Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

On 04/07/17 17:10, Richard Biener wrote:
> On April 7, 2017 3:37:30 PM GMT+02:00, Bernd Edlinger <bernd.edlinger@hotmail.de> wrote:
>> On 04/07/17 08:54, Richard Biener wrote:
>>> On Thu, 6 Apr 2017, Bernd Edlinger wrote:
>>>> I think get_alias_set(t) will return 0 for typeless_storage
>>>> types, and therefore has_zero_child will be set anyway.
>>>> I think both mean the same thing in the end, but it depends on
>>>> what typeless_storage should actually mean, and we have
>>>> not yet the same idea about it.
>>>
>>> But has_zero_child does not do what we like it to because otherwise
>>> in the PR using the char[] array member would have worked!
>>>
>>> has_zero_child doesn't do that on purpose of course, but this means
>>> returing alias-set zero for the typeless storage _member_ doesn't
>>> suffice.
>>>
>>
>> I see you have a certain idea how to solve the C++17 issue.
>> And yes, I apologize, if I tried to pee on your tree :)
>
> We do have the need to support this part of the C++ standard.  For other user code may_alias suffices and I see no reason to haste inventing sth new without a single convincing testcase.  GCC/Language extensions should not be added without a good reason.
>
> I didn't propose to expose the type flag to users at all.
>
> Richard.
>

Well, actually you did:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671#c100

But I won't argue if you prefer to do it as you like,
after all TBAA is your area.  So that is fine for me.

Attached is the latest version of my patch, I fixed
the issue with the type-flag conversion, and it is
already fully functional.


Bernd.

Attachment: changelog-typeless-storage.txt
Description: changelog-typeless-storage.txt

Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi	(revision 246678)
+++ gcc/doc/extend.texi	(working copy)
@@ -6656,6 +6656,36 @@
 @option{-fstrict-aliasing}, which is on by default at @option{-O2} or
 above.
 
+@item typeless_storage
+@cindex @code{typeless_storage} type attribute
+In the context of section 6.5 paragraph 6 of the C11 standard,
+an object of this type behaves as if it has no declared type.
+In the context of section 6.5 paragraph 7 of the C11 standard,
+an object or a pointer if this type behaves as if it were a
+character type.
+This attribute is similar to the @code{may_alias} attribute,
+except that it is not restricted to pointers.
+
+Example of use:
+
+@smallexample
+typedef int __attribute__((__typeless_storage__)) int_a;
+
+int
+main (void)
+@{
+  int_a a = 0x12345678;
+  short *b = (short *) &a;
+
+  b[1] = 0;
+
+  if (a == 0x12345678)
+    abort();
+
+  exit(0);
+@}
+@end smallexample
+
 @item packed
 @cindex @code{packed} type attribute
 This attribute, attached to @code{struct} or @code{union} type
Index: gcc/alias.c
===================================================================
--- gcc/alias.c	(revision 246678)
+++ gcc/alias.c	(working copy)
@@ -879,6 +879,10 @@ get_alias_set (tree t)
       t = TREE_TYPE (t);
     }
 
+  /* Honor the typeless_storage type attribute.  */
+  if (TYPE_TYPELESS_STORAGE (t))
+    return 0;
+
   /* Variant qualifiers don't affect the alias set, so get the main
      variant.  */
   t = TYPE_MAIN_VARIANT (t);
@@ -1234,7 +1238,8 @@ record_component_aliases (tree type)
 		/* VECTOR_TYPE and ARRAY_TYPE share the alias set with their
 		   element type and that type has to be normalized to void *,
 		   too, in the case it is a pointer. */
-		while (!canonical_type_used_p (t) && !POINTER_TYPE_P (t))
+		while (!canonical_type_used_p (t) && !POINTER_TYPE_P (t)
+		       && !TYPE_TYPELESS_STORAGE (t))
 		  {
 		    gcc_checking_assert (TYPE_STRUCTURAL_EQUALITY_P (t));
 		    t = TREE_TYPE (t);
Index: gcc/c-family/c-attribs.c
===================================================================
--- gcc/c-family/c-attribs.c	(revision 246678)
+++ gcc/c-family/c-attribs.c	(working copy)
@@ -113,6 +113,7 @@ static tree handle_vector_size_attribute (tree *,
 					  bool *);
 static tree handle_nonnull_attribute (tree *, tree, tree, int, bool *);
 static tree handle_nothrow_attribute (tree *, tree, tree, int, bool *);
+static tree handle_typeless_storage_attribute (tree *, tree, tree, int, bool *);
 static tree handle_cleanup_attribute (tree *, tree, tree, int, bool *);
 static tree handle_warn_unused_result_attribute (tree *, tree, tree, int,
 						 bool *);
@@ -265,6 +266,8 @@ const struct attribute_spec c_common_attribute_tab
   { "nothrow",                0, 0, true,  false, false,
 			      handle_nothrow_attribute, false },
   { "may_alias",	      0, 0, false, true, false, NULL, false },
+  { "typeless_storage",       0, 0, false, true, false,
+			      handle_typeless_storage_attribute, false },
   { "cleanup",		      1, 1, true, false, false,
 			      handle_cleanup_attribute, false },
   { "warn_unused_result",     0, 0, false, true, true,
@@ -2879,6 +2882,24 @@ handle_nothrow_attribute (tree *node, tree name, t
   return NULL_TREE;
 }
 
