Add warn_if_not_aligned attribute

Add warn_if_not_aligned attribute as well as  command line options:
-Wif-not-aligned and -Wpacked-not-aligned.

__attribute__((warn_if_not_aligned(N))) causes compiler to issue a
warning if the field in a struct or union is not aligned to N:

typedef unsigned long long __u64
  __attribute__((aligned(4),warn_if_not_aligned(8)));

struct foo
{
  int i1;
  int i2;
  __u64 x;
};

__u64 is aligned to 4 bytes.  But inside struct foo, __u64 should be
aligned at 8 bytes.  It is used to define struct foo in such a way that
struct foo has the same layout and x has the same alignment when __u64
is aligned at either 4 or 8 bytes.

Since struct foo is normally aligned to 4 bytes, a warning will be issued:

warning: alignment 4 of 'struct foo' is less than 8

Align struct foo to 8 bytes:

struct foo
{
  int i1;
  int i2;
  __u64 x;
} __attribute__((aligned(8)));

silences the warning.  It also warns the field with misaligned offset:

struct foo
{
  int i1;
  int i2;
  int i3;
  __u64 x;
} __attribute__((aligned(8)));

warning: 'x' offset 12 in 'struct foo' isn't aligned to 8

This warning is controlled by -Wif-not-aligned and is enabled by default.

When -Wpacked-not-aligned is used, the same warning is also issued for
the field with explicitly specified alignment in a packed struct or union:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

warning: alignment 1 of 'struct S' is less than 8

This warning is disabled by default and enabled by -Wall.

gcc/

	PR c/53037
	* print-tree.c (print_node): Support DECL_WARN_IF_NOT_ALIGN
	and TYPE_WARN_IF_NOT_ALIGN.
	* stor-layout.c (do_type_align): Merge DECL_WARN_IF_NOT_ALIGN.
	(handle_warn_if_not_align): New.
	(place_union_field): Call handle_warn_if_not_align.
	(place_field): Call handle_warn_if_not_align.  Copy
	TYPE_WARN_IF_NOT_ALIGN.
	(finish_builtin_struct): Copy TYPE_WARN_IF_NOT_ALIGN.
	(layout_type): Likewise.
	* tree-core.h (tree_type_common): Add warn_if_not_align.  Set
	spare to 18.
	(tree_decl_common): Add warn_if_not_align.
	* tree.c (build_range_type_1): Copy TYPE_WARN_IF_NOT_ALIGN.
	* tree.h (TYPE_WARN_IF_NOT_ALIGN): New.
	(SET_TYPE_WARN_IF_NOT_ALIGN): Likewise.
	(DECL_WARN_IF_NOT_ALIGN): Likewise.
	(SET_DECL_WARN_IF_NOT_ALIGN): Likewise.
	* doc/extend.texi: Document warn_if_not_aligned attribute.
	* doc/invoke.texi: Document -Wif-not-aligned and
	-Wpacked-not-aligned.

gcc/c-family/

	PR c/53037
	* c-attribs.c (handle_warn_if_not_aligned_attribute): New.
	(c_common_attribute_table): Add warn_if_not_aligned.
	(handle_aligned_attribute): Renamed to ...
	(common_handle_aligned_attribute): Remove argument, name, and add
	argument, warn_if_not_aligned.  Handle warn_if_not_aligned.
	(handle_aligned_attribute): New.
	* c.opt: Add -Wif-not-aligned and -Wpacked-not-aligned.

gcc/c/

	PR c/53037
	* c-decl.c (merge_decls): Also merge DECL_WARN_IF_NOT_ALIGN.
	(check_bitfield_type_and_width): Don't allow bit-field with
	warn_if_not_aligned type.

gcc/cp/

	PR c/53037
	* decl.c (duplicate_decls): Also merge DECL_WARN_IF_NOT_ALIGN.
	* decl2.c (grokbitfield): Don't allow bit-field with
	warn_if_not_aligned type.

gcc/testsuite/

	PR c/53037
	* c-c++-common/pr53037-5.c: New test.
	* g++.dg/pr53037-1.C: Likewise.
	* g++.dg/pr53037-2.C: Likewise.
	* g++.dg/pr53037-3.C: Likewise.
	* g++.dg/pr53037-4.C: Likewise.
	* gcc.dg/pr53037-1.c: Likewise.
	* gcc.dg/pr53037-2.c: Likewise.
	* gcc.dg/pr53037-3.c: Likewise.
	* gcc.dg/pr53037-4.c: Likewise.

