c++: Implement P0466R5 __cpp_lib_is_layout_compatible compiler helpers [PR101539]

The following patch implements __is_layout_compatible trait and
__builtin_is_corresponding_member helper function for the
std::is_corresponding_member template function.

As the current definition of layout compatible type has various problems,
which result e.g. in corresponding members in layout compatible types having
different member offsets, the patch anticipates some changes to the C++
standard:
1) class or enumeral types aren't layout compatible if they have different
   alignment or size
2) if two members have different offsets, they can't be corresponding members
   ([[no_unique_address]] with empty types can change that, or alignas
   on the member decls)
3) in unions, bitfields can't correspond to non-unions, or bitfields can't
   correspond to bitfields with different widths, or members with
   [[no_unique_address]] can't correspond to members without that attribute

__builtin_is_corresponding_member for anonymous structs (GCC extension) will
recurse into the anonymous structs.  For anonymous unions it will emit
a sorry if it can't prove such member types can't appear in the
anonymous unions or anonymous aggregates in that union, because
corresponding member is defined only using common initial sequence which is
only defined for std-layout non-union class types and so I have no idea what
to do otherwise in that case.

2021-08-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101539
gcc/c-family/
	* c-common.h (enum rid): Add RID_IS_LAYOUT_COMPATIBLE.
	* c-common.c (c_common_reswords): Add __is_layout_compatible.
gcc/cp/
	* cp-tree.h (enum cp_trait_kind): Add CPTK_IS_LAYOUT_COMPATIBLE.
	(enum cp_built_in_function): Add CP_BUILT_IN_IS_CORRESPONDING_MEMBER.
	(fold_builtin_is_corresponding_member, next_common_initial_seqence,
	layout_compatible_type_p): Declare.
	* parser.c (cp_parser_primary_expression): Handle
	RID_IS_LAYOUT_COMPATIBLE.
	(cp_parser_trait_expr): Likewise.
	* cp-objcp-common.c (names_builtin_p): Likewise.
	* constraint.cc (diagnose_trait_expr): Handle
	CPTK_IS_LAYOUT_COMPATIBLE.
	* decl.c (cxx_init_decl_processing): Register
	__builtin_is_corresponding_member builtin.
	* constexpr.c (cxx_eval_builtin_function_call): Handle
	CP_BUILT_IN_IS_CORRESPONDING_MEMBER builtin.
	* semantics.c (is_corresponding_member_union,
	is_corresponding_member_aggr, fold_builtin_is_corresponding_member):
	New functions.
	(trait_expr_value): Handle CPTK_IS_LAYOUT_COMPATIBLE.
	(finish_trait_expr): Likewise.
	* typeck.c (next_common_initial_seqence, layout_compatible_type_p):
	New functions.
	* cp-gimplify.c (cp_gimplify_expr): Fold
	CP_BUILT_IN_IS_CORRESPONDING_MEMBER.
	(cp_fold): Likewise.
	* tree.c (builtin_valid_in_constant_expr_p): Handle
	CP_BUILT_IN_IS_CORRESPONDING_MEMBER.
	* cxx-pretty-print.c (pp_cxx_trait_expression): Handle
	CPTK_IS_LAYOUT_COMPATIBLE.
	* class.c (remove_zero_width_bit_fields): Remove.
	(layout_class_type): Don't call it.
gcc/testsuite/
	* g++.dg/cpp2a/is-corresponding-member1.C: New test.
	* g++.dg/cpp2a/is-corresponding-member2.C: New test.
	* g++.dg/cpp2a/is-corresponding-member3.C: New test.
	* g++.dg/cpp2a/is-corresponding-member4.C: New test.
	* g++.dg/cpp2a/is-corresponding-member5.C: New test.
	* g++.dg/cpp2a/is-corresponding-member6.C: New test.
	* g++.dg/cpp2a/is-corresponding-member7.C: New test.
	* g++.dg/cpp2a/is-corresponding-member8.C: New test.
	* g++.dg/cpp2a/is-layout-compatible1.C: New test.
	* g++.dg/cpp2a/is-layout-compatible2.C: New test.
	* g++.dg/cpp2a/is-layout-compatible3.C: New test.

