arm: Implement Armv8.1-M low overhead loops

arm: Implement Armv8.1-M low overhead loops

gcc/ChangeLog

2020-06-18  Andrea Corallo  <andrea.corallo@arm.com>
	    Mihail-Calin Ionescu  <mihail.ionescu@arm.com>
	    Iain Apreotesei  <iain.apreotesei@arm.com>

	* config/arm/arm-protos.h (arm_target_insn_ok_for_lob): New
	prototype.
	* config/arm/arm.c (TARGET_INVALID_WITHIN_DOLOOP): Define.
	(arm_invalid_within_doloop): Implement invalid_within_doloop hook.
	(arm_target_insn_ok_for_lob): New function.
	* config/arm/arm.h (TARGET_HAVE_LOB): Define macro.
	* config/arm/thumb2.md (*doloop_end_internal, doloop_begin)
	(dls_insn): Add new patterns.
	(doloop_end): Modify to select LR when LOB is available.
	* config/arm/unspecs.md: Add new unspec.
	* doc/sourcebuild.texi (arm_v8_1_lob_ok)
	(arm_thumb2_ok_no_arm_v8_1_lob): Document new target supports
	options.

gcc/testsuite/ChangeLog

2020-06-18  Andrea Corallo  <andrea.corallo@arm.com>
	    Mihail-Calin Ionescu  <mihail.ionescu@arm.com>
	    Iain Apreotesei  <iain.apreotesei@arm.com>

	* gcc.target/arm/lob.h: New header.
	* gcc.target/arm/lob1.c: New testcase.
	* gcc.target/arm/lob2.c: Likewise.
	* gcc.target/arm/lob3.c: Likewise.
	* gcc.target/arm/lob4.c: Likewise.
	* gcc.target/arm/lob5.c: Likewise.
	* gcc.target/arm/lob6.c: Likewise.
	* gcc.target/arm/unsigned-extend-2.c: Do not run when generating
	low loop overhead.
	* gcc.target/arm/ivopts.c: Fix check for low loop overhead.
	* lib/target-supports.exp (check_effective_target_arm_v8_1_lob)
	(check_effective_target_arm_thumb2_ok_no_arm_v8_1_lob): New procs.