[V2,1/1] Add a new warning option -Wstrict-flex-arrays.

diff --git a/gcc/attribs.cc b/gcc/attribs.cc
index 27dea748561..095def4d6b0 100644
--- a/gcc/attribs.cc
+++ b/gcc/attribs.cc
@@ -2456,6 +2456,36 @@  init_attr_rdwr_indices (rdwr_map *rwm, tree attrs)
     }
 }
 
+/* Get the LEVEL of the strict_flex_array for the ARRAY_FIELD based on the
+   values of attribute strict_flex_array and the flag_strict_flex_arrays.  */
+unsigned int
+strict_flex_array_level_of (tree array_field)
+{
+  gcc_assert (TREE_CODE (array_field) == FIELD_DECL);
+  unsigned int strict_flex_array_level = flag_strict_flex_arrays;
+
+  tree attr_strict_flex_array
+    = lookup_attribute ("strict_flex_array", DECL_ATTRIBUTES (array_field));
+  /* 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;
+    }
+  return strict_flex_array_level;
+}
+
+
 /* Return the access specification for a function parameter PARM
    or null if the current function has no such specification.  */
 
diff --git a/gcc/attribs.h b/gcc/attribs.h
index 1dc16e4bc4e..742811e6fda 100644
--- a/gcc/attribs.h
+++ b/gcc/attribs.h
@@ -398,4 +398,6 @@  extern void init_attr_rdwr_indices (rdwr_map *, tree);
 extern attr_access *get_parm_access (rdwr_map &, tree,
 				     tree = current_function_decl);
 
+extern unsigned int strict_flex_array_level_of (tree);
+
 #endif // GCC_ATTRIBS_H
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 0d0ad0a6374..33edeefd285 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -976,6 +976,11 @@  Wstringop-truncation
 C ObjC C++ LTO ObjC++ Var(warn_stringop_truncation) Warning Init (1) LangEnabledBy(C ObjC C++ LTO ObjC++, Wall)
 Warn about truncation in string manipulation functions like strncat and strncpy.
 
+Wstrict-flex-arrays
+C C++ Var(warn_strict_flex_arrays) Warning
+Warn about inproper usages of flexible array members
+according to the level of -fstrict-flex-arrays.
+
 Wsuggest-attribute=format
 C ObjC C++ ObjC++ Var(warn_suggest_attribute_format) Warning
 Warn about functions which might be candidates for format attributes.
diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 4adb89e4aaf..f8cfb1d1090 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9040,7 +9040,6 @@  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;
@@ -9067,26 +9066,7 @@  is_flexible_array_member_p (bool is_last_field,
   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_array",
-						  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;
-    }
+  unsigned int strict_flex_array_level = strict_flex_array_level_of (x);
 
   switch (strict_flex_array_level)
     {
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index e312b5cef3d..5f5a6562f97 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -398,7 +398,7 @@  Objective-C and Objective-C++ Dialects}.
 -Wstrict-aliasing=n  -Wstrict-overflow  -Wstrict-overflow=@var{n} @gol
 -Wstring-compare @gol
 -Wno-stringop-overflow -Wno-stringop-overread @gol
--Wno-stringop-truncation @gol
+-Wno-stringop-truncation -Wstrict-flex-arrays @gol
 -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{|}malloc@r{]} @gol
 -Wswitch  -Wno-switch-bool  -Wswitch-default  -Wswitch-enum @gol
 -Wno-switch-outside-range  -Wno-switch-unreachable  -Wsync-nand @gol
@@ -7835,6 +7835,31 @@  however, are not suitable arguments to functions that expect
 such arrays GCC issues warnings unless it can prove that the use is
 safe.  @xref{Common Variable Attributes}.
 
+@item -Wstrict-flex-arrays
+@opindex Wstrict-flex-arrays
+@opindex Wno-strict-flex-arrays
+Warn about inproper usages of flexible array members
+according to the @var{level} of the @code{strict_flex_array (@var{level})}
+attribute attached to the trailing array field of a structure if it's
+available, otherwise according to the @var{level} of the option
+@option{-fstrict-flex-arrays=@var{level}}.
+
+This option is effective only when @var{level} is bigger than 0.  Otherwise,
+it will be ignored with a warning.
+
+when @var{level}=1, warnings will be issued for a trailing array reference
+of a structure that have 2 or more elements if the trailing array is referenced
+as a flexible array member.
+
+when @var{level}=2, in addition to @var{level}=1, additional warnings will be
+issued for a trailing one-element array reference of a structure
+if the array is referenced as a flexible array member.
+
+when @var{level}=3, in addition to @var{level}=2, additional warnings will be
+issued for a trailing zero-length array reference of a structure
+if the array is referenced as a flexible array member.
+
+
 @item -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{|}cold@r{|}malloc@r{]}
 @opindex Wsuggest-attribute=
 @opindex Wno-suggest-attribute=
@@ -8040,17 +8065,34 @@  is enabled by @option{-Wall}.  It is more effective when @option{-ftree-vrp}
 is active (the default for @option{-O2} and above) but a subset of instances
 are issued even without optimization.
 