+/* Handle a "typeless_storage" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_typeless_storage_attribute (tree *node, tree name, tree ARG_UNUSED (args),
+				   int ARG_UNUSED (flags), bool *no_add_attrs)
+{
+  if (TYPE_P (*node))
+    TYPE_TYPELESS_STORAGE (*node) = 1;
+  else
+    {
+      warning (OPT_Wattributes, "%qE attribute ignored", name);
+      *no_add_attrs = true;
+    }
+
+  return NULL_TREE;
+}
+
 /* Handle a "cleanup" attribute; arguments as in
    struct attribute_spec.handler.  */
 
Index: gcc/cp/class.c
===================================================================
--- gcc/cp/class.c	(revision 246678)
+++ gcc/cp/class.c	(working copy)
@@ -2097,6 +2097,7 @@ fixup_attribute_variants (tree t)
       /* These are the two fields that check_qualified_type looks at and
 	 are affected by attributes.  */
       TYPE_ATTRIBUTES (variants) = attrs;
+      TYPE_TYPELESS_STORAGE (variants) = TYPE_TYPELESS_STORAGE (t);
       unsigned valign = align;
       if (TYPE_USER_ALIGN (variants))
 	valign = MAX (valign, TYPE_ALIGN (variants));
@@ -7345,6 +7346,12 @@ finish_struct_1 (tree t)
      the class or perform any other required target modifications.  */
   targetm.cxx.adjust_class_at_definition (t);
 
+  if (cxx_dialect >= cxx1z && cxx_type_contains_byte_buffer (t))
+    {
+      TYPE_TYPELESS_STORAGE (t) = 1;
+      fixup_attribute_variants (t);
+    }
+
   maybe_suppress_debug_info (t);
 
   if (flag_vtable_verify)
Index: gcc/cp/cp-tree.h
===================================================================
--- gcc/cp/cp-tree.h	(revision 246678)
+++ gcc/cp/cp-tree.h	(working copy)
@@ -6858,6 +6858,7 @@ extern tree finish_binary_fold_expr          (tree
 extern void require_complete_eh_spec_types	(tree, tree);
 extern void cxx_incomplete_type_diagnostic	(location_t, const_tree,
 						 const_tree, diagnostic_t);
+extern bool cxx_type_contains_byte_buffer	(tree);
 inline void
 cxx_incomplete_type_diagnostic (const_tree value, const_tree type,
 				diagnostic_t diag_kind)
Index: gcc/cp/decl.c
===================================================================
--- gcc/cp/decl.c	(revision 246678)
+++ gcc/cp/decl.c	(working copy)
@@ -14081,10 +14081,11 @@ start_enum (tree name, tree enumtype, tree underly
 	  enumtype = pushtag (name, enumtype, /*tag_scope=*/ts_current);
 
 	  /* std::byte aliases anything.  */
-	  if (enumtype != error_mark_node
+	  if (cxx_dialect >= cxx1z
+	      && enumtype != error_mark_node
 	      && TYPE_CONTEXT (enumtype) == std_node
 	      && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
-	    TYPE_ALIAS_SET (enumtype) = 0;
+	    TYPE_TYPELESS_STORAGE (enumtype) = 1;
 	}
       else
 	  enumtype = xref_tag (enum_type, name, /*tag_scope=*/ts_current,
Index: gcc/cp/pt.c
===================================================================
--- gcc/cp/pt.c	(revision 246678)
+++ gcc/cp/pt.c	(working copy)
@@ -8853,7 +8853,8 @@ lookup_template_class_1 (tree d1, tree arglist, tr
 	{
 	  static const char *tags[] = {"abi_tag", "may_alias"};
 
