): Print new bit decl_not_flexarray.
	* tree-core.h (struct tree_decl_common): New bit field
	decl_not_flexarray.
	* tree-streamer-in.cc (unpack_ts_decl_common_value_fields): Stream
	in new bit decl_not_flexarray.
	* tree-streamer-out.cc (pack_ts_decl_common_value_fields): Stream
	out new bit decl_not_flexarray.
	* tree.cc (array_at_struct_end_p): Update it with the new bit field
	decl_not_flexarray.
	* tree.h (DECL_NOT_FLEXARRAY): New flag.

gcc/testsuite/ChangeLog:

	* g++.dg/strict-flex-array-1.C: New test.
	* gcc.dg/strict-flex-array-1.c: New test.
---
 gcc/c-family/c-attribs.cc                  |  47 ++++++++
 gcc/c-family/c.opt                         |   7 ++
 gcc/c/c-decl.cc                            | 130 +++++++++++++++++++--
 gcc/cp/module.cc                           |   2 +
 gcc/doc/extend.texi                        |  25 ++++
 gcc/doc/invoke.texi                        |  27 ++++-
 gcc/print-tree.cc                          |   8 +-
 gcc/testsuite/g++.dg/strict-flex-array-1.C |  31 +++++
 gcc/testsuite/gcc.dg/strict-flex-array-1.c |  31 +++++
 gcc/tree-core.h                            |   5 +-
 gcc/tree-streamer-in.cc                    |   1 +
 gcc/tree-streamer-out.cc                   |   1 +
 gcc/tree.cc                                |  45 +++++--
 gcc/tree.h                                 |  14 ++-
 14 files changed, 346 insertions(+), 28 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/strict-flex-array-1.C
 create mode 100644 gcc/testsuite/gcc.dg/strict-flex-array-1.c

diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index 8bb80e251dc2..0becd4bbe8d2 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -101,6 +101,8 @@ static tree handle_special_var_sec_attribute (tree *, tree, tree, int, bool *);
 static tree handle_aligned_attribute (tree *, tree, tree, int, bool *);
 static tree handle_warn_if_not_aligned_attribute (tree *, tree, tree,
 						  int, bool *);
+static tree handle_strict_flex_arrays_attribute (tree *, tree, tree,
+						 int, bool *);
 static tree handle_weak_attribute (tree *, tree, tree, int, bool *) ;
 static tree handle_noplt_attribute (tree *, tree, tree, int, bool *) ;
 static tree handle_alias_ifunc_attribute (bool, tree *, tree, tree, bool *);
@@ -368,6 +370,8 @@ const struct attribute_spec c_common_attribute_table[] =
 	                      attr_aligned_exclusions },
   { "warn_if_not_aligned",    0, 1, false, false, false, false,
 			      handle_warn_if_not_aligned_attribute, NULL },
+  { "strict_flex_arrays",      1, 1, false, false, false, false,
+			      handle_strict_flex_arrays_attribute, NULL },
   { "weak",                   0, 0, true,  false, false, false,
 			      handle_weak_attribute, NULL },
   { "noplt",                   0, 0, true,  false, false, false,
@@ -2505,6 +2509,49 @@ handle_warn_if_not_aligned_attribute (tree *node, tree name,
 					  no_add_attrs, true);
 }
 
+/* Handle a "strict_flex_arrays" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_strict_flex_arrays_attribute (tree *node, tree name,
+				     tree args, int ARG_UNUSED (flags),
+				     bool *no_add_attrs)
+{
+  tree decl = *node;
+  tree argval = TREE_VALUE (args);
+
+  /* This attribute only applies to field decls of a structure.  */
+  if (TREE_CODE (decl) != FIELD_DECL)
+    {
+      error_at (DECL_SOURCE_LOCATION (decl),
+		"%qE attribute may not be specified for %q+D", name, decl);
+      *no_add_attrs = true;
+    }
+  /* This attribute only applies to field with array type.  */
+  else if (TREE_CODE (TREE_TYPE (decl)) != ARRAY_TYPE)
+    {
+      error_at (DECL_SOURCE_LOCATION (decl),
+		"%qE attribute may not be specified for a non-array field",
+		name);
+      *no_add_attrs = true;
+    }
+  else if (TREE_CODE (argval) != INTEGER_CST)
+    {
+      error_at (DECL_SOURCE_LOCATION (decl),
+		"%qE attribute argument not an integer", name);
+      *no_add_attrs = true;
+    }
+  else if (!tree_fits_uhwi_p (argval) || tree_to_uhwi (argval) > 3)
+    {
+      error_at (DECL_SOURCE_LOCATION (decl),
+		"%qE attribute argument %qE is not an integer constant"
+		" between 0 and 3", name, argval);
+      *no_add_attrs = true;
+    }
+
+  return NULL_TREE;
+}
+
 /* Handle a "weak" attribute; arguments as in
    struct attribute_spec.handler.  */
 
