c++: Handle __builtin_clear_padding on non-trivially-copyable types [PR102586]

On Fri, Feb 11, 2022 at 07:55:50PM +0100, Jakub Jelinek via Gcc-patches wrote:
> Something like the https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c16
> will still be needed with adjusted testcase from
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15 such that
> __builtin_clear_padding is called directly on var addresses rather than
> in separate functions.

Here is an updated version of the
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15
patch which uses FIELD_DECL in the langhook instead of its TREE_TYPE,
and the testcases have been adjusted for the builtin accepting
pointers to non-trivially-copyable types only if it is address of a
declaration.

2022-04-07  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102586
gcc/
	* langhooks.h (struct lang_hooks_for_types): Add classtype_as_base
	langhook.
	* langhooks-def.h (LANG_HOOKS_CLASSTYPE_AS_BASE): Define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Add it.
	* gimple-fold.cc (clear_padding_type): Use ftype instead of
	TREE_TYPE (field) some more.  For artificial FIELD_DECLs without
	name try the lang_hooks.types.classtype_as_base langhook and
	if it returns non-NULL, use that instead of ftype for recursive call.
gcc/cp/
	* cp-objcp-common.h (cp_classtype_as_base): Declare.
	(LANG_HOOKS_CLASSTYPE_AS_BASE): Redefine.
	* cp-objcp-common.cc (cp_classtype_as_base): New function.
gcc/testsuite/
	* g++.dg/torture/builtin-clear-padding-5.C: New test.
	* g++.dg/cpp2a/builtin-clear-padding1.C (bar): Uncomment one
	call that is now accepted.

7 files changed, 76 insertions, 5 deletions

diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
index 8f74802..0b70d55 100644
--- a/gcc/cp/cp-objcp-common.cc
+++ b/gcc/cp/cp-objcp-common.cc
@@ -280,6 +280,22 @@ cp_unit_size_without_reusable_padding (tree type)
   return TYPE_SIZE_UNIT (type);
 }
 
+/* Returns type corresponding to FIELD's type when FIELD is a C++ base class
+   i.e., type without virtual base classes or tail padding.  Returns
+   NULL_TREE otherwise.  */
+
+tree
+cp_classtype_as_base (const_tree field)
+{
+  if (DECL_FIELD_IS_BASE (field))
+    {
+      tree type = TREE_TYPE (field);
+      if (TYPE_LANG_SPECIFIC (type))
+	return CLASSTYPE_AS_BASE (type);
+    }
+  return NULL_TREE;
+}
+
 /* Stubs to keep c-opts.cc happy.  */
 void
 push_file_scope (void)
diff --git a/gcc/cp/cp-objcp-common.h b/gcc/cp/cp-objcp-common.h
index 4c13731..3c04e5c 100644
--- a/gcc/cp/cp-objcp-common.h
+++ b/gcc/cp/cp-objcp-common.h
@@ -31,6 +31,7 @@ extern int cp_decl_dwarf_attribute (const_tree, int);
 extern int cp_type_dwarf_attribute (const_tree, int);
 extern void cp_common_init_ts (void);
 extern tree cp_unit_size_without_reusable_padding (tree);
+extern tree cp_classtype_as_base (const_tree);
 extern tree cp_get_global_decls ();
 extern tree cp_pushdecl (tree);
 extern void cp_register_dumps (gcc::dump_manager *);
@@ -167,6 +168,8 @@ extern tree cxx_simulate_record_decl (location_t, const char *,
 #define LANG_HOOKS_TYPE_DWARF_ATTRIBUTE cp_type_dwarf_attribute
 #undef LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING
 #define LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING cp_unit_size_without_reusable_padding
+#undef LANG_HOOKS_CLASSTYPE_AS_BASE
+#define LANG_HOOKS_CLASSTYPE_AS_BASE cp_classtype_as_base
 
