DWARF: Allow hard frame pointer even if frame pointer isn't used

DWARF: Allow hard frame pointer even if frame pointer isn't used

r251028

commit cd557ff63f388ad27c376d0a225e74d3594a6f9d
Author: hjl <hjl@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Thu Aug 10 15:29:05 2017 +0000

    i386: Don't use frame pointer without stack access

    When there is no stack access, there is no need to use frame pointer
    even if -fno-omit-frame-pointer is used and caller's frame pointer is
    unchanged.

frame pointer may not be available even if -fno-omit-frame-pointer is
used.  When this happened, arg pointer may be eliminated by hard frame
pointer.  Since hard frame pointer is encoded with DW_OP_fbreg which
uses the DW_AT_frame_base attribute, not hard frame pointer directly,
we should allow hard frame pointer when generating DWARF info even if
frame pointer isn't used.

gcc/

	PR debug/86593
	* dwarf2out.c (based_loc_descr): Allow hard frame pointer even
	if frame pointer isn't used.
	(compute_frame_pointer_to_fb_displacement): Likewise.

gcc/testsuite/

	PR debug/86593
	* g++.dg/pr86593.C: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@264096 138bc75d-0d04-0410-961f-82ee72b054a4