-	  for (unsigned ix = 0; ix != 2; ix++)
+	  TYPE_TYPELESS_STORAGE (t) |= TYPE_TYPELESS_STORAGE (template_type);
+	  for (unsigned ix = 0; ix < sizeof (tags) / sizeof (tags[0]); ix++)
 	    {
 	      tree attributes
 		= lookup_attribute (tags[ix], TYPE_ATTRIBUTES (template_type));
Index: gcc/cp/typeck2.c
===================================================================
--- gcc/cp/typeck2.c	(revision 246678)
+++ gcc/cp/typeck2.c	(working copy)
@@ -2234,5 +2234,29 @@ require_complete_eh_spec_types (tree fntype, tree
     }
 }
 
+/* True iff type either is or contains a byte buffer (which can be used for
+   storing any trivially copyable type).  */
+
+bool
+cxx_type_contains_byte_buffer (tree type)
+{
+  if (TREE_CODE (type) == ARRAY_TYPE
+      && (cxx_type_contains_byte_buffer (TREE_TYPE (type))
+	  || TREE_TYPE (type) == unsigned_char_type_node
+	  || (TREE_CODE (TREE_TYPE (type)) == ENUMERAL_TYPE
+	      && TYPE_CONTEXT (TREE_TYPE (type)) == std_node
+	      && !strcmp ("byte", TYPE_NAME_STRING (TREE_TYPE (type))))))
+    return true;
+
+  if (CLASS_TYPE_P (type))
+    for (tree field = next_initializable_field (TYPE_FIELDS (type));
+	 field;
+	 field = next_initializable_field (DECL_CHAIN (field)))
+      if (cxx_type_contains_byte_buffer (TREE_TYPE (field)))
+	return true;
+
+  return false;
+}
+
 
 #include "gt-cp-typeck2.h"
Index: gcc/lto/lto.c
===================================================================
--- gcc/lto/lto.c	(revision 246678)
+++ gcc/lto/lto.c	(working copy)
@@ -1164,6 +1164,7 @@ compare_tree_sccs_1 (tree t1, tree t2, tree **map)
 	compare_values (TYPE_NONALIASED_COMPONENT);
       compare_values (TYPE_PACKED);
       compare_values (TYPE_RESTRICT);
+      compare_values (TYPE_TYPELESS_STORAGE);
       compare_values (TYPE_USER_ALIGN);
       compare_values (TYPE_READONLY);
       compare_values (TYPE_PRECISION);
Index: gcc/lto-streamer-out.c
===================================================================
--- gcc/lto-streamer-out.c	(revision 246678)
+++ gcc/lto-streamer-out.c	(working copy)
@@ -1134,6 +1134,7 @@ hash_tree (struct streamer_tree_cache_d *cache, ha
       hstate.add_flag (TYPE_NEEDS_CONSTRUCTING (t));
       hstate.add_flag (TYPE_PACKED (t));
       hstate.add_flag (TYPE_RESTRICT (t));
+      hstate.add_flag (TYPE_TYPELESS_STORAGE (t));
       hstate.add_flag (TYPE_USER_ALIGN (t));
       hstate.add_flag (TYPE_READONLY (t));
       if (RECORD_OR_UNION_TYPE_P (t))
Index: gcc/tree-core.h
===================================================================
--- gcc/tree-core.h	(revision 246678)
+++ gcc/tree-core.h	(working copy)
@@ -1216,6 +1216,9 @@ struct GTY(()) tree_base {
        DECL_NONALIASED in
 	  VAR_DECL
 
+       TYPE_TYPELESS_STORAGE in
+	  all types
+
    deprecated_flag:
 