+By default, the trailing array of a structure will be treated as a flexible
+array member by @option{-Warray-bounds} or @option{-Warray-bounds=@var{n}}
+if it is declared as either a flexible array member per C99 standard onwards
+(@samp{[]}), a GCC zero-length array extension (@samp{[0]}), or an one-element
+array (@samp{[1]}). As a result, out of bounds subscripts or offsets into
+zero-length arrays or one-element arrays are not warned by default.
+
+You can add the option @option{-fstrict-flex-arrays} or
+@option{-fstrict-flex-arrays=@var{level}} to control how this
+option treat trailing array of a structure as a flexible array member:
+
+when @var{level}<=1, no change to the default behavior.
+
+when @var{level}=2, additional warnings will be issued for out of bounds
+subscripts or offsets into one-element arrays;
+
+when @var{level}=3, in addition to @var{level}=2, additional warnings will be
+issued for out of bounds subscripts or offsets into zero-length arrays.
+
 @table @gcctabopt
 @item -Warray-bounds=1
 This is the default warning level of @option{-Warray-bounds} and is enabled
 by @option{-Wall}; higher levels are not, and must be explicitly requested.
 
 @item -Warray-bounds=2
-This warning level also warns about out of bounds accesses to trailing
-struct members of one-element array types (@pxref{Zero Length}) and about
-the intermediate results of pointer arithmetic that may yield out of bounds
-values.  This warning level may give a larger number of false positives and
-is deactivated by default.
+This warning level also warns about the intermediate results of pointer
+arithmetic that may yield out of bounds values. This warning level may
+give a larger number of false positives and is deactivated by default.
 @end table
 
 @item -Warray-compare
diff --git a/gcc/gimple-array-bounds.cc b/gcc/gimple-array-bounds.cc
index eae49ab3910..7cf1c56ac3f 100644
--- a/gcc/gimple-array-bounds.cc
+++ b/gcc/gimple-array-bounds.cc
@@ -160,38 +160,17 @@  trailing_array (tree arg, tree *pref)
   return array_ref_flexible_size_p (arg);
 }
 
-/* Checks one ARRAY_REF in REF, located at LOCUS. Ignores flexible
-   arrays and "struct" hacks. If VRP can determine that the array
-   subscript is a constant, check if it is outside valid range.  If
-   the array subscript is a RANGE, warn if it is non-overlapping with
-   valid range.  IGNORE_OFF_BY_ONE is true if the ARRAY_REF is inside
-   a ADDR_EXPR.  Return  true if a warning has been issued or if
-   no-warning is set.  */
+/* Acquire the upper bound and upper bound plus one for the array
+   reference REF and record them into UP_BOUND and UP_BOUND_P1.
+   Set *DECL to the decl or expresssion REF refers to.  */
 
-bool
-array_bounds_checker::check_array_ref (location_t location, tree ref,
-				       gimple *stmt, bool ignore_off_by_one)
+static void
+get_up_bounds_for_array_ref (tree ref, tree *decl,
+			     tree *up_bound, tree *up_bound_p1)
 {
-  if (warning_suppressed_p (ref, OPT_Warray_bounds))
-    /* Return true to have the caller prevent warnings for enclosing
-       refs.  */
-    return true;
-
-  tree low_sub = TREE_OPERAND (ref, 1);
-  tree up_sub = low_sub;
-  tree up_bound = array_ref_up_bound (ref);
-
-  /* Referenced decl if one can be determined.  */
-  tree decl = NULL_TREE;
-
-  /* Set for accesses to interior zero-length arrays.  */
-  special_array_member sam{ };
-
-  tree up_bound_p1;
-
-  if (!up_bound
-      || TREE_CODE (up_bound) != INTEGER_CST
-      || (warn_array_bounds < 2 && trailing_array (ref, &decl)))
+  if (!(*up_bound)
+      || TREE_CODE (*up_bound) != INTEGER_CST
+      || trailing_array (ref, decl))
     {
       /* Accesses to trailing arrays via pointers may access storage
 	 beyond the types array bounds.  For such arrays, or for flexible
@@ -203,8 +182,8 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
       if (TREE_CODE (eltsize) != INTEGER_CST
 	  || integer_zerop (eltsize))
 	{
-	  up_bound = NULL_TREE;
-	  up_bound_p1 = NULL_TREE;
+	  *up_bound = NULL_TREE;
+	  *up_bound_p1 = NULL_TREE;
 	}
       else
 	{
@@ -217,7 +196,7 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 	    {
 	      /* Try to determine the size of the trailing array from
 		 its initializer (if it has one).  */
-	      if (tree refsize = component_ref_size (arg, &sam))
+	      if (tree refsize = component_ref_size (arg))
 		if (TREE_CODE (refsize) == INTEGER_CST)
 		  maxbound = refsize;
 	    }
@@ -236,7 +215,7 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 		    {
 		      /* Try to determine the size from a pointer to
 			 an array if BASE is one.  */
-		      if (tree size = get_ref_size (base, &decl))
+		      if (tree size = get_ref_size (base, decl))
 			maxbound = size;
 		    }
 		  else if (!compref && DECL_P (base))
@@ -244,7 +223,7 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 		      if (TREE_CODE (basesize) == INTEGER_CST)
 			{
 			  maxbound = basesize;
-			  decl = base;
+			  *decl = base;
 			}
 