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 4515664aa590..e053bd939559 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -2072,6 +2072,13 @@ fsized-deallocation
 C++ ObjC++ Var(flag_sized_deallocation) Init(-1)
 Enable C++14 sized deallocation support.
 
+fstrict-flex-arrays
+C C++ Common Alias(fstrict-flex-arrays=,3,0)
+
+fstrict-flex-arrays=
+C C++ Common Joined RejectNegative UInteger Var(flag_strict_flex_arrays) Init(0) IntegerRange(0,3)
+-fstrict-flex-arrays=<level>    Treat the trailing array of a structure as a flexible array in a stricter way.  The default is treating all trailing arrays of structures as flexible arrays.
+
 fsquangle
 C++ ObjC++ WarnRemoved
 
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 34f8feda897f..ee90808bcb0e 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -5005,6 +5005,41 @@ set_array_declarator_inner (struct c_declarator *decl,
   return decl;
 }
 
+/* Determine whether TYPE is a ISO C99 flexible array memeber type "[]".  */
+static bool
+flexible_array_member_type_p (const_tree type)
+{
+  if (TREE_CODE (type) == ARRAY_TYPE
+      && TYPE_SIZE (type) == NULL_TREE
+      && TYPE_DOMAIN (type) != NULL_TREE
+      && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
+    return true;
+
+  return false;
+}
+
+/* Determine whether TYPE is a one-element array type "[1]".  */
+static bool
+one_element_array_type_p (const_tree type)
+{
+  if (TREE_CODE (type) != ARRAY_TYPE)
+    return false;
+  return integer_zerop (array_type_nelts (type));
+}
+
+/* Determine whether TYPE is a zero-length array type "[0]".  */
+static bool
+zero_length_array_type_p (const_tree type)
+{
+  if (TREE_CODE (type) == ARRAY_TYPE)
+    if (tree type_size = TYPE_SIZE_UNIT (type))
+      if ((integer_zerop (type_size))
+	   && TYPE_DOMAIN (type) != NULL_TREE
+	   && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
+	return true;
+  return false;
+}
+
 /* INIT is a constructor that forms DECL's initializer.  If the final
    element initializes a flexible array field, add the size of that
    initializer to DECL's size.  */
@@ -5019,10 +5054,7 @@ add_flexible_array_elts_to_size (tree decl, tree init)
 
   elt = CONSTRUCTOR_ELTS (init)->last ().value;
   type = TREE_TYPE (elt);
-  if (TREE_CODE (type) == ARRAY_TYPE
-      && TYPE_SIZE (type) == NULL_TREE
-      && TYPE_DOMAIN (type) != NULL_TREE
-      && TYPE_MAX_VALUE (TYPE_DOMAIN (type)) == NULL_TREE)
+  if (flexible_array_member_type_p (type))
     {
       complete_array_type (&type, elt, false);
       DECL_SIZE (decl)
@@ -8710,6 +8742,81 @@ finish_incomplete_vars (tree incomplete_vars, bool toplevel)
     }
 }
 