        TREE_DEPRECATED in
Index: gcc/tree-streamer-in.c
===================================================================
--- gcc/tree-streamer-in.c	(revision 246678)
+++ gcc/tree-streamer-in.c	(working copy)
@@ -367,6 +367,7 @@ unpack_ts_type_common_value_fields (struct bitpack
   TYPE_NEEDS_CONSTRUCTING (expr) = (unsigned) bp_unpack_value (bp, 1);
   TYPE_PACKED (expr) = (unsigned) bp_unpack_value (bp, 1);
   TYPE_RESTRICT (expr) = (unsigned) bp_unpack_value (bp, 1);
+  TYPE_TYPELESS_STORAGE (expr) = (unsigned) bp_unpack_value (bp, 1);
   TYPE_USER_ALIGN (expr) = (unsigned) bp_unpack_value (bp, 1);
   TYPE_READONLY (expr) = (unsigned) bp_unpack_value (bp, 1);
   if (RECORD_OR_UNION_TYPE_P (expr))
Index: gcc/tree-streamer-out.c
===================================================================
--- gcc/tree-streamer-out.c	(revision 246678)
+++ gcc/tree-streamer-out.c	(working copy)
@@ -316,6 +316,7 @@ pack_ts_type_common_value_fields (struct bitpack_d
   bp_pack_value (bp, TYPE_NEEDS_CONSTRUCTING (expr), 1);
   bp_pack_value (bp, TYPE_PACKED (expr), 1);
   bp_pack_value (bp, TYPE_RESTRICT (expr), 1);
+  bp_pack_value (bp, TYPE_TYPELESS_STORAGE (expr), 1);
   bp_pack_value (bp, TYPE_USER_ALIGN (expr), 1);
   bp_pack_value (bp, TYPE_READONLY (expr), 1);
   /* We used to stream TYPE_ALIAS_SET == 0 information to let frontends mark
Index: gcc/tree.h
===================================================================
--- gcc/tree.h	(revision 246678)
+++ gcc/tree.h	(working copy)
@@ -1944,6 +1944,11 @@ extern machine_mode element_mode (const_tree t);
    the term.  */
 #define TYPE_RESTRICT(NODE) (TYPE_CHECK (NODE)->type_common.restrict_flag)
 
+/* Nonzero if the type should behave like a character type
+   with respect to aliasing sementics.  */
+#define TYPE_TYPELESS_STORAGE(NODE) \
+  (TYPE_CHECK (NODE)->base.nothrow_flag)
+
 /* If nonzero, type's name shouldn't be emitted into debug info.  */
 #define TYPE_NAMELESS(NODE) (TYPE_CHECK (NODE)->base.u.bits.nameless_flag)
 
Index: gcc/testsuite/c-c++-common/attr-typeless-storage-1.c
===================================================================
--- gcc/testsuite/c-c++-common/attr-typeless-storage-1.c	(revision 0)
+++ gcc/testsuite/c-c++-common/attr-typeless-storage-1.c	(working copy)
@@ -0,0 +1,29 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall" } */
+
+typedef int T __attribute__((typeless_storage));
+
+extern T t, v;
+extern T *p;
+extern int *p;
+
+extern int *p2;
+extern T *p2;
+
+void fn1 (T);
+void fn1 (int);
+
+void fn2 (int);
+void fn2 (T);
+
+/* Ensure that the composite types have typeless_storage.  */
+void
+f (long *i)
+{
+  *i = *(__typeof (*p) *) &p;
+  asm ("" : : "r" (*p));
+  *i = *(__typeof (*p2) *) &p2;
+  asm ("" : : "r" (*p2));
+  t = v;
+  asm ("" : : "r" (t));
+}
Index: gcc/testsuite/c-c++-common/attr-typeless-storage-2.c
===================================================================
--- gcc/testsuite/c-c++-common/attr-typeless-storage-2.c	(revision 0)
+++ gcc/testsuite/c-c++-common/attr-typeless-storage-2.c	(working copy)
@@ -0,0 +1,17 @@
+/* We used to reject this because types differentiating only in
+   TYPE_REF_CAN_ALIAS_ALL were deemed incompatible.  */
+/* { dg-do compile } */
+
+struct sockaddr;
+struct sockaddr *f (void);
+
+struct __attribute__((typeless_storage)) sockaddr { int j; };
+struct sockaddr *
+f (void)
+{
+  return
+#ifndef __cplusplus
+    (void *)
+#endif
+    0;
+}
Index: gcc/testsuite/gcc.c-torture/execute/typeless-storage-1.c
===================================================================
--- gcc/testsuite/gcc.c-torture/execute/typeless-storage-1.c	(revision 0)
+++ gcc/testsuite/gcc.c-torture/execute/typeless-storage-1.c	(working copy)
@@ -0,0 +1,20 @@
+/* Tests that the typeless_storage attribute works as expected.  */
+ 
+extern void abort(void);
+extern void exit(int);
+
+typedef int __attribute__((__typeless_storage__)) int_a;
+
+int
+main (void)
+{
+  int_a a = 0x12345678;
+  short *b = (short*) &a;
+
+  b[1] = 0;
+
+  if (a == 0x12345678)
+    abort();
+
+  exit(0);
+}