 		  if (known_gt (off, 0))
@@ -256,40 +235,49 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 	  else
 	    maxbound = fold_convert (sizetype, maxbound);
 
-	  up_bound_p1 = int_const_binop (TRUNC_DIV_EXPR, maxbound, eltsize);
+	  *up_bound_p1 = int_const_binop (TRUNC_DIV_EXPR, maxbound, eltsize);
 
-	  if (up_bound_p1 != NULL_TREE)
-	    up_bound = int_const_binop (MINUS_EXPR, up_bound_p1,
+	  if (*up_bound_p1 != NULL_TREE)
+	    *up_bound = int_const_binop (MINUS_EXPR, *up_bound_p1,
 					build_int_cst (ptrdiff_type_node, 1));
 	  else
-	    up_bound = NULL_TREE;
+	    *up_bound = NULL_TREE;
 	}
     }
   else
-    up_bound_p1 = int_const_binop (PLUS_EXPR, up_bound,
-				   build_int_cst (TREE_TYPE (up_bound), 1));
+    *up_bound_p1 = int_const_binop (PLUS_EXPR, *up_bound,
+				   build_int_cst (TREE_TYPE (*up_bound), 1));
+  return;
+}
 
-  tree low_bound = array_ref_low_bound (ref);
+/* Given the LOW_SUB_ORG, LOW_SUB and UP_SUB, and the computed UP_BOUND
+   and UP_BOUND_P1, check whether the array reference REF is out of bound.
+   When out of bounds, set OUT_OF_BOUND to true.
+   Issue warnings if FOR_ARRAY_BOUND is true.
+   return TRUE if warnings are issued.  */
 
+static bool
+check_out_of_bounds_and_warn (location_t location, tree ref,
+			      tree low_sub_org, tree low_sub, tree up_sub,
+			      tree up_bound, tree up_bound_p1,
+			      const value_range *vr,
+			      bool ignore_off_by_one, bool for_array_bound,
+			      bool *out_of_bound)
+{
+  tree low_bound = array_ref_low_bound (ref);
   tree artype = TREE_TYPE (TREE_OPERAND (ref, 0));
 
   bool warned = false;
+  *out_of_bound = false;
 
   /* Empty array.  */
   if (up_bound && tree_int_cst_equal (low_bound, up_bound_p1))
-    warned = warning_at (location, OPT_Warray_bounds,
-			 "array subscript %E is outside array bounds of %qT",
-			 low_sub, artype);
-
-  const value_range *vr = NULL;
-  if (TREE_CODE (low_sub) == SSA_NAME)
     {
-      vr = get_value_range (low_sub, stmt);
-      if (!vr->undefined_p () && !vr->varying_p ())
-	{
-	  low_sub = vr->kind () == VR_RANGE ? vr->max () : vr->min ();
-	  up_sub = vr->kind () == VR_RANGE ? vr->min () : vr->max ();
-	}
+      *out_of_bound = true;
+      if (for_array_bound)
+	warned = warning_at (location, OPT_Warray_bounds,
+			     "array subscript %E is outside array"
+			     " bounds of %qT", low_sub_org, artype);
     }
 
