[arm][PR88167] Fix __builtin_return_address returns invalid address

[arm][PR88167] Fix __builtin_return_address returns invalid address

This patch fixes a problem with the thumb1 prologue code where the link
register could be unconditionally used as a scratch register even if the
return value was still live at the end of the prologue.

Additionally, the patch improves the code generated when we are not
using many low call-saved registers to make use of any unused call
clobbered registers to help with the saving of high registers that
cannot be pushed directly (quite rare in normal code as the register
allocator correctly prefers low registers).

2019-05-08  Mihail Ionescu  <mihail.ionescu@arm.com>
	    Richard Earnshaw  <rearnsha@arm.com>

gcc:

	PR target/88167
	* config/arm/arm.c (thumb1_prologue_unused_call_clobbered_lo_regs): New
	function.
	(thumb1_epilogue_unused_call_clobbered_lo_regs): New function.
	(thumb1_compute_save_core_reg_mask): Don't force a spare work
	register if both the epilogue and prologue can use call-clobbered
	regs.
	(thumb1_unexpanded_epilogue): Use
	thumb1_epilogue_unused_call_clobbered_lo_regs.  Reverse the logic for
	picking temporaries for restoring high regs to match that of the
	prologue where possible.
	(thumb1_expand_prologue): Add any usable call-clobbered low registers to
	the list of work registers.  Detect if the return address is still live
	at the end of the prologue and avoid using it for a work register if so.
	If the return address is not live, add LR to the list of pushable regs
	after the first pass.

gcc/testsuite:

	PR target/88167
	* gcc.target/arm/pr88167-1.c: New test.
	* gcc.target/arm/pr88167-2.c: New test.

Co-Authored-By: Richard Earnshaw <rearnsha@arm.com>
From-SVN: r271012