This patch implements support for marking CALL_EXPRs
as being mandatory for tail-call-optimization. expand_call
tries harder to perform the optimization on such CALL_EXPRs,
and issues an error if it fails.

Currently this flag isn't accessible from any frontend,
so the patch uses a plugin for testing the functionality.

gcc/ChangeLog:
	* calls.c (maybe_complain_about_tail_call): New function.
	(initialize_argument_information): Call
	maybe_complain_about_tail_call when clearing *may_tailcall.
	(can_implement_as_sibling_call_p): Call
	maybe_complain_about_tail_call when returning false.
	(expand_call): Read CALL_EXPR_MUST_TAIL_CALL and, if set,
	ensure try_tail_call is set.  Call maybe_complain_about_tail_call
	if tail-call optimization fails.
	* cfgexpand.c (expand_call_stmt): Initialize
	CALL_EXPR_MUST_TAIL_CALL from gimple_call_must_tail_p.
	* gimple-pretty-print.c (dump_gimple_call): Dump
	gimple_call_must_tail_p.
	* gimple.c (gimple_build_call_from_tree): Call
	gimple_call_set_must_tail with the value of
	CALL_EXPR_MUST_TAIL_CALL.
	* gimple.h (enum gf_mask): Add GF_CALL_MUST_TAIL_CALL.
	(gimple_call_set_must_tail): New function.
	(gimple_call_must_tail_p): New function.
	* print-tree.c (print_node): Update printing of TREE_STATIC
	to reflect its use for CALL_EXPR_MUST_TAIL_CALL.
	* tree-core.h (struct tree_base): Add MUST_TAIL_CALL to the
	trailing comment listing applicable flags.
	* tree.h (CALL_EXPR_MUST_TAIL_CALL): New macro.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/must-tail-call-1.c: New test case.
	* gcc.dg/plugin/must-tail-call-2.c: New test case.
	* gcc.dg/plugin/must_tail_call_plugin.c: New file.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add the above.

From-SVN: r236514