   if (warned)
@@ -303,24 +291,107 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 	      : tree_int_cst_le (up_bound, up_sub))
 	  && TREE_CODE (low_sub) == INTEGER_CST
 	  && tree_int_cst_le (low_sub, low_bound))
-	warned = warning_at (location, OPT_Warray_bounds,
-			     "array subscript [%E, %E] is outside "
-			     "array bounds of %qT",
-			     low_sub, up_sub, artype);
+	{
+	  *out_of_bound = true;
+	  if (for_array_bound)
+	    warned = warning_at (location, OPT_Warray_bounds,
+				 "array subscript [%E, %E] is outside "
+				 "array bounds of %qT",
+				 low_sub, up_sub, artype);
+	}
     }
   else if (up_bound
 	   && TREE_CODE (up_sub) == INTEGER_CST
 	   && (ignore_off_by_one
 	       ? !tree_int_cst_le (up_sub, up_bound_p1)
 	       : !tree_int_cst_le (up_sub, up_bound)))
-    warned = warning_at (location, OPT_Warray_bounds,
-			 "array subscript %E is above array bounds of %qT",
-			 up_sub, artype);
+    {
+      *out_of_bound = true;
+      if (for_array_bound)
+	warned = warning_at (location, OPT_Warray_bounds,
+			     "array subscript %E is above array bounds of %qT",
+			     up_sub, artype);
+    }
   else if (TREE_CODE (low_sub) == INTEGER_CST
 	   && tree_int_cst_lt (low_sub, low_bound))
-    warned = warning_at (location, OPT_Warray_bounds,
-			 "array subscript %E is below array bounds of %qT",
-			 low_sub, artype);
+    {
+      *out_of_bound = true;
+      if (for_array_bound)
+	warned = warning_at (location, OPT_Warray_bounds,
+			     "array subscript %E is below array bounds of %qT",
+			     low_sub, artype);
+    }
+  return warned;
+}
+
+/* Checks one ARRAY_REF in REF, located at LOCUS.  Ignores flexible
+   arrays and "struct" hacks.  If VRP can determine that the array
+   subscript is a constant, check if it is outside valid range.  If
+   the array subscript is a RANGE, warn if it is non-overlapping with
+   valid range.  IGNORE_OFF_BY_ONE is true if the ARRAY_REF is inside
+   a ADDR_EXPR.  Return  true if a warning has been issued or if
+   no-warning is set.  */
+
+bool
+array_bounds_checker::check_array_ref (location_t location, tree ref,
+				       gimple *stmt, bool ignore_off_by_one)
+{
+  if (warning_suppressed_p (ref, OPT_Warray_bounds))
+    /* Return true to have the caller prevent warnings for enclosing
+       refs.  */
+    return true;
+
+  /* Upper bound and Upper bound plus one for -Warray-bounds.  */
+  tree up_bound = array_ref_up_bound (ref);
+  tree up_bound_p1 = NULL_TREE;
+
+  /* Referenced decl if one can be determined.  */
+  tree decl = NULL_TREE;
+
+  /* Set to the type of the special array member for a COMPONENT_REF.  */
+  special_array_member sam{ };
+
+  tree arg = TREE_OPERAND (ref, 0);
+  const bool compref = TREE_CODE (arg) == COMPONENT_REF;
+
+  unsigned int strict_flex_array_level = flag_strict_flex_arrays;
+
+  if (compref)
+    {
+      /* Try to determine special array member type for this COMPONENT_REF.  */
+      sam = component_ref_sam_type (arg);
+      /* Get the level of strict_flex_array for this array field.  */
+      tree afield_decl = TREE_OPERAND (arg, 1);
+      strict_flex_array_level = strict_flex_array_level_of (afield_decl);
+    }
+
+  get_up_bounds_for_array_ref (ref, &decl, &up_bound, &up_bound_p1);
+
+  bool warned = false;
+  bool out_of_bound = false;
+
+  tree artype = TREE_TYPE (TREE_OPERAND (ref, 0));
+  tree low_sub_org = TREE_OPERAND (ref, 1);
+  tree up_sub = low_sub_org;
+  tree low_sub = low_sub_org;
+
+  const value_range *vr = NULL;
+  if (TREE_CODE (low_sub_org) == SSA_NAME)
+    {
+      vr = get_value_range (low_sub_org, stmt);
+      if (!vr->undefined_p () && !vr->varying_p ())
+	{
+	  low_sub = vr->kind () == VR_RANGE ? vr->max () : vr->min ();
+	  up_sub = vr->kind () == VR_RANGE ? vr->min () : vr->max ();
+	}
+    }
+
+  warned = check_out_of_bounds_and_warn (location, ref,
+					 low_sub_org, low_sub, up_sub,
+					 up_bound, up_bound_p1, vr,
+					 ignore_off_by_one, warn_array_bounds,
+					 &out_of_bound);
+
 
   if (!warned && sam == special_array_member::int_0)
     warned = warning_at (location, OPT_Wzero_length_bounds,
@@ -331,19 +402,56 @@  array_bounds_checker::check_array_ref (location_t location, tree ref,
 			       "of an interior zero-length array %qT")),
 			 low_sub, artype);
 
-  if (warned)
+  if (warned || out_of_bound)
     {
-      if (dump_file && (dump_flags & TDF_DETAILS))
+      if (warned && dump_file && (dump_flags & TDF_DETAILS))
 	{
 	  fprintf (dump_file, "Array bound warning for ");
 	  dump_generic_expr (MSG_NOTE, TDF_SLIM, ref);
 	  fprintf (dump_file, "\n");
 	}
 