1 files changed, 268 insertions, 0 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 0198d2d..e191aa3 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10693,6 +10693,258 @@ fold_builtin_is_pointer_inverconvertible_with_class (location_t loc, int nargs,
 		      build_zero_cst (TREE_TYPE (arg)));
 }
 
+/* Helper function for is_corresponding_member_aggr.  Return true if
+   MEMBERTYPE pointer-to-data-member ARG can be found in anonymous
+   union or structure BASETYPE.  */
+
+static bool
+is_corresponding_member_union (tree basetype, tree membertype, tree arg)
+{
+  for (tree field = TYPE_FIELDS (basetype); field; field = DECL_CHAIN (field))
+    if (TREE_CODE (field) != FIELD_DECL || DECL_BIT_FIELD_TYPE (field))
+      continue;
+    else if (same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (field),
+							membertype))
+      {
+	if (TREE_CODE (arg) != INTEGER_CST
+	    || tree_int_cst_equal (arg, byte_position (field)))
+	  return true;
+      }
+    else if (ANON_AGGR_TYPE_P (TREE_TYPE (field)))
+      {
+	tree narg = arg;
+	if (TREE_CODE (basetype) != UNION_TYPE
+	    && TREE_CODE (narg) == INTEGER_CST)
+	  narg = size_binop (MINUS_EXPR, arg, byte_position (field));
+	if (is_corresponding_member_union (TREE_TYPE (field),
+					   membertype, narg))
+	  return true;
+      }
+  return false;
+}
+
+/* Helper function for fold_builtin_is_corresponding_member call.
+   Return boolean_false_node if MEMBERTYPE1 BASETYPE1::*ARG1 and
+   MEMBERTYPE2 BASETYPE2::*ARG2 aren't corresponding members,
+   boolean_true_node if they are corresponding members, or for
+   non-constant ARG2 the highest member offset for corresponding
+   members.  */
+
+static tree
+is_corresponding_member_aggr (location_t loc, tree basetype1, tree membertype1,
+			      tree arg1, tree basetype2, tree membertype2,
+			      tree arg2)
+{
+  tree field1 = TYPE_FIELDS (basetype1);
+  tree field2 = TYPE_FIELDS (basetype2);
+  tree ret = boolean_false_node;
+  while (1)
+    {
+      bool r = next_common_initial_seqence (field1, field2);
+      if (field1 == NULL_TREE || field2 == NULL_TREE)
+	break;
+      if (r
+	  && same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (field1),
+							membertype1)
+	  && same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (field2),
+							membertype2))
+	{
+	  tree pos = byte_position (field1);
+	  if (TREE_CODE (arg1) == INTEGER_CST
+	      && tree_int_cst_equal (arg1, pos))
+	    {
+	      if (TREE_CODE (arg2) == INTEGER_CST)
+		return boolean_true_node;
+	      return pos;
+	    }
+	  else if (TREE_CODE (arg1) != INTEGER_CST)
+	    ret = pos;
+	}
+      else if (ANON_AGGR_TYPE_P (TREE_TYPE (field1))
+	       && ANON_AGGR_TYPE_P (TREE_TYPE (field2)))
+	{
+	  if ((!lookup_attribute ("no_unique_address",
+				  DECL_ATTRIBUTES (field1)))
+	      != !lookup_attribute ("no_unique_address",
+				    DECL_ATTRIBUTES (field2)))
+	    break;
+	  if (!tree_int_cst_equal (bit_position (field1),
+				   bit_position (field2)))
+	    break;
+	  bool overlap = true;
+	  tree pos = byte_position (field1);
+	  if (TREE_CODE (arg1) == INTEGER_CST)
+	    {
+	      tree off1 = fold_convert (sizetype, arg1);
+	      tree sz1 = TYPE_SIZE_UNIT (TREE_TYPE (field1));
+	      if (tree_int_cst_lt (off1, pos)
+		  || tree_int_cst_le (size_binop (PLUS_EXPR, pos, sz1), off1))
+		overlap = false;
+	    }
+	  if (TREE_CODE (arg2) == INTEGER_CST)
+	    {
