Hi,
this is related to the P1 codegen bug PR79671:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
Therefore I would like to add this now although
the trunk is already at in stage 4.
I propose to add a new type attribute always_alias that
works like may_alias but unlike may_alias it should
also make instances alias anything. It is also exposed
in C/C++ as __attribute__((always_alias)).
For C++17 the FE automatically introduces this attribute
at std::byte and all class/struct/union which contain
either a std::byte array or an unsigned char array.
Note that the patch [1] needs to be applied as well,
because otherwise the new attribute will trip several
failures in the libstdc++ testsuite.
Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
Is it OK for trunk?
Thanks
Bernd.
[1] https://gcc.gnu.org/ml/gcc-patches/2017-04/msg00189.html
gcc
2017-04-05 Bernd Edlinger <bernd.edlin...@hotmail.de>
* doc/extend.texi: Document the always_alias type attribute.
* alias.c (get_alias_set): Honor the always_alias attribute.
(record_component_aliases): Don't ignore the always_alias attribute.
* tree.c (build_pointer_type_for_mode,
build_reference_type_for_mode): Add the always_alias attribute.
gcc/c-family
2017-04-05 Bernd Edlinger <bernd.edlin...@hotmail.de>
* c-attribs.c (c_common_attribute_tab): Add the always_alias attribute.
gcc/cp
2017-04-05 Bernd Edlinger <bernd.edlin...@hotmail.de>
* class.c (fixup_attribute_variants): Add the always_alias attribute.
(finish_struct_1): Add an always_alias attribute if required by C++17.
* decl.c (start_enum): Likewise.
* pt.c (lookup_template_class_1): Add the always_alias attribute.
* typeck2.c (cxx_type_contains_byte_buffer): New function.
* cp-tree.h (cxx_type_contains_byte_buffer): Declare.
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 always_alias type attribute. */
+ if (lookup_attribute ("always_alias", 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 ("always_alias",
+ TYPE_ATTRIBUTES (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)
@@ -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 },
+ { "always_alias", 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 ("always_alias", 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 ("always_alias", TYPE_ATTRIBUTES (t)))
+ {
+ TYPE_ATTRIBUTES (t)
+ = tree_cons (get_identifier ("always_alias"),
+ 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 ("always_alias",
+ TYPE_ATTRIBUTES (enumtype)))
+ TYPE_ATTRIBUTES (enumtype)
+ = tree_cons (get_identifier ("always_alias"),
+ 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 +8851,9 @@ 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", "always_alias"};
- 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;
+}
+
#include "gt-cp-typeck2.h"
Index: gcc/doc/extend.texi
===================================================================
--- gcc/doc/extend.texi (revision 246678)
+++ gcc/doc/extend.texi (working copy)
@@ -6656,6 +6656,11 @@ declaration, the above program would abort when co
@option{-fstrict-aliasing}, which is on by default at @option{-O2} or
above.
+@item always_alias
+@cindex @code{always_alias} type attribute
+Same as @code{may_alias}, but additionally applies to instances of
+types with this attribute.
+
@item packed
@cindex @code{packed} type attribute
This attribute, attached to @code{struct} or @code{union} type
Index: gcc/testsuite/c-c++-common/attr-always-alias-1.c
===================================================================
--- gcc/testsuite/c-c++-common/attr-always-alias-1.c (revision 0)
+++ gcc/testsuite/c-c++-common/attr-always-alias-1.c (working copy)
@@ -0,0 +1,29 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall" } */
+
+typedef int T __attribute__((always_alias));
+
+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 always_alias. */
+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/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 ("always_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 (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type))
+ || lookup_attribute ("always_alias", TYPE_ATTRIBUTES (to_type)))
can_alias_all = true;
/* In some cases, languages will have things that aren't a REFERENCE_TYPE