+      /* issue warnings for -Wstrict-flex-arrays according to the level of
+	 flag_strict_flex_arrays.  */
+      if (out_of_bound && warn_strict_flex_arrays)
+	switch (strict_flex_array_level)
+	  {
+	    case 3:
+	      /* Issue additional warnings for trailing arrays [0].  */
+	      if (sam == special_array_member::trail_0)
+		warned = warning_at (location, OPT_Wstrict_flex_arrays,
+				     "trailing array %qT should not be used as "
+				     "a flexible array member for level 3",
+				     artype);
+	      /* FALLTHROUGH.  */
+	    case 2:
+	      /* Issue additional warnings for trailing arrays [1].  */
+	      if (sam == special_array_member::trail_1)
+		warned = warning_at (location, OPT_Wstrict_flex_arrays,
+				     "trailing array %qT should not be used as "
+				     "a flexible array member for level 2 and "
+				     "above", artype);
+	      /* FALLTHROUGH.  */
+	    case 1:
+	      /* Issue warnings for trailing arrays [n].  */
+	      if (sam == special_array_member::trail_n)
+		warned = warning_at (location, OPT_Wstrict_flex_arrays,
+				     "trailing array %qT should not be used as "
+				     "a flexible array member for level 1 and "
+				     "above", artype);
+	      break;
+	    case 0:
+	      /* Do nothing.  */
+	      break;
+	    default:
+	      gcc_unreachable ();
+	  }
+
       /* Avoid more warnings when checking more significant subscripts
 	 of the same expression.  */
       ref = TREE_OPERAND (ref, 0);
       suppress_warning (ref, OPT_Warray_bounds);
+      suppress_warning (ref, OPT_Wstrict_flex_arrays);
 
       if (decl)
 	ref = decl;
diff --git a/gcc/opts.cc b/gcc/opts.cc
index 73fc97756e4..8db53ad6c77 100644
--- a/gcc/opts.cc
+++ b/gcc/opts.cc
@@ -1411,6 +1411,14 @@  finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
       opts->x_profile_flag = 0;
     }
 
+  if (opts->x_warn_strict_flex_arrays)
+    if (opts->x_flag_strict_flex_arrays == 0)
+      {
+	opts->x_warn_strict_flex_arrays = 0;
+	warning_at (UNKNOWN_LOCATION, 0,
+		    "%<-Wstrict-flex-arrays%> is ignored when"
+		    " %<-fstrict-flex-arrays%> does not present");
+      }
 
   diagnose_options (opts, opts_set, loc);
 }
diff --git a/gcc/testsuite/c-c++-common/Wstrict-flex-arrays.c b/gcc/testsuite/c-c++-common/Wstrict-flex-arrays.c
new file mode 100644
index 00000000000..72b4b7c6406
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wstrict-flex-arrays.c
@@ -0,0 +1,9 @@ 
+/* Test the usage of option -Wstrict-flex-arrays.  */
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wstrict-flex-arrays" } */
+
+int main(int argc, char *argv[])
+{
+    return 0;
+}
+/* { dg-warning "is ignored when \'-fstrict-flex-arrays\' does not present" "" { target *-*-* } 0 } */
diff --git a/gcc/testsuite/gcc.dg/Warray-bounds-11.c b/gcc/testsuite/gcc.dg/Warray-bounds-11.c
index c9fc461942f..3ba2bb67fa0 100644
--- a/gcc/testsuite/gcc.dg/Warray-bounds-11.c
+++ b/gcc/testsuite/gcc.dg/Warray-bounds-11.c
@@ -78,7 +78,7 @@  void foo(int (*a)[3])
 
 	h->j[4] = 1;	// flexible array member
 	h0->j[4] = 1;	// zero-sized array extension
-	h1->j[4] = 1;	/* { dg-warning "subscript 4 is above array bound" } */
+	h1->j[4] = 1;	/* { dg-bogus "subscript 4 is above array bound" } */
 	h3->j[4] = 1;	/* { dg-warning "subscript 4 is above array bound" } */
 
