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

[Richard Biener <rguenther@suse.de>]

On Wed, 5 Apr 2017, 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.
> 
> I have changed the documentation as Richi suggested, but
> I am not too sure what to say here.

The alias.c changes are not sufficient.  I think what you want is
sth like

Index: gcc/alias.c
===================================================================
--- gcc/alias.c	(revision 246678)
+++ gcc/alias.c	(working copy)
@@ -136,6 +136,9 @@ struct GTY(()) alias_set_entry {
   bool is_pointer;
   /* Nonzero if is_pointer or if one of childs have has_pointer set.  */
   bool has_pointer;
+  /* Nonzero if we have a child serving as typeless storage (or are
+     such storage ourselves).  */
+  bool has_typeless_storage;
 
   /* The children of the alias set.  These are not just the immediate
      children, but, in fact, all descendants.  So, if we have:
@@ -419,7 +422,8 @@ alias_set_subset_of (alias_set_type set1
   /* Check if set1 is a subset of set2.  */
   ase2 = get_alias_set_entry (set2);
   if (ase2 != 0
-      && (ase2->has_zero_child
+      && (ase2->has_typeless_storage
+	  || ase2->has_zero_child
 	  || (ase2->children && ase2->children->get (set1))))
     return true;
 
@@ -825,6 +829,7 @@ init_alias_set_entry (alias_set_type set
   ase->has_zero_child = false;
   ase->is_pointer = false;
   ase->has_pointer = false;
+  ase->has_typeless_storage = false;
   gcc_checking_assert (!get_alias_set_entry (set));
   (*alias_sets)[set] = ase;
   return ase;
@@ -955,6 +960,7 @@ get_alias_set (tree t)
      Just be pragmatic here and make sure the array and its element
      type get the same alias set assigned.  */
   else if (TREE_CODE (t) == ARRAY_TYPE
+	   && ! TYPE_TYPELESS_STORAGE (t)
 	   && (!TYPE_NONALIASED_COMPONENT (t)
 	       || TYPE_STRUCTURAL_EQUALITY_P (t)))
     set = get_alias_set (TREE_TYPE (t));
@@ -1094,6 +1100,15 @@ get_alias_set (tree t)
 
   TYPE_ALIAS_SET (t) = set;
 
+  if (TREE_CODE (t) == ARRAY_TYPE
+      && TYPE_TYPELESS_STORAGE (t))
+    {
+      alias_set_entry *ase = get_alias_set_entry (set);
+      if (!ase)
+	ase = init_alias_set_entry (set);
+      ase->has_typeless_storage = true;
+    }
+
   /* If this is an aggregate type or a complex type, we must record any
      component aliasing information.  */
   if (AGGREGATE_TYPE_P (t) || TREE_CODE (t) == COMPLEX_TYPE)
@@ -1173,6 +1188,8 @@ record_alias_subset (alias_set_type supe
 	    superset_entry->has_zero_child = true;
           if (subset_entry->has_pointer)
 	    superset_entry->has_pointer = true;
+	  if (subset_entry->has_typeless_storage)
+	    superset_entry->has_typeless_storage = true;
 
 	  if (subset_entry->children)
 	    {


please also restrict TYPE_TYPELESS_STORAGE to ARRAY_TYPEs (otherwise
more complications will arise).

Index: gcc/cp/class.c
===================================================================
--- gcc/cp/class.c      (revision 246678)
+++ gcc/cp/class.c      (working copy)
@@ -2083,7 +2083,8 @@ 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 = TYPE_TYPELESS_STORAGE (t)
+                  || lookup_attribute ("may_alias", attrs);

   if (may_alias)
     fixup_may_alias (t);
@@ -7345,6 +7348,12 @@ 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);
...

I don't think you need all this given alias.c only looks at
TYPE_MAIN_VARIANTs.

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

not needed (but also not sufficient - you need to handle arrays of
byte somewhere).

@@ -1491,6 +1491,7 @@ struct GTY(()) tree_type_common {
   unsigned needs_constructing_flag : 1;
   unsigned transparent_aggr_flag : 1;
   unsigned restrict_flag : 1;
+  unsigned typeless_storage_flag : 1;
   unsigned contains_placeholder_bits : 2;

   ENUM_BITFIELD(machine_mode) mode : 8;

bits are grouped in groups of 8 bits, this breaks it.

@@ -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 (TYPE_TYPELESS_STORAGE (to_type)
+      || lookup_attribute ("may_alias", 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 (TYPE_TYPELESS_STORAGE (to_type)
+      || lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
     can_alias_all = true;

   /* In some cases, languages will have things that aren't a

not needed.

+/* Nonzero if the type should behave like a character type
+   with respect to aliasing sementics.  */
+#define TYPE_TYPELESS_STORAGE(NODE) \
+  (TYPE_CHECK (NODE)->type_common.typeless_storage_flag)

ARRAY_TYPE_CHECK (NODE)->

Richard.