+
+/* Determine whether the FIELD_DECL X is a flexible array member according to
+   the following info:
+  A. whether the FIELD_DECL X is the last field of the DECL_CONTEXT;
+  B. whether the FIELD_DECL is an array that is declared as "[]", "[0]",
+     or "[1]";
+  C. flag_strict_flex_arrays;
+  D. the attribute strict_flex_array that is attached to the field
+     if presenting.
+  Return TRUE when it's a flexible array member, FALSE otherwise.  */
+
+static bool
+is_flexible_array_member_p (bool is_last_field,
+			    tree x)
+{
+  /* if not the last field, return false.  */
+  if (!is_last_field)
+    return false;
+
+  /* if not an array field, return false.  */
+  if (TREE_CODE (TREE_TYPE (x)) != ARRAY_TYPE)
+    return false;
+
+  bool is_zero_length_array = zero_length_array_type_p (TREE_TYPE (x));
+  bool is_one_element_array = one_element_array_type_p (TREE_TYPE (x));
+  bool is_flexible_array = flexible_array_member_type_p (TREE_TYPE (x));
+
+  unsigned int strict_flex_array_level = flag_strict_flex_arrays;
+
+  tree attr_strict_flex_array = lookup_attribute ("strict_flex_arrays",
+						  DECL_ATTRIBUTES (x));
+  /* if there is a strict_flex_array attribute attached to the field,
+     override the flag_strict_flex_arrays.  */
+  if (attr_strict_flex_array)
+    {
+      /* get the value of the level first from the attribute.  */
+      unsigned HOST_WIDE_INT attr_strict_flex_array_level = 0;
+      gcc_assert (TREE_VALUE (attr_strict_flex_array) != NULL_TREE);
+      attr_strict_flex_array = TREE_VALUE (attr_strict_flex_array);
+      gcc_assert (TREE_VALUE (attr_strict_flex_array) != NULL_TREE);
+      attr_strict_flex_array = TREE_VALUE (attr_strict_flex_array);
+      gcc_assert (tree_fits_uhwi_p (attr_strict_flex_array));
+      attr_strict_flex_array_level = tree_to_uhwi (attr_strict_flex_array);
+
+      /* the attribute has higher priority than flag_struct_flex_array.  */
+      strict_flex_array_level = attr_strict_flex_array_level;
+    }
+
+  switch (strict_flex_array_level)
+    {
+      case 0:
+	/* default, all trailing arrays are flexiable array members.  */
+	return true;
+      case 1:
+	/* Level 1: all "[1]", "[0]", and "[]" are flexiable array members.  */
+	if (is_one_element_array)
+	  return true;
+	/* FALLTHROUGH.  */
+      case 2:
+	/* Level 2: all "[0]", and "[]" are flexiable array members.  */
+	if (is_zero_length_array)
+	  return true;
+	/* FALLTHROUGH.  */
+      case 3:
+	/* Level 3: Only "[]" are flexible array members.  */
+	if (is_flexible_array)
+	  return true;
+	break;
+      default:
+	gcc_unreachable ();
+    }
+  return false;
+}
+
+
 /* Fill in the fields of a RECORD_TYPE or UNION_TYPE node, T.
    LOC is the location of the RECORD_TYPE or UNION_TYPE's definition.
    FIELDLIST is a chain of FIELD_DECL nodes for the fields.
@@ -8771,6 +8878,11 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
   bool saw_named_field = false;
   for (x = fieldlist; x; x = DECL_CHAIN (x))
     {
+      /* whether this field is the last field of the structure or union.
+	 for UNION, any field is the last field of it.  */
+      bool is_last_field = (DECL_CHAIN (x) == NULL_TREE)
+			    || (TREE_CODE (t) == UNION_TYPE);
+
       if (TREE_TYPE (x) == error_mark_node)
 	continue;
 
@@ -8809,10 +8921,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 	DECL_PACKED (x) = 1;
 
       /* Detect flexible array member in an invalid context.  */
-      if (TREE_CODE (TREE_TYPE (x)) == ARRAY_TYPE
-	  && TYPE_SIZE (TREE_TYPE (x)) == NULL_TREE
-	  && TYPE_DOMAIN (TREE_TYPE (x)) != NULL_TREE
-	  && TYPE_MAX_VALUE (TYPE_DOMAIN (TREE_TYPE (x))) == NULL_TREE)
+      if (flexible_array_member_type_p (TREE_TYPE (x)))
 	{
 	  if (TREE_CODE (t) == UNION_TYPE)
 	    {
@@ -8820,7 +8929,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 			"flexible array member in union");
 	      TREE_TYPE (x) = error_mark_node;
 	    }
-	  else if (DECL_CHAIN (x) != NULL_TREE)
+	  else if (!is_last_field)
 	    {
 	      error_at (DECL_SOURCE_LOCATION (x),
 			"flexible array member not at end of struct");
@@ -8840,6 +8949,9 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 	pedwarn (DECL_SOURCE_LOCATION (x), OPT_Wpedantic,
 		 "invalid use of structure with flexible array member");
 