 	struct h0b* h0b = malloc(sizeof(struct h) + 3 * sizeof(int));
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-2.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-2.c
new file mode 100644
index 00000000000..59d5a5fcb23
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-2.c
@@ -0,0 +1,46 @@ 
+/* Test -Wstrict-flex-arrays.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=2" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-3.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-3.c
new file mode 100644
index 00000000000..2f66151e927
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-3.c
@@ -0,0 +1,46 @@ 
+/* Test -Wstrict-flex-arrays.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=3" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+    trailing_0->c[1] = 1; /* { dg-warning "should not be used as a flexible array member for level 3" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 3" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-4.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-4.c
new file mode 100644
index 00000000000..8ea6236b4f7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-4.c
@@ -0,0 +1,47 @@ 
+/* Test -Wstrict-flex-arrays + -Warray-bounds.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=1 -Warray-bounds" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    			 /*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_1->c[2] = 2; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-5.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-5.c
new file mode 100644
index 00000000000..00f4037d03c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-5.c
@@ -0,0 +1,49 @@ 
+/* Test -Wstrict-flex-arrays + -Warray-bounds.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=2 -Warray-bounds" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    			 /*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+    			 /*{ dg-warning "array subscript 2 is above array bounds of" "" { target *-*-* } .-1 } */
+
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-6.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-6.c
new file mode 100644
index 00000000000..190dd0d89c2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-6.c
@@ -0,0 +1,49 @@ 
+/* Test -Wstrict-flex-arrays.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=3 -Warray-bounds" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+   			/*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */ 
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+   			/*{ dg-warning "array subscript 2 is above array bounds of" "" { target *-*-* } .-1 } */ 
+    trailing_0->c[1] = 1; /* { dg-warning "should not be used as a flexible array member for level 3" } */
+   			/*{ dg-warning "array subscript 1 is outside array bounds of" "" { target *-*-* } .-1 } */ 
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 3" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-7.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-7.c
new file mode 100644
index 00000000000..f422705e09f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-7.c
@@ -0,0 +1,48 @@ 
+/* Test -Wstrict-flex-arrays + -Warray-bounds=2.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=1 -Warray-bounds=2" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    			 /*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_1->c[2] = 2; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    			 /*{ dg-bogus "array subscript 2 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-8.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-8.c
new file mode 100644
index 00000000000..e9bbd03dd9a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-8.c
@@ -0,0 +1,48 @@ 
+/* Test -Wstrict-flex-arrays + -Warray-bounds=2.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=2 -Warray-bounds=2" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    			 /*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+    			  /*{ dg-warning "array subscript 2 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 2 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-9.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-9.c
new file mode 100644
index 00000000000..54e8de4b04f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays-9.c
@@ -0,0 +1,49 @@ 
+/* Test -Wstrict-flex-arrays + -Warray-bounds=2.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=3 -Warray-bounds=2" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+   			/*{ dg-warning "array subscript 5 is above array bounds of" "" { target *-*-* } .-1 } */ 
+    trailing_1->c[2] = 2; /* { dg-warning "should not be used as a flexible array member for level 2 and above" } */
+    			/*{ dg-warning "array subscript 2 is above array bounds of" "" { target *-*-* } .-1 } */
+    trailing_0->c[1] = 1; /* { dg-warning "should not be used as a flexible array member for level 3" } */
+    			/*{ dg-warning "array subscript 1 is outside array bounds of" "" { target *-*-* } .-1 } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 3" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/Wstrict-flex-arrays.c b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays.c
new file mode 100644
index 00000000000..43a9098f138
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wstrict-flex-arrays.c
@@ -0,0 +1,46 @@ 
+/* Test -Wstrict-flex-arrays.  */
+/* { dg-do run } */
+/* { dg-options "-O2 -Wstrict-flex-arrays -fstrict-flex-arrays=1" } */
+
+struct trailing_array_1 {
+    int a;
+    int b;
+    int c[4]; 
+};
+
+struct trailing_array_2 {
+    int a;
+    int b;
+    int c[1]; 
+};
+
+struct trailing_array_3 {
+    int a;
+    int b;
+    int c[0];
+};
+struct trailing_array_4 {
+    int a;
+    int b;
+    int c[];
+};
+
+void __attribute__((__noinline__)) stuff(
+    struct trailing_array_1 *normal,
+    struct trailing_array_2 *trailing_1,
+    struct trailing_array_3 *trailing_0,
+    struct trailing_array_4 *trailing_flex)
+{
+    normal->c[5] = 5; 	/*{ dg-warning "should not be used as a flexible array member for level 1 and above" } */
+    trailing_1->c[2] = 2; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    trailing_0->c[1] = 1; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+    trailing_flex->c[10] = 10; /* { dg-bogus "should not be used as a flexible array member for level 1 and above" } */
+
+}
+
+int main(int argc, char *argv[])
+{
+    stuff((void *)argv[0], (void *)argv[0], (void *)argv[0], (void *)argv[0]);
+
+    return 0;
+}
diff --git a/gcc/tree-vrp.cc b/gcc/tree-vrp.cc
index 3846dc1d849..e6c6c5a301d 100644
--- a/gcc/tree-vrp.cc
+++ b/gcc/tree-vrp.cc
@@ -1087,7 +1087,7 @@  execute_ranger_vrp (struct function *fun, bool warn_array_bounds_p,
   if (dump_file && (dump_flags & TDF_DETAILS))
     ranger->dump (dump_file);
 
