Hazard barrier return support

Hazard barrier return support

This patch allows a function to request clearing of all instruction and execution
hazards upon normal return via __attribute__ ((use_hazard_barrier_return)).

2017-04-25  Prachi Godbole  <prachi.godbole@imgtec.com>

gcc/ChangeLog:

	* config/mips/mips.h (machine_function): New variable
	use_hazard_barrier_return_p.
	* config/mips/mips.md (UNSPEC_JRHB): New unspec.
	(mips_hb_return_internal): New insn pattern.
	* config/mips/mips.cc (mips_attribute_table): Add attribute
	use_hazard_barrier_return.
	(mips_use_hazard_barrier_return_p): New static function.
	(mips_function_attr_inlinable_p): Likewise.
	(mips_compute_frame_info): Set use_hazard_barrier_return_p.
	Emit error for unsupported architecture choice.
	(mips_function_ok_for_sibcall, mips_can_use_return_insn):
	Return false for use_hazard_barrier_return.
	(mips_expand_epilogue): Emit hazard barrier return.
	* doc/extend.texi: Document use_hazard_barrier_return.

gcc/testsuite/ChangeLog:

	* gcc.target/mips/hazard-barrier-return-attribute.c: New test.

Signed-off-by: Junxian Zhu <zhujunxian@oss.cipunited.com>