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

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

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On 04/05/17 23:02, Bernd Edlinger wrote:
> On 04/05/17 19:22, Bernd Edlinger wrote:
>> On 04/05/17 18:08, Jakub Jelinek wrote:
>>
>> Yes, exactly.  I really want to reach the deadline for gcc-7.
>> Fixing the name is certainly the most important first step,
>> and if everybody agrees on "typeless_storage", for the name
>> I can start with adjusting the name, and look into how
>> to use a spare type-flag that should be a mechanical change.
>>
>
> Jakub, I just renamed the attribute and reworked the patch
> as you suggested, reg-testing is not yet completed, but
> it looks good so far.  I also added a few more tests.
>

Aehm, sorry, actually I ran into a problem with the latest
patch version, where I tried to convert the TYPE_ATTRIBUTE
into a TYPE_FLAG here:
https://gcc.gnu.org/ml/gcc-patches/2017-04/msg00254.html

That is for instance with g++.dg/cpp1z/init-statement6.C an
internal error: "same canonical type node for different types"
happened, and I was not able to fix it immediately.

Although I would have liked it better this way, I think this
can be fixed separately, unless someone sees an obvious thinko
in the previous version.

So in the moment I restored the typeless_storage as an
ordinary TYPE_ATTRIBUTE, but at least it bootstraps and
causes no test regressions, as always with
languages=all,ada,go,obj-c++


Thanks
Bernd.

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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 (lookup_attribute ("typeless_storage", TYPE_ATTRIBUTES (t)))
+    return 0;
+
   /* Variant qualifiers don't affect the alias set, so get the main
      variant.  */
   t = TYPE_MAIN_VARIANT (t);
@@ -1234,7 +1238,9 @@ 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)
+		       && !lookup_attribute ("typeless_storage",
+					     TYPE_ATTRIBUTES (t)))
 		  {
 		    gcc_checking_assert (TYPE_STRUCTURAL_EQUALITY_P (t));
 		    t = TREE_TYPE (t);
Index: gcc/tree.c
===================================================================
--- gcc/tree.c	(revision 246678)
+++ gcc/tree.c	(working copy)
@@ -8041,7 +8041,8 @@ build_pointer_type_for_mode (tree to_type, machine
 
   /* If the pointed-to type has the may_alias attribute set, force
      a TYPE_REF_CAN_ALIAS_ALL pointer to be generated.  */
-  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
+  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type))
+      || lookup_attribute ("typeless_storage", TYPE_ATTRIBUTES (to_type)))
     can_alias_all = true;
 
   /* In some cases, languages will have things that aren't a POINTER_TYPE
@@ -8110,7 +8111,8 @@ build_reference_type_for_mode (tree to_type, machi
 
   /* If the pointed-to type has the may_alias attribute set, force
      a TYPE_REF_CAN_ALIAS_ALL pointer to be generated.  */
-  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
+  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type))
+      || lookup_attribute ("typeless_storage", TYPE_ATTRIBUTES (to_type)))
     can_alias_all = true;
 
   /* In some cases, languages will have things that aren't a REFERENCE_TYPE
Index: gcc/c-family/c-attribs.c
===================================================================
--- gcc/c-family/c-attribs.c	(revision 246678)
+++ gcc/c-family/c-attribs.c	(working copy)
@@ -265,6 +265,7 @@ 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, NULL, false },
   { "cleanup",		      1, 1, true, false, false,
 			      handle_cleanup_attribute, false },
   { "warn_unused_result",     0, 0, false, true, true,
Index: gcc/cp/class.c
===================================================================
--- gcc/cp/class.c	(revision 246678)
+++ gcc/cp/class.c	(working copy)
@@ -2083,7 +2082,8 @@ fixup_attribute_variants (tree t)
   tree attrs = TYPE_ATTRIBUTES (t);
   unsigned align = TYPE_ALIGN (t);
   bool user_align = TYPE_USER_ALIGN (t);
-  bool may_alias = lookup_attribute ("may_alias", attrs);
+  bool may_alias = lookup_attribute ("may_alias", attrs)
+		   || lookup_attribute ("typeless_storage", attrs);
 
   if (may_alias)
     fixup_may_alias (t);
@@ -7345,6 +7346,15 @@ 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)
+      && !lookup_attribute ("typeless_storage", TYPE_ATTRIBUTES (t)))
+    {
+      TYPE_ATTRIBUTES (t)
+	= tree_cons (get_identifier ("typeless_storage"),
+		     NULL_TREE, TYPE_ATTRIBUTES (t));
+      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 +14081,15 @@ 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;
+	      && !strcmp ("byte", TYPE_NAME_STRING (enumtype))
+	      && !lookup_attribute ("typeless_storage",
+				    TYPE_ATTRIBUTES (enumtype)))
+	    TYPE_ATTRIBUTES (enumtype)
+	      = tree_cons (get_identifier ("typeless_storage"),
+			   NULL_TREE, TYPE_ATTRIBUTES (enumtype));
 	}
       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)
@@ -8851,9 +8851,10 @@ lookup_template_class_1 (tree d1, tree arglist, tr
       if (OVERLOAD_TYPE_P (t)
 	  && !DECL_ALIAS_TEMPLATE_P (gen_tmpl))
 	{
-	  static const char *tags[] = {"abi_tag", "may_alias"};
+	  static const char *tags[] = {"abi_tag", "may_alias",
+				       "typeless_storage"};
 
-	  for (unsigned ix = 0; ix != 2; ix++)
+	  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;
+}
+
 \f
 #include "gt-cp-typeck2.h"
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);
+}