+	      tree off2 = fold_convert (sizetype, arg2);
+	      tree sz2 = TYPE_SIZE_UNIT (TREE_TYPE (field2));
+	      if (tree_int_cst_lt (off2, pos)
+		  || tree_int_cst_le (size_binop (PLUS_EXPR, pos, sz2), off2))
+		overlap = false;
+	    }
+	  if (overlap
+	      && NON_UNION_CLASS_TYPE_P (TREE_TYPE (field1))
+	      && NON_UNION_CLASS_TYPE_P (TREE_TYPE (field2)))
+	    {
+	      tree narg1 = arg1;
+	      if (TREE_CODE (arg1) == INTEGER_CST)
+		narg1 = size_binop (MINUS_EXPR,
+				    fold_convert (sizetype, arg1), pos);
+	      tree narg2 = arg2;
+	      if (TREE_CODE (arg2) == INTEGER_CST)
+		narg2 = size_binop (MINUS_EXPR,
+				    fold_convert (sizetype, arg2), pos);
+	      tree t1 = TREE_TYPE (field1);
+	      tree t2 = TREE_TYPE (field2);
+	      tree nret = is_corresponding_member_aggr (loc, t1, membertype1,
+							narg1, t2, membertype2,
+							narg2);
+	      if (nret != boolean_false_node)
+		{
+		  if (nret == boolean_true_node)
+		    return nret;
+		  if (TREE_CODE (arg1) == INTEGER_CST)
+		    return size_binop (PLUS_EXPR, nret, pos);
+		  ret = size_binop (PLUS_EXPR, nret, pos);
+		}
+	    }
+	  else if (overlap
+		   && TREE_CODE (TREE_TYPE (field1)) == UNION_TYPE
+		   && TREE_CODE (TREE_TYPE (field2)) == UNION_TYPE)
+	    {
+	      tree narg1 = arg1;
+	      if (TREE_CODE (arg1) == INTEGER_CST)
+		narg1 = size_binop (MINUS_EXPR,
+				    fold_convert (sizetype, arg1), pos);
+	      tree narg2 = arg2;
+	      if (TREE_CODE (arg2) == INTEGER_CST)
+		narg2 = size_binop (MINUS_EXPR,
+				    fold_convert (sizetype, arg2), pos);
+	      if (is_corresponding_member_union (TREE_TYPE (field1),
+						 membertype1, narg1)
+		  && is_corresponding_member_union (TREE_TYPE (field2),
+						    membertype2, narg2))
+		{
+		  sorry_at (loc, "%<__builtin_is_corresponding_member%> "
+				 "not well defined for anonymous unions");
+		  return boolean_false_node;
+		}
+	    }
+	}
+      if (!r)
+	break;
+      field1 = DECL_CHAIN (field1);
+      field2 = DECL_CHAIN (field2);
+    }
+  return ret;
+}
+
+/* Fold __builtin_is_corresponding_member call.  */
+
+tree
+fold_builtin_is_corresponding_member (location_t loc, int nargs,
+				      tree *args)
+{
+  /* Unless users call the builtin directly, the following 3 checks should be
+     ensured from std::is_corresponding_member function template.  */
+  if (nargs != 2)
+    {
+      error_at (loc, "%<__builtin_is_corresponding_member%> "
+		     "needs two arguments");
+      return boolean_false_node;
+    }
+  tree arg1 = args[0];
+  tree arg2 = args[1];
+  if (error_operand_p (arg1) || error_operand_p (arg2))
+    return boolean_false_node;
+  if (!TYPE_PTRMEM_P (TREE_TYPE (arg1))
+      || !TYPE_PTRMEM_P (TREE_TYPE (arg2)))
+    {
+      error_at (loc, "%<__builtin_is_corresponding_member%> "
+		     "argument is not pointer to member");
+      return boolean_false_node;
+    }
+
+  if (!TYPE_PTRDATAMEM_P (TREE_TYPE (arg1))
+      || !TYPE_PTRDATAMEM_P (TREE_TYPE (arg2)))
+    return boolean_false_node;
+
+  tree membertype1 = TREE_TYPE (TREE_TYPE (arg1));
+  tree basetype1 = TYPE_OFFSET_BASETYPE (TREE_TYPE (arg1));
+  if (!complete_type_or_else (basetype1, NULL_TREE))
+    return boolean_false_node;
+
+  tree membertype2 = TREE_TYPE (TREE_TYPE (arg2));
+  tree basetype2 = TYPE_OFFSET_BASETYPE (TREE_TYPE (arg2));
+  if (!complete_type_or_else (basetype2, NULL_TREE))
+    return boolean_false_node;
+
+  if (!NON_UNION_CLASS_TYPE_P (basetype1)