-  if (warn_array_bounds && warn_array_bounds_p)
+  if ((warn_array_bounds || warn_strict_flex_arrays) && warn_array_bounds_p)
     {
       // Set all edges as executable, except those ranger says aren't.
       int non_exec_flag = ranger->non_executable_edge_flag;
diff --git a/gcc/tree.cc b/gcc/tree.cc
index 254b2373dcf..b40c95ae8c4 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -12719,15 +12719,21 @@  array_ref_up_bound (tree exp)
    int test (uint8_t *p, uint32_t t[1][1], int n) {
    for (int i = 0; i < 4; i++, p++)
      t[i][0] = ...;
+
+   If non-null, set IS_TRAILING_ARRAY to true if the ref is the above case A.
 */
 
 bool
-array_ref_flexible_size_p (tree ref)
+array_ref_flexible_size_p (tree ref, bool *is_trailing_array /* = NULL */)
 {
-  /* the TYPE for this array referece.  */
+  /* The TYPE for this array referece.  */
   tree atype = NULL_TREE;
-  /* the FIELD_DECL for the array field in the containing structure.  */
+  /* The FIELD_DECL for the array field in the containing structure.  */
   tree afield_decl = NULL_TREE;
+  /* Whether this array is the trailing array of a structure.  */
+  bool is_trailing_array_tmp = false;
+  if (!is_trailing_array)
+    is_trailing_array = &is_trailing_array_tmp;
 
   if (TREE_CODE (ref) == ARRAY_REF
       || TREE_CODE (ref) == ARRAY_RANGE_REF)
@@ -12815,7 +12821,10 @@  array_ref_flexible_size_p (tree ref)
   if (! TYPE_SIZE (atype)
       || ! TYPE_DOMAIN (atype)
       || ! TYPE_MAX_VALUE (TYPE_DOMAIN (atype)))
-    return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+    {
+      *is_trailing_array = afield_decl && TREE_CODE (afield_decl) == FIELD_DECL;
+      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).  */
@@ -12837,9 +12846,17 @@  array_ref_flexible_size_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 afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+	{
+	  *is_trailing_array
+	    = afield_decl && TREE_CODE (afield_decl) == FIELD_DECL;
+	  return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+	}
       if (! get_addr_base_and_unit_offset (ref_to_array, &offset))
-	return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+	{
+	  *is_trailing_array
+	    = afield_decl && TREE_CODE (afield_decl) == FIELD_DECL;
+	  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)))
@@ -12847,11 +12864,16 @@  array_ref_flexible_size_p (tree ref)
 		     + 2)
 		    * wi::to_offset (TYPE_SIZE_UNIT (TREE_TYPE (atype))),
 		    wi::to_offset (DECL_SIZE_UNIT (ref)) - offset))
-	return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+	{
+	  *is_trailing_array
+	    = afield_decl && TREE_CODE (afield_decl) == FIELD_DECL;
+	  return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
+	}
 
       return false;
     }
 
+  *is_trailing_array = afield_decl && TREE_CODE (afield_decl) == FIELD_DECL;
   return afield_decl ? !DECL_NOT_FLEXARRAY (afield_decl) : true;
 }
 
@@ -12913,11 +12935,63 @@  get_initializer_for (tree init, tree decl)
   return NULL_TREE;
 }
 
+/* Determines the special array member type for the array reference REF.  */
+special_array_member
+component_ref_sam_type (tree ref)
+{
+  special_array_member sam_type = special_array_member::none;
+
+  tree member = TREE_OPERAND (ref, 1);
+  tree memsize = DECL_SIZE_UNIT (member);
+  if (memsize)
+    {
+      tree memtype = TREE_TYPE (member);
+      if (TREE_CODE (memtype) != ARRAY_TYPE)
+	return sam_type;
+
+      bool trailing = false;
+      (void)array_ref_flexible_size_p (ref, &trailing);
+      bool zero_length = integer_zerop (memsize);
+      if (!trailing && !zero_length)
+	/* MEMBER is an interior array with
+	  more than one element.  */
+	return special_array_member::int_n;
+
+      if (zero_length)
+	{
+	  if (trailing)
+	    return special_array_member::trail_0;
+	  else
+	    return special_array_member::int_0;
+	}
+
+      if (!zero_length)
+	if (tree dom = TYPE_DOMAIN (memtype))
+	  if (tree min = TYPE_MIN_VALUE (dom))
+	    if (tree max = TYPE_MAX_VALUE (dom))
+	      if (TREE_CODE (min) == INTEGER_CST
+		  && TREE_CODE (max) == INTEGER_CST)
+		{
+		  offset_int minidx = wi::to_offset (min);
+		  offset_int maxidx = wi::to_offset (max);
+		  offset_int neltsm1 = maxidx - minidx;
+		  if (neltsm1 > 0)
+		    /* MEMBER is a trailing array with more than
+		       one elements.  */
+		    return special_array_member::trail_n;
+
+		  if (neltsm1 == 0)
+		    return special_array_member::trail_1;
+		}
+    }
+
+  return sam_type;
+}
+
 /* Determines the size of the member referenced by the COMPONENT_REF
    REF, using its initializer expression if necessary in order to
    determine the size of an initialized flexible array member.
-   If non-null, set *ARK when REF refers to an interior zero-length
-   array or a trailing one-element array.
+   If non-null, set *SAM to the type of special array member.
    Returns the size as sizetype (which might be zero for an object
    with an uninitialized flexible array member) or null if the size
    cannot be determined.  */
@@ -12930,7 +13004,7 @@  component_ref_size (tree ref, special_array_member *sam /* = NULL */)
   special_array_member sambuf;
   if (!sam)
     sam = &sambuf;
-  *sam = special_array_member::none;
+  *sam = component_ref_sam_type (ref);
 
