c++: Support &typeid(x) == &typeid(y) and typeid(x) == typeid(y) in constant evaluati...

If the tinfo vars are emitted in the current TU, they are emitted at the end
of the compilation, and for some types they are exported from
libstdc++/libsupc++ and not emitted in the current TU at all.

The following patch allows constant folding of comparisons of typeid
addresses and makes it possible to implement P1328R1 - making type_info
operator== constexpr (Jonathan has a patch for that).

As mentioned in the PR, the varpool/middle-end code is trying to be
conservative with address comparisons of different vars if those vars
don't bind locally, because of possible aliases in other TUs etc.
and so while match.pd folds &typeid(int) == &typeid(int) because
it is equality comparison with the same operands, for different typeids
it doesn't fold it.

On Wed, Dec 08, 2021 at 08:53:03AM -0500, Jason Merrill wrote:
> Would it make sense to assume that DECL_ARTIFICIAL variables can't be
> aliases?  If not, could we have some way of marking a variable as
> non-aliasing, perhaps an attribute?

I think DECL_ARTIFICIAL vars generally can overlap.

The following patch adds a GCC internal attribute "non overlapping"
and uses it in symtab_node::equal_address_to.
Not sure what plans has Honza in that area and whether it would be useful
to make the attribute public and let users assert that some variable will
never overlap with other variables, won't have aliases etc.

> During constant evaluation, the operator== could compare the type_info
> address instead of the __name address, reducing this to the previous
> problem.

Ah, indeed, good idea.  FYI, clang++ seems to constant fold
&typeid(x) != &typeid(y) already, so Jonathan could use it even for
clang++ in the constexpr operator==.  But it folds even
extern int &a, &b;
constexpr bool c = &a != &b;
regardless of whether some other TU has
int a;
int b __attribute__((alias (a));
or not.

2022-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103600
gcc/
	* symtab.c (symtab_node::equal_address_to): Return 0 if one of
	VAR_DECLs has "non overlapping" attribute and rs1 != rs2.
gcc/c-family/
	* c-attribs.c (handle_non_overlapping_attribute): New function.
	(c_common_attribute_table): Add "non overlapping" attribute.
gcc/cp/
	* rtti.c (get_tinfo_decl_direct): Add "non overlapping" attribute
	to DECL_TINFO_P VAR_DECLs.
gcc/testsuite/
	* g++.dg/cpp0x/constexpr-typeid2.C: New test.

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index bd420369a9180262145b37e96013bb8fa360cd20..dbb892e0ec6dbd6cc3335a70b3374697da0256b6 100644 (file)
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -159,6 +159,7 @@ static tree handle_omp_declare_variant_attribute (tree *, tree, tree, int,
 static tree handle_simd_attribute (tree *, tree, tree, int, bool *);
 static tree handle_omp_declare_target_attribute (tree *, tree, tree, int,
                                                 bool *);
+static tree handle_non_overlapping_attribute (tree *, tree, tree, int, bool *);
 static tree handle_designated_init_attribute (tree *, tree, tree, int, bool *);
 static tree handle_patchable_function_entry_attribute (tree *, tree, tree,
                                                       int, bool *);
@@ -512,6 +513,8 @@ const struct attribute_spec c_common_attribute_table[] =
                              handle_omp_declare_target_attribute, NULL },
   { "omp declare target block", 0, 0, true, false, false, false,
                              handle_omp_declare_target_attribute, NULL },
+  { "non overlapping",       0, 0, true, false, false, false,
+                             handle_non_overlapping_attribute, NULL },
   { "alloc_align",           1, 1, false, true, true, false,
                              handle_alloc_align_attribute,
                              attr_alloc_exclusions },
@@ -3765,6 +3768,15 @@ handle_omp_declare_target_attribute (tree *, tree, tree, int, bool *)
   return NULL_TREE;
 }
 
+/* Handle an "non overlapping" attribute; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_non_overlapping_attribute (tree *, tree, tree, int, bool *)
+{
+  return NULL_TREE;
+}
+
 /* Handle a "returns_twice" attribute; arguments as in
    struct attribute_spec.handler.  */
 

diff --git a/gcc/cp/rtti.c b/gcc/cp/rtti.c
index 30ae18a0e5f32f441d54d373384c3e29e16eaafa..a4dedc31f0ab632181bbf86cd6e4223eebf2a697 100644 (file)
--- a/gcc/cp/rtti.c
+++ b/gcc/cp/rtti.c
@@ -475,6 +475,15 @@ get_tinfo_decl_direct (tree type, tree name, int pseudo_ix)
       DECL_IGNORED_P (d) = 1;
       TREE_READONLY (d) = 1;
       TREE_STATIC (d) = 1;
+      /* Tell equal_address_to that different tinfo decls never
+        overlap.  */
+      if (vec_safe_is_empty (unemitted_tinfo_decls))
+       DECL_ATTRIBUTES (d)
+         = build_tree_list (get_identifier ("non overlapping"),
+                            NULL_TREE);
+      else
+       DECL_ATTRIBUTES (d)
+         = DECL_ATTRIBUTES ((*unemitted_tinfo_decls)[0]);
 
       /* Mark the variable as undefined -- but remember that we can
         define it later if we need to do so.  */

diff --git a/gcc/symtab.c b/gcc/symtab.c
index d037fe68e7e5612507ef7d2fd0e7dba97e93df73..f9e4571d396c628b00b21685ae6fd1a79d391fcc 100644 (file)
--- a/gcc/symtab.c
+++ b/gcc/symtab.c
@@ -2276,6 +2276,14 @@ symtab_node::equal_address_to (symtab_node *s2, bool memory_accessed)
       return 0;
     }
 
+  /* If the FE tells us at least one of the decls will never be aliased nor
+     overlapping with other vars in some other way, return 0.  */
+  if (VAR_P (decl)
+      && rs1 != rs2
+      && (lookup_attribute ("non overlapping", DECL_ATTRIBUTES (decl))
+         || lookup_attribute ("non overlapping", DECL_ATTRIBUTES (s2->decl))))
+    return 0;
+
   /* TODO: Alias oracle basically assume that addresses of global variables
      are different unless they are declared as alias of one to another while
      the code folding comparisons doesn't.

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-typeid2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-typeid2.C
new file mode 100644 (file)
index 0000000..78c6b8e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-typeid2.C
@@ -0,0 +1,14 @@
+// PR c++/103600
+// { dg-do compile { target c++11 } }
+
+#include <typeinfo>
+
+struct S { int i; };
+namespace {
+  struct T { int i; };
+};
+constexpr bool a = &typeid (int) == &typeid (int);
+constexpr bool b = &typeid (int) == &typeid (long);
+constexpr bool c = &typeid (double) != &typeid (int);
+constexpr bool d = &typeid (S) != &typeid (T);
+static_assert (a && !b && c && d, "");