+      /* Set DECL_NOT_FLEXARRAY flag for FIELD_DECL x.  */
+      DECL_NOT_FLEXARRAY (x) = !is_flexible_array_member_p (is_last_field, x);
+
       if (DECL_NAME (x)
 	  || RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
 	saw_named_field = true;
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index f27f4d091e5e..75ee2514f66b 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -5414,6 +5414,7 @@ trees_out::core_bools (tree t)
       WB (t->decl_common.decl_by_reference_flag);
       WB (t->decl_common.decl_read_flag);
       WB (t->decl_common.decl_nonshareable_flag);
+      WB (t->decl_common.decl_not_flexarray);
     }
 
   if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))
@@ -5558,6 +5559,7 @@ trees_in::core_bools (tree t)
       RB (t->decl_common.decl_by_reference_flag);
       RB (t->decl_common.decl_read_flag);
       RB (t->decl_common.decl_nonshareable_flag);
+      RB (t->decl_common.decl_not_flexarray);
     }
 
   if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 0fedab96610b..62328d77c78c 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -7459,6 +7459,31 @@ This warning can be disabled by @option{-Wno-if-not-aligned}.
 The @code{warn_if_not_aligned} attribute can also be used for types
 (@pxref{Common Type Attributes}.)
 
+@cindex @code{strict_flex_arrays} variable attribute
+@item strict_flex_arrays (@var{level})
+The @code{strict_flex_arrays} attribute should be attached to the trailing
+array field of a structure.  It specifies the level of strictness of
+treating the trailing array field of a structure as a flexible array
+member. @var{level} must be an integer betwen 0 to 3.
+
+@var{level}=0 is the least strict level, all trailing arrays of structures
+are treated as flexible array members. @var{level}=3 is the strictest level,
+only when the trailing array is declared as a flexible array member per C99
+standard onwards (@samp{[]}), it is treated as a flexible array member.
+
+There are two more levels in between 0 and 3, which are provided to support
+older codes that use GCC zero-length array extension (@samp{[0]}) or one-size
+array as flexible array member (@samp{[1]}):
+When @var{level} is 1, the trailing array is treated as a flexible array member
+when it is declared as either @samp{[]}, @samp{[0]}, or @samp{[1]};
+When @var{level} is 2, the trailing array is treated as a flexible array member
+when it is declared as either @samp{[]}, or @samp{[0]}.
+
+This attribute can be used with or without the @option{-fstrict-flex-arrays}.
+When both the attribute and the option present at the same time, the level of
+the strictness for the specific trailing array field is determined by the
+attribute.
+
 @item alloc_size (@var{position})
 @itemx alloc_size (@var{position-1}, @var{position-2})
 @cindex @code{alloc_size} variable attribute
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 9d662e353163..db1a68cd545c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -207,7 +207,8 @@ in the following sections.
 -fopenmp  -fopenmp-simd @gol
 -fpermitted-flt-eval-methods=@var{standard} @gol
 -fplan9-extensions  -fsigned-bitfields  -funsigned-bitfields @gol
--fsigned-char  -funsigned-char  -fsso-struct=@var{endianness}}
+-fsigned-char  -funsigned-char -fstrict-flex-arrays[=@var{n}] @gol
+-fsso-struct=@var{endianness}}
 
 @item C++ Language Options
 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
@@ -2826,6 +2827,30 @@ The type @code{char} is always a distinct type from each of
 @code{signed char} or @code{unsigned char}, even though its behavior
 is always just like one of those two.
 
