c++: Add __builtin_bit_cast to implement std::bit_cast [PR93121]

The following patch adds __builtin_bit_cast builtin, similarly to
clang or MSVC which implement std::bit_cast using such an builtin too.
It checks the various std::bit_cast requirements, when not constexpr
evaluated acts pretty much like VIEW_CONVERT_EXPR of the source argument
to the destination type and the hardest part is obviously the constexpr
evaluation.
I've left out PDP11 handling of those, couldn't figure out how exactly are
bitfields laid out there

2020-12-03  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/93121
	* fold-const.h (native_encode_initializer): Add mask argument
	defaulted to nullptr.
	(find_bitfield_repr_type): Declare.
	(native_interpret_aggregate): Declare.
	* fold-const.c (find_bitfield_repr_type): New function.
	(native_encode_initializer): Add mask argument and support for
	filling it.  Handle also some bitfields without integral
	DECL_BIT_FIELD_REPRESENTATIVE.
	(native_interpret_aggregate): New function.
	* gimple-fold.h (clear_type_padding_in_mask): Declare.
	* gimple-fold.c (struct clear_padding_struct): Add clear_in_mask
	member.
	(clear_padding_flush): Handle buf->clear_in_mask.
	(clear_padding_union): Copy clear_in_mask.  Don't error if
	buf->clear_in_mask is set.
	(clear_padding_type): Don't error if buf->clear_in_mask is set.
	(clear_type_padding_in_mask): New function.
	(gimple_fold_builtin_clear_padding): Set buf.clear_in_mask to false.
	* doc/extend.texi (__builtin_bit_cast): Document.

	* c-common.h (enum rid): Add RID_BUILTIN_BIT_CAST.
	* c-common.c (c_common_reswords): Add __builtin_bit_cast.

	* cp-tree.h (cp_build_bit_cast): Declare.
	* cp-tree.def (BIT_CAST_EXPR): New tree code.
	* cp-objcp-common.c (names_builtin_p): Handle RID_BUILTIN_BIT_CAST.
	(cp_common_init_ts): Handle BIT_CAST_EXPR.
	* cxx-pretty-print.c (cxx_pretty_printer::postfix_expression):
	Likewise.
	* parser.c (cp_parser_postfix_expression): Handle
	RID_BUILTIN_BIT_CAST.
	* semantics.c (cp_build_bit_cast): New function.
	* tree.c (cp_tree_equal): Handle BIT_CAST_EXPR.
	(cp_walk_subtrees): Likewise.
	* pt.c (tsubst_copy): Likewise.
	* constexpr.c (check_bit_cast_type, cxx_eval_bit_cast): New functions.
	(cxx_eval_constant_expression): Handle BIT_CAST_EXPR.
	(potential_constant_expression_1): Likewise.
	* cp-gimplify.c (cp_genericize_r): Likewise.

	* g++.dg/cpp2a/bit-cast1.C: New test.
	* g++.dg/cpp2a/bit-cast2.C: New test.
	* g++.dg/cpp2a/bit-cast3.C: New test.
	* g++.dg/cpp2a/bit-cast4.C: New test.
	* g++.dg/cpp2a/bit-cast5.C: New test.

1 files changed, 26 insertions, 0 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index ef4d73d..103567c 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -7250,6 +7250,32 @@ cp_parser_postfix_expression (cp_parser *parser, bool address_p, bool cast_p,
 				     tf_warning_or_error);
       }
 
+    case RID_BUILTIN_BIT_CAST:
+      {
+	tree expression;
+	tree type;
+	/* Consume the `__builtin_bit_cast' token.  */
+	cp_lexer_consume_token (parser->lexer);
+	/* Look for the opening `('.  */
+	matching_parens parens;
+	parens.require_open (parser);
+	location_t type_location
+	  = cp_lexer_peek_token (parser->lexer)->location;
+	/* Parse the type-id.  */
+	{
+	  type_id_in_expr_sentinel s (parser);
+	  type = cp_parser_type_id (parser);
+	}
+	/* Look for the `,'.  */
+	cp_parser_require (parser, CPP_COMMA, RT_COMMA);
+	/* Now, parse the assignment-expression.  */
+	expression = cp_parser_assignment_expression (parser);
+	/* Look for the closing `)'.  */
+	parens.require_close (parser);
+	return cp_build_bit_cast (type_location, type, expression,
+				  tf_warning_or_error);
+      }
+
     default:
       {
 	tree type;