 #undef LANG_HOOKS_OMP_PREDETERMINED_SHARING
 #define LANG_HOOKS_OMP_PREDETERMINED_SHARING cxx_omp_predetermined_sharing
diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index 97880a5..ac22adf 100644
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -4747,7 +4747,7 @@ clear_padding_type (clear_padding_struct *buf, tree type,
 				      "have well defined padding bits for %qs",
 			    field, "__builtin_clear_padding");
 	      }
-	    else if (is_empty_type (TREE_TYPE (field)))
+	    else if (is_empty_type (ftype))
 	      continue;
 	    else
 	      {
@@ -4758,8 +4758,9 @@ clear_padding_type (clear_padding_struct *buf, tree type,
 		gcc_assert (pos >= 0 && fldsz >= 0 && pos >= cur_pos);
 		clear_padding_add_padding (buf, pos - cur_pos);
 		cur_pos = pos;
-		clear_padding_type (buf, TREE_TYPE (field),
-				    fldsz, for_auto_init);
+		if (tree asbase = lang_hooks.types.classtype_as_base (field))
+		  ftype = asbase;
+		clear_padding_type (buf, ftype, fldsz, for_auto_init);
 		cur_pos += fldsz;
 	      }
 	  }
diff --git a/gcc/langhooks-def.h b/gcc/langhooks-def.h
index 49c8f58..e226395 100644
--- a/gcc/langhooks-def.h
+++ b/gcc/langhooks-def.h
@@ -216,6 +216,7 @@ extern tree lhd_unit_size_without_reusable_padding (tree);
 #define LANG_HOOKS_GET_FIXED_POINT_TYPE_INFO NULL
 #define LANG_HOOKS_TYPE_DWARF_ATTRIBUTE	lhd_type_dwarf_attribute
 #define LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING lhd_unit_size_without_reusable_padding
+#define LANG_HOOKS_CLASSTYPE_AS_BASE	hook_tree_const_tree_null
 
 #define LANG_HOOKS_FOR_TYPES_INITIALIZER { \
   LANG_HOOKS_MAKE_TYPE, \
@@ -243,7 +244,8 @@ extern tree lhd_unit_size_without_reusable_padding (tree);
   LANG_HOOKS_GET_DEBUG_TYPE, \
   LANG_HOOKS_GET_FIXED_POINT_TYPE_INFO, \
   LANG_HOOKS_TYPE_DWARF_ATTRIBUTE, \
-  LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING \
+  LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING, \
+  LANG_HOOKS_CLASSTYPE_AS_BASE \
 }
 
 /* Declaration hooks.  */
diff --git a/gcc/langhooks.h b/gcc/langhooks.h
index 0eec1b0..4731f08 100644
--- a/gcc/langhooks.h
+++ b/gcc/langhooks.h
@@ -188,6 +188,11 @@ struct lang_hooks_for_types
   /* Returns a tree for the unit size of T excluding tail padding that
      might be used by objects inheriting from T.  */
   tree (*unit_size_without_reusable_padding) (tree);
+
+  /* Returns type corresponding to FIELD's type when FIELD is a C++ base class
+     i.e., type without virtual base classes or tail padding.  Returns
+     NULL_TREE otherwise.  */
+  tree (*classtype_as_base) (const_tree);
 };
 
 /* Language hooks related to decls and the symbol table.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C b/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C
index 7b1282e..ed74ebf 100644
--- a/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C
+++ b/gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C
@@ -43,7 +43,7 @@ bar ()
   __builtin_clear_padding (&c2);
   __builtin_clear_padding (&c3);
   __builtin_clear_padding (&c4);
-//  __builtin_clear_padding (&c5);
+  __builtin_clear_padding (&c5);
   __builtin_clear_padding (&c6);
   __builtin_clear_padding (&c7);
   __builtin_clear_padding (&c8);
diff --git a/gcc/testsuite/g++.dg/torture/builtin-clear-padding-5.C b/gcc/testsuite/g++.dg/torture/builtin-clear-padding-5.C
new file mode 100644
index 0000000..b5f0191
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/builtin-clear-padding-5.C
@@ -0,0 +1,44 @@
+// PR tree-optimization/102586
+// { dg-options "-Wno-inaccessible-base" }
+
+struct C0 {};
+struct C1 {};
+struct C2 : C1, virtual C0 {};
+struct C3 : virtual C2, C1 { virtual int foo () { return 1; } };
+struct C4 : virtual C3, C1 { virtual int foo () { return 2; } };
+struct C5 : C4 { virtual int foo () { return 3; } };
+struct C6 { char c; };
+struct C7 : virtual C6, virtual C3, C1 { virtual int foo () { return 4; } };
+struct C8 : C7 { virtual int foo () { return 5; } };
+
+__attribute__((noipa)) int
+bar (C5 *p)
+{
+  return p->foo ();
+}
+
+__attribute__((noipa)) int
+baz (C3 *p)
+{
+  return p->foo ();
+}
+
+__attribute__((noipa)) int
+qux (C8 *p)
+{
+  return p->foo ();
+}
+
+int
+main ()
+{
+  C5 c5;
+  C8 c8;
+  c8.c = 42;
+  __builtin_clear_padding (&c5);
+  __builtin_clear_padding (&c8);
+  if (bar (&c5) != 3 || baz (&c5) != 3)
+    __builtin_abort ();
+  if (qux (&c8) != 5 || baz (&c8) != 5 || c8.c != 42)
+    __builtin_abort ();
+}