+      || !NON_UNION_CLASS_TYPE_P (basetype2)
+      || !std_layout_type_p (basetype1)
+      || !std_layout_type_p (basetype2))
+    return boolean_false_node;
+
+  /* If the member types aren't layout compatible, then they
+     can't be corresponding members.  */
+  if (!layout_compatible_type_p (membertype1, membertype2))
+    return boolean_false_node;
+
+  if (TREE_CODE (arg1) == PTRMEM_CST)
+    arg1 = cplus_expand_constant (arg1);
+  if (TREE_CODE (arg2) == PTRMEM_CST)
+    arg2 = cplus_expand_constant (arg2);
+
+  if (null_member_pointer_value_p (arg1)
+      || null_member_pointer_value_p (arg2))
+    return boolean_false_node;
+
+  if (TREE_CODE (arg1) == INTEGER_CST
+      && TREE_CODE (arg2) == INTEGER_CST
+      && !tree_int_cst_equal (arg1, arg2))
+    return boolean_false_node;
+
+  if (TREE_CODE (arg2) == INTEGER_CST
+      && TREE_CODE (arg1) != INTEGER_CST)
+    {
+      std::swap (arg1, arg2);
+      std::swap (membertype1, membertype2);
+      std::swap (basetype1, basetype2);
+    }
+
+  tree ret = is_corresponding_member_aggr (loc, basetype1, membertype1, arg1,
+					   basetype2, membertype2, arg2);
+  if (TREE_TYPE (ret) == boolean_type_node)
+    return ret;
+  /* If both arg1 and arg2 are INTEGER_CSTs, is_corresponding_member_aggr
+     already returns boolean_{true,false}_node whether those particular
+     members are corresponding members or not.  Otherwise, if only
+     one of them is INTEGER_CST (canonicalized to first being INTEGER_CST
+     above), it returns boolean_false_node if it is certainly not a
+     corresponding member and otherwise we need to do a runtime check that
+     those two OFFSET_TYPE offsets are equal.
+     If neither of the operands is INTEGER_CST, is_corresponding_member_aggr
+     returns the largest offset at which the members would be corresponding
+     members, so perform arg1 <= ret && arg1 == arg2 runtime check.  */
+  gcc_assert (TREE_CODE (arg2) != INTEGER_CST);
+  if (TREE_CODE (arg1) == INTEGER_CST)
+    return fold_build2 (EQ_EXPR, boolean_type_node, arg1,
+			fold_convert (TREE_TYPE (arg1), arg2));
+  ret = fold_build2 (LE_EXPR, boolean_type_node,
+		     fold_convert (pointer_sized_int_node, arg1),
+		     fold_convert (pointer_sized_int_node, ret));
+  return fold_build2 (TRUTH_AND_EXPR, boolean_type_node, ret,
+		      fold_build2 (EQ_EXPR, boolean_type_node, arg1,
+				   fold_convert (TREE_TYPE (arg1), arg2)));
+}
+
 /* Actually evaluates the trait.  */
 
 static bool
@@ -10783,6 +11035,9 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_FINAL:
       return CLASS_TYPE_P (type1) && CLASSTYPE_FINAL (type1);
 
+    case CPTK_IS_LAYOUT_COMPATIBLE:
+      return layout_compatible_type_p (type1, type2);
+
     case CPTK_IS_LITERAL_TYPE:
       return literal_type_p (type1);
 
@@ -10930,6 +11185,19 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
     case CPTK_IS_SAME_AS:
       break;
 
+    case CPTK_IS_LAYOUT_COMPATIBLE:
+      if (!array_of_unknown_bound_p (type1)
+	  && TREE_CODE (type1) != VOID_TYPE
+	  && !complete_type_or_else (type1, NULL_TREE))
+	/* We already issued an error.  */
+	return error_mark_node;
+      if (!array_of_unknown_bound_p (type2)
+	  && TREE_CODE (type2) != VOID_TYPE
+	  && !complete_type_or_else (type2, NULL_TREE))
+	/* We already issued an error.  */
+	return error_mark_node;
+      break;
+
     default:
       gcc_unreachable ();
     }