+@item -fstrict-flex-arrays
+@opindex fstrict-flex-arrays
+@opindex fno-strict-flex-arrays
+Treat the trailing array of a structure as a flexible array member in a
+stricter way.
+The positive form is equivalent to @option{-fstrict-flex-arrays=3}, which is the
+strictest.  A trailing array is treated as a flexible array member only when it
+is declared as a flexible array member per C99 standard onwards.
+The negative form is equivalent to @option{-fstrict-flex-arrays=0}, which is the
+least strict.  All trailing arrays of structures are treated as flexible array
+members.
+
+@item -fstrict-flex-arrays=@var{level}
+@opindex fstrict-flex-arrays=@var{level}
+Treat the trailing array of a structure as a flexible array member in a
+stricter way.  The value of @var{level} controls the level of strictness.
+
+The possible values of @var{level} are the same as for the
+@code{strict_flex_arrays} attribute (@pxref{Variable Attributes}).
+
+You can control this behavior for a specific trailing array field of a
+structure by using the variable attribute @code{strict_flex_arrays} attribute
+(@pxref{Variable Attributes}).
+
 @item -fsso-struct=@var{endianness}
 @opindex fsso-struct
 Set the default scalar storage order of structures and unions to the
diff --git a/gcc/print-tree.cc b/gcc/print-tree.cc
index 6d45a4a59669..58a98250cc4f 100644
--- a/gcc/print-tree.cc
+++ b/gcc/print-tree.cc
@@ -517,8 +517,12 @@ print_node (FILE *file, const char *prefix, tree node, int indent,
 	  fprintf (file, " align:%d warn_if_not_align:%d",
 		   DECL_ALIGN (node), DECL_WARN_IF_NOT_ALIGN (node));
 	  if (code == FIELD_DECL)
-	    fprintf (file, " offset_align " HOST_WIDE_INT_PRINT_UNSIGNED,
-		     DECL_OFFSET_ALIGN (node));
+	    {
+	      fprintf (file, " offset_align " HOST_WIDE_INT_PRINT_UNSIGNED,
+		       DECL_OFFSET_ALIGN (node));
+	      fprintf (file, " decl_not_flexarray: %d",
+		       DECL_NOT_FLEXARRAY (node));
+	    }
 
 	  if (code == FUNCTION_DECL && fndecl_built_in_p (node))
 	    {
diff --git a/gcc/testsuite/g++.dg/strict-flex-array-1.C b/gcc/testsuite/g++.dg/strict-flex-array-1.C
new file mode 100644
index 000000000000..cd6a65d2b7e6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/strict-flex-array-1.C
@@ -0,0 +1,31 @@
+/* testing the correct usage of attribute strict_flex_array.  */   
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+
+int x __attribute__ ((strict_flex_arrays (1))); /* { dg-error "'strict_flex_arrays' attribute may not be specified for 'x'" } */
+
+struct trailing {
+    int a;
+    int c __attribute ((strict_flex_arrays)); /* { dg-error "wrong number of arguments specified for 'strict_flex_arrays' attribute" } */
+};
+
+struct trailing_1 {
+    int a;
+    int b;
+    int c __attribute ((strict_flex_arrays (2))); /* { dg-error "'strict_flex_arrays' attribute may not be specified for a non-array field" } */
+};
+
+extern int d;
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1] __attribute ((strict_flex_arrays (d))); /* { dg-error "'strict_flex_arrays' attribute argument not an integer" } */
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0] __attribute ((strict_flex_arrays (5))); /* { dg-error "'strict_flex_arrays' attribute argument '5' is not an integer constant between 0 and 3" } */
+};
diff --git a/gcc/testsuite/gcc.dg/strict-flex-array-1.c b/gcc/testsuite/gcc.dg/strict-flex-array-1.c
new file mode 100644
index 000000000000..cd6a65d2b7e6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/strict-flex-array-1.c
@@ -0,0 +1,31 @@
+/* testing the correct usage of attribute strict_flex_array.  */   
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+
+int x __attribute__ ((strict_flex_arrays (1))); /* { dg-error "'strict_flex_arrays' attribute may not be specified for 'x'" } */
+
+struct trailing {
+    int a;
+    int c __attribute ((strict_flex_arrays)); /* { dg-error "wrong number of arguments specified for 'strict_flex_arrays' attribute" } */
+};
+
+struct trailing_1 {
+    int a;
+    int b;
+    int c __attribute ((strict_flex_arrays (2))); /* { dg-error "'strict_flex_arrays' attribute may not be specified for a non-array field" } */
+};
+
+extern int d;
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1] __attribute ((strict_flex_arrays (d))); /* { dg-error "'strict_flex_arrays' attribute argument not an integer" } */
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0] __attribute ((strict_flex_arrays (5))); /* { dg-error "'strict_flex_arrays' attribute argument '5' is not an integer constant between 0 and 3" } */
+};
diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index 80c2bcb333dd..18bd6d82a32d 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1823,7 +1823,10 @@ struct GTY(()) tree_decl_common {
      TYPE_WARN_IF_NOT_ALIGN.  */
   unsigned int warn_if_not_align : 6;
 