   /* The object/argument referenced by the COMPONENT_REF and its type.  */
   tree arg = TREE_OPERAND (ref, 0);
@@ -12951,43 +13025,46 @@  component_ref_size (tree ref, special_array_member *sam /* = NULL */)
 	return (tree_int_cst_equal (memsize, TYPE_SIZE_UNIT (memtype))
 		? memsize : NULL_TREE);
 
-      bool trailing = array_ref_flexible_size_p (ref);
-      bool zero_length = integer_zerop (memsize);
-      if (!trailing && !zero_length)
-	/* MEMBER is either an interior array or is an array with
-	   more than one element.  */
+      /* 2-or-more elements arrays are treated as normal arrays by default.  */
+      if (*sam == special_array_member::int_n
+	  || *sam == special_array_member::trail_n)
 	return memsize;
 
-      if (zero_length)
+      /* flag_strict_flex_arrays will control how to treat
+	 the trailing arrays as flexiable array members.  */
+
+      tree afield_decl = TREE_OPERAND (ref, 1);
+      unsigned int strict_flex_array_level
+	= strict_flex_array_level_of (afield_decl);
+
+      switch (strict_flex_array_level)
 	{
-	  if (trailing)
-	    *sam = special_array_member::trail_0;
-	  else
-	    {
-	      *sam = special_array_member::int_0;
-	      memsize = NULL_TREE;
-	    }
+	  case 3:
+	    /* Treaing 0-length trailing arrays as normal array.  */
+	    if (*sam == special_array_member::trail_0)
+	      return size_zero_node;
+	    /* FALLTHROUGH.  */
+	  case 2:
+	    /* Treating 1-element trailing arrays as normal array.  */
+	    if (*sam == special_array_member::trail_1)
+	      return memsize;
+	    /* FALLTHROUGH.  */
+	  case 1:
+	    /* Treating 2-or-more elements trailing arrays as normal
+	       array.  */
+	    if (*sam == special_array_member::trail_n)
+	      return memsize;
+	    /* FALLTHROUGH.  */
+	  case 0:
+	    break;
+	  default:
+	    gcc_unreachable ();
 	}
 
-      if (!zero_length)
-	if (tree dom = TYPE_DOMAIN (memtype))
-	  if (tree min = TYPE_MIN_VALUE (dom))
-	    if (tree max = TYPE_MAX_VALUE (dom))
-	      if (TREE_CODE (min) == INTEGER_CST
-		  && TREE_CODE (max) == INTEGER_CST)
-		{
-		  offset_int minidx = wi::to_offset (min);
-		  offset_int maxidx = wi::to_offset (max);
-		  offset_int neltsm1 = maxidx - minidx;
-		  if (neltsm1 > 0)
-		    /* MEMBER is an array with more than one element.  */
-		    return memsize;
-
-		  if (neltsm1 == 0)
-		    *sam = special_array_member::trail_1;
-		}
+	if (*sam == special_array_member::int_0)
+	  memsize = NULL_TREE;
 
-      /* For a reference to a zero- or one-element array member of a union
+      /* For a reference to a flexible array member of a union
 	 use the size of the union instead of the size of the member.  */
       if (TREE_CODE (argtype) == UNION_TYPE)
 	memsize = TYPE_SIZE_UNIT (argtype);
diff --git a/gcc/tree.h b/gcc/tree.h
index 4a19de1c94d..23223ca0c87 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -5553,22 +5553,26 @@  extern tree array_ref_low_bound (tree);
 /* Returns true if REF is an array reference, a component reference,
    or a memory reference to an array whose actual size might be larger
    than its upper bound implies.  */
-extern bool array_ref_flexible_size_p (tree);
+extern bool array_ref_flexible_size_p (tree, bool * = NULL);
 
 /* Return a tree representing the offset, in bytes, of the field referenced
    by EXP.  This does not include any offset in DECL_FIELD_BIT_OFFSET.  */
 extern tree component_ref_field_offset (tree);
 
-/* Describes a "special" array member due to which component_ref_size
-   returns null.  */
+/* Describes a "special" array member for a COMPONENT_REF.  */
 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.  */
+    trail_1,	/* Trailing array member with one element.  */
+    trail_n,	/* Trailing array member with two or more elements.  */
+    int_n	/* Interior array member with one or more elements.  */
   };
 
+/* Determines the special array member type for a COMPONENT_REF.  */
+extern special_array_member component_ref_sam_type (tree);
+
 /* Return the size of the member referenced by the COMPONENT_REF, using
    its initializer expression if necessary in order to determine the size
    of an initialized flexible array member.  The size might be zero for