From-SVN: r251180

1 files changed, 63 insertions, 8 deletions

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index ad2289c..5f79468 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -89,6 +89,8 @@ static tree handle_destructor_attribute (tree *, tree, tree, int, bool *);
 static tree handle_mode_attribute (tree *, tree, tree, int, bool *);
 static tree handle_section_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_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 *);
@@ -217,6 +219,9 @@ const struct attribute_spec c_common_attribute_table[] =
 			      handle_section_attribute, false },
   { "aligned",                0, 1, false, false, false,
 			      handle_aligned_attribute, false },
+  { "warn_if_not_aligned",    0, 1, false, false, false,
+			      handle_warn_if_not_aligned_attribute,
+			      false },
   { "weak",                   0, 0, true,  false, false,
 			      handle_weak_attribute, false },
   { "noplt",                   0, 0, true,  false, false,
@@ -1666,12 +1671,13 @@ check_cxx_fundamental_alignment_constraints (tree node,
   return !alignment_too_large_p;
 }
 
-/* Handle a "aligned" attribute; arguments as in
-   struct attribute_spec.handler.  */
+/* Common codes shared by handle_warn_if_not_aligned_attribute and
+   handle_aligned_attribute.  */
 
 static tree
-handle_aligned_attribute (tree *node, tree ARG_UNUSED (name), tree args,
-			  int flags, bool *no_add_attrs)
+common_handle_aligned_attribute (tree *node, tree args, int flags,
+				 bool *no_add_attrs,
+				 bool warn_if_not_aligned_p)
 {
   tree decl = NULL_TREE;
   tree *type = NULL;
@@ -1720,8 +1726,16 @@ handle_aligned_attribute (tree *node, tree ARG_UNUSED (name), tree args,
       else
 	*type = build_variant_type_copy (*type);
 
-      SET_TYPE_ALIGN (*type, (1U << i) * BITS_PER_UNIT);
-      TYPE_USER_ALIGN (*type) = 1;
+      if (warn_if_not_aligned_p)
+	{
+	  SET_TYPE_WARN_IF_NOT_ALIGN (*type, (1U << i) * BITS_PER_UNIT);
+	  warn_if_not_aligned_p = false;
+	}
+      else
+	{
+	  SET_TYPE_ALIGN (*type, (1U << i) * BITS_PER_UNIT);
+	  TYPE_USER_ALIGN (*type) = 1;
+	}
     }
   else if (! VAR_OR_FUNCTION_DECL_P (decl)
 	   && TREE_CODE (decl) != FIELD_DECL)
@@ -1754,13 +1768,54 @@ handle_aligned_attribute (tree *node, tree ARG_UNUSED (name), tree args,
     }
   else
     {
-      SET_DECL_ALIGN (decl, (1U << i) * BITS_PER_UNIT);
-      DECL_USER_ALIGN (decl) = 1;
+      if (warn_if_not_aligned_p)
+	{
+	  if (TREE_CODE (decl) == FIELD_DECL && !DECL_INITIAL (decl))
+	    {
+	      SET_DECL_WARN_IF_NOT_ALIGN (decl, (1U << i) * BITS_PER_UNIT);
+	      warn_if_not_aligned_p = false;
+	    }
+	}
+      else
+	{
+	  SET_DECL_ALIGN (decl, (1U << i) * BITS_PER_UNIT);
+	  DECL_USER_ALIGN (decl) = 1;
+	}
+    }
+
+  if (warn_if_not_aligned_p)
+    {
+      error ("%<warn_if_not_aligned%> may not be specified for %q+D",
+	     decl);
+      *no_add_attrs = true;
     }
 
   return NULL_TREE;
 }
 
+/* Handle a "aligned" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_aligned_attribute (tree *node, tree ARG_UNUSED (name), tree args,
+			  int flags, bool *no_add_attrs)
+{
+  return common_handle_aligned_attribute (node, args, flags,
+					 no_add_attrs, false);
+}
+
+/* Handle a "warn_if_not_aligned" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_warn_if_not_aligned_attribute (tree *node, tree ARG_UNUSED (name),
+				      tree args, int flags,
+				      bool *no_add_attrs)
+{
+  return common_handle_aligned_attribute (node, args, flags,
+					  no_add_attrs, true);
+}
+
 /* Handle a "weak" attribute; arguments as in
    struct attribute_spec.handler.  */