-  /* 14 bits unused.  */
+  /* In FIELD_DECL, this is DECL_NOT_FLEXARRAY.  */
+  unsigned int decl_not_flexarray : 1;
+
+  /* 13 bits unused.  */
 
   /* UID for points-to sets, stable over copying from inlining.  */
   unsigned int pt_uid;
diff --git a/gcc/tree-streamer-in.cc b/gcc/tree-streamer-in.cc
index 219cf5e7ef45..57923da3741e 100644
--- a/gcc/tree-streamer-in.cc
+++ b/gcc/tree-streamer-in.cc
@@ -261,6 +261,7 @@ unpack_ts_decl_common_value_fields (struct bitpack_d *bp, tree expr)
       else
 	SET_DECL_FIELD_ABI_IGNORED (expr, val);
       expr->decl_common.off_align = bp_unpack_value (bp, 8);
+      DECL_NOT_FLEXARRAY (expr) = (unsigned) bp_unpack_value (bp, 1);
     }
 
   else if (VAR_P (expr))
diff --git a/gcc/tree-streamer-out.cc b/gcc/tree-streamer-out.cc
index 9b114dc05bb1..68a2818a9f93 100644
--- a/gcc/tree-streamer-out.cc
+++ b/gcc/tree-streamer-out.cc
@@ -229,6 +229,7 @@ pack_ts_decl_common_value_fields (struct bitpack_d *bp, tree expr)
       else
 	bp_pack_value (bp, DECL_FIELD_ABI_IGNORED (expr), 1);
       bp_pack_value (bp, expr->decl_common.off_align, 8);
+      bp_pack_value (bp, DECL_NOT_FLEXARRAY (expr), 1);
     }
 
   else if (VAR_P (expr))
diff --git a/gcc/tree.cc b/gcc/tree.cc
index 2f488e4467c4..e4664eab54ef 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -12688,14 +12688,30 @@ array_ref_up_bound (tree exp)
 }
 
 /* Returns true if REF is an array reference, component reference,
-   or memory reference to an array at the end of a structure.
-   If this is the case, the array may be allocated larger
-   than its upper bound implies.  */
+   or memory reference to an array whose actual size might be larger
+   than its upper bound implies, there are multiple cases:
+   A. a ref to a flexible array member at the end of a structure;
+   B. a ref to an array with a different type against the original decl;
+      for example:
 
+   short a[16] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
+   (*((char(*)[16])&a[0]))[i+8]
+
+   C. a ref to an array that was passed as a parameter;
+      for example:
+
+   int test (uint8_t *p, uint32_t t[1][1], int n) {
+   for (int i = 0; i < 4; i++, p++)
+     t[i][0] = ...;
+
+   FIXME, the name of this routine need to be changed to be more accurate.  */
 bool
 array_at_struct_end_p (tree ref)
 {
-  tree atype;
+  /* the TYPE for this array referece.  */
+  tree atype = NULL_TREE;
+  /* the FIELD_DECL for the array field in the containing structure.  */
+  tree afield_decl = NULL_TREE;
 
   if (TREE_CODE (ref) == ARRAY_REF
       || TREE_CODE (ref) == ARRAY_RANGE_REF)
@@ -12705,7 +12721,10 @@ array_at_struct_end_p (tree ref)
     }
   else if (TREE_CODE (ref) == COMPONENT_REF
 	   && TREE_CODE (TREE_TYPE (TREE_OPERAND (ref, 1))) == ARRAY_TYPE)
-    atype = TREE_TYPE (TREE_OPERAND (ref, 1));
+    {
+      atype = TREE_TYPE (TREE_OPERAND (ref, 1));
+      afield_decl = TREE_OPERAND (ref, 1);
+    }
   else if (TREE_CODE (ref) == MEM_REF)
     {
       tree arg = TREE_OPERAND (ref, 0);
@@ -12717,6 +12736,7 @@ array_at_struct_end_p (tree ref)
 	  if (tree fld = last_field (argtype))
 	    {
 	      atype = TREE_TYPE (fld);
+	      afield_decl = fld;
 	      if (TREE_CODE (atype) != ARRAY_TYPE)
 		return false;
 	      if (VAR_P (arg) && DECL_SIZE (fld))
@@ -12770,13 +12790,16 @@ array_at_struct_end_p (tree ref)
       ref = TREE_OPERAND (ref, 0);
     }
 
-  /* The array now is at struct end.  Treat flexible arrays as
+  gcc_assert (!afield_decl
+	      || (afield_decl && TREE_CODE (afield_decl) == FIELD_DECL));
+
+  /* The array now is at struct end.  Treat flexible array member as
      always subject to extend, even into just padding constrained by
      an underlying decl.  */
   if (! TYPE_SIZE (atype)
       || ! TYPE_DOMAIN (atype)
       || ! TYPE_MAX_VALUE (TYPE_DOMAIN (atype)))
-    return true;
+    return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
 
   /* If the reference is based on a declared entity, the size of the array
      is constrained by its given domain.  (Do not trust commons PR/69368).  */
@@ -12798,9 +12821,9 @@ array_at_struct_end_p (tree ref)
       if (TREE_CODE (TYPE_SIZE_UNIT (TREE_TYPE (atype))) != INTEGER_CST
 	  || TREE_CODE (TYPE_MAX_VALUE (TYPE_DOMAIN (atype))) != INTEGER_CST
           || TREE_CODE (TYPE_MIN_VALUE (TYPE_DOMAIN (atype))) != INTEGER_CST)
-	return true;
+	return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
       if (! get_addr_base_and_unit_offset (ref_to_array, &offset))
-	return true;
+	return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
 
       /* If at least one extra element fits it is a flexarray.  */
       if (known_le ((wi::to_offset (TYPE_MAX_VALUE (TYPE_DOMAIN (atype)))
@@ -12808,12 +12831,12 @@ array_at_struct_end_p (tree ref)
 		     + 2)
 		    * wi::to_offset (TYPE_SIZE_UNIT (TREE_TYPE (atype))),
 		    wi::to_offset (DECL_SIZE_UNIT (ref)) - offset))
-	return true;
+	return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
 
       return false;
     }
 
-  return true;
+  return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
 }
 
 /* Return a tree representing the offset, in bytes, of the field referenced
diff --git a/gcc/tree.h b/gcc/tree.h
index 266e24a05633..81076b045003 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -3005,6 +3005,12 @@ extern void decl_value_expr_insert (tree, tree);
 #define DECL_PADDING_P(NODE) \
   (FIELD_DECL_CHECK (NODE)->decl_common.decl_flag_3)
 
+/* Used in a FIELD_DECL to indicate whether this field is not a flexible
+   array member. This is only valid for the last array type field of a
+   structure.  */
+#define DECL_NOT_FLEXARRAY(NODE) \
+  (FIELD_DECL_CHECK (NODE)->decl_common.decl_not_flexarray)
+
 /* A numeric unique identifier for a LABEL_DECL.  The UID allocation is
    dense, unique within any one function, and may be used to index arrays.
    If the value is -1, then no UID has been assigned.  */
@@ -5543,10 +5549,10 @@ extern tree component_ref_field_offset (tree);
    returns null.  */
 enum struct special_array_member
   {
-   none,      /* Not a special array member.  */
-   int_0,     /* Interior array member with size zero.  */
-   trail_0,   /* Trailing array member with size zero.  */
-   trail_1    /* Trailing array member with one element.  */
+    none,	/* Not a special array member.  */
+    int_0,	/* Interior array member with size zero.  */
+    trail_0,	/* Trailing array member with size zero.  */
+    trail_1	/* Trailing array member with one element.  */
   };
 
 /* Return the size of the member referenced by the COMPONENT_REF, using
-- 
2.31.1