Fix inline frame unwinding breakage

Fix inline frame unwinding breakage

There has been some breakage for aarch64-linux, arm-linux and s390-linux in
terms of inline frame unwinding. There may be other targets, but these are
the ones i'm aware of.

The following testcases started to show numerous failures and trigger internal
errors in GDB after commit 1009d92fc621bc4d017029b90a5bfab16e17fde5,
"Find tailcall frames before inline frames".

gdb.opt/inline-break.exp
gdb.opt/inline-cmds.exp
gdb.python/py-frame-inline.exp
gdb.reverse/insn-reverse.exp

The internal errors were of this kind:

binutils-gdb/gdb/frame.c:579: internal-error: frame_id get_frame_id(frame_info*): Assertion `fi->level == 0' failed.

After a lengthy investigation to try and find the cause of these assertions,
it seems we're dealing with some fragile/poorly documented code to handle inline
frames and we are attempting to unwind from this fragile section of code.

Before commit 1009d92fc621bc4d017029b90a5bfab16e17fde5, the tailcall sniffer
was invoked from dwarf2_frame_prev_register. By the time we invoke the
dwarf2_frame_prev_register function, we've probably already calculated the
frame id (via compute_frame_id).

After said commit, the call to dwarf2_tailcall_sniffer_first was moved to
dwarf2_frame_cache. This is very early in a frame creation process, and
we're still calculating the frame ID (so compute_frame_id is in the call
stack).

This would be fine for regular frames, but the above testcases all deal
with some inline frames.

The particularity of inline frames is that their frame ID's depend on
the previous frame's ID, and the previous frame's ID relies in the inline
frame's registers. So it is a bit of a messy situation.

We have comments in various parts of the code warning about some of these
particularities.

In the case of dwarf2_tailcall_sniffer_first, we attempt to unwind the PC,
which goes through various functions until we eventually invoke
frame_unwind_got_register. This function will eventually attempt to create
a lazy value for a particular register, and this lazy value will require
a valid frame ID.  Since the inline frame doesn't have a valid frame ID
yet (remember we're still calculating the previous frame's ID so we can tell
what the inline frame ID is) we will call compute_frame_id for the inline
frame (level 0).

We'll eventually hit the assertion above, inside get_frame_id:

--
      /* If we haven't computed the frame id yet, then it must be that
         this is the current frame.  Compute it now, and stash the
         result.  The IDs of other frames are computed as soon as
         they're created, in order to detect cycles.  See
         get_prev_frame_if_no_cycle.  */
      gdb_assert (fi->level == 0);
--

It seems to me we shouldn't have reached this assertion without having the
inline frame ID already calculated. In fact, it seems we even start recursing
a bit when we invoke get_prev_frame_always within inline_frame_this_id. But
a check makes us quit the recursion and proceed to compute the id.

Here's the call stack for context:

 #0  get_prev_frame_always_1 (this_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:2109
 RECURSION - #1  0x0000aaaaaae1d098 in get_prev_frame_always (this_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:2124
 #2  0x0000aaaaaae95768 in inline_frame_this_id (this_frame=0xaaaaab85a670, this_cache=0xaaaaab85a688, this_id=0xaaaaab85a6d0)
     at ../../../repos/binutils-gdb/gdb/inline-frame.c:165
 #3  0x0000aaaaaae1916c in compute_frame_id (fi=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:550
 #4  0x0000aaaaaae19318 in get_frame_id (fi=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:582
 #5  0x0000aaaaaae13480 in value_of_register_lazy (frame=0xaaaaab85a730, regnum=30) at ../../../repos/binutils-gdb/gdb/findvar.c:296
 #6  0x0000aaaaaae16c00 in frame_unwind_got_register (frame=0xaaaaab85a730, regnum=30, new_regnum=30) at ../../../repos/binutils-gdb/gdb/frame-unwind.c:268
 #7  0x0000aaaaaad52604 in dwarf2_frame_prev_register (this_frame=0xaaaaab85a730, this_cache=0xaaaaab85a748, regnum=30)
     at ../../../repos/binutils-gdb/gdb/dwarf2/frame.c:1296
 #8  0x0000aaaaaae1ae68 in frame_unwind_register_value (next_frame=0xaaaaab85a730, regnum=30) at ../../../repos/binutils-gdb/gdb/frame.c:1229
 #9  0x0000aaaaaae1b304 in frame_unwind_register_unsigned (next_frame=0xaaaaab85a730, regnum=30) at ../../../repos/binutils-gdb/gdb/frame.c:1320
 #10 0x0000aaaaaab76574 in aarch64_dwarf2_prev_register (this_frame=0xaaaaab85a730, this_cache=0xaaaaab85a748, regnum=32)
     at ../../../repos/binutils-gdb/gdb/aarch64-tdep.c:1114
 #11 0x0000aaaaaad52724 in dwarf2_frame_prev_register (this_frame=0xaaaaab85a730, this_cache=0xaaaaab85a748, regnum=32)
     at ../../../repos/binutils-gdb/gdb/dwarf2/frame.c:1316
 #12 0x0000aaaaaae1ae68 in frame_unwind_register_value (next_frame=0xaaaaab85a730, regnum=32) at ../../../repos/binutils-gdb/gdb/frame.c:1229
 #13 0x0000aaaaaae1b304 in frame_unwind_register_unsigned (next_frame=0xaaaaab85a730, regnum=32) at ../../../repos/binutils-gdb/gdb/frame.c:1320
 #14 0x0000aaaaaae16a84 in default_unwind_pc (gdbarch=0xaaaaab81edc0, next_frame=0xaaaaab85a730) at ../../../repos/binutils-gdb/gdb/frame-unwind.c:223
 #15 0x0000aaaaaae32124 in gdbarch_unwind_pc (gdbarch=0xaaaaab81edc0, next_frame=0xaaaaab85a730) at ../../../repos/binutils-gdb/gdb/gdbarch.c:3074
 #16 0x0000aaaaaad4f15c in dwarf2_tailcall_sniffer_first (this_frame=0xaaaaab85a730, tailcall_cachep=0xaaaaab85a830, entry_cfa_sp_offsetp=0x0)
     at ../../../repos/binutils-gdb/gdb/dwarf2/frame-tailcall.c:388
 #17 0x0000aaaaaad520c0 in dwarf2_frame_cache (this_frame=0xaaaaab85a730, this_cache=0xaaaaab85a748) at ../../../repos/binutils-gdb/gdb/dwarf2/frame.c:1190
 #18 0x0000aaaaaad52204 in dwarf2_frame_this_id (this_frame=0xaaaaab85a730, this_cache=0xaaaaab85a748, this_id=0xaaaaab85a790)
     at ../../../repos/binutils-gdb/gdb/dwarf2/frame.c:1218
 #19 0x0000aaaaaae1916c in compute_frame_id (fi=0xaaaaab85a730) at ../../../repos/binutils-gdb/gdb/frame.c:550
 #20 0x0000aaaaaae1c958 in get_prev_frame_if_no_cycle (this_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:1927
 #21 0x0000aaaaaae1cc44 in get_prev_frame_always_1 (this_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:2006
 FIRST CALL - #22 0x0000aaaaaae1d098 in get_prev_frame_always (this_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:2124
 #23 0x0000aaaaaae18f68 in skip_artificial_frames (frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:495
 #24 0x0000aaaaaae193e8 in get_stack_frame_id (next_frame=0xaaaaab85a670) at ../../../repos/binutils-gdb/gdb/frame.c:596
 #25 0x0000aaaaaae87a54 in process_event_stop_test (ecs=0xffffffffefc8) at ../../../repos/binutils-gdb/gdb/infrun.c:6857
 #26 0x0000aaaaaae86bdc in handle_signal_stop (ecs=0xffffffffefc8) at ../../../repos/binutils-gdb/gdb/infrun.c:6381
 #27 0x0000aaaaaae84fd0 in handle_inferior_event (ecs=0xffffffffefc8) at ../../../repos/binutils-gdb/gdb/infrun.c:5578
 #28 0x0000aaaaaae81588 in fetch_inferior_event (client_data=0x0) at ../../../repos/binutils-gdb/gdb/infrun.c:4020
 #29 0x0000aaaaaae5f7fc in inferior_event_handler (event_type=INF_REG_EVENT, client_data=0x0) at ../../../repos/binutils-gdb/gdb/inf-loop.c:43
 #30 0x0000aaaaaae8d768 in infrun_async_inferior_event_handler (data=0x0) at ../../../repos/binutils-gdb/gdb/infrun.c:9377
 #31 0x0000aaaaaabff970 in check_async_event_handlers () at ../../../repos/binutils-gdb/gdb/async-event.c:291
 #32 0x0000aaaaab27cbec in gdb_do_one_event () at ../../../repos/binutils-gdb/gdbsupport/event-loop.cc:194
 #33 0x0000aaaaaaef1894 in start_event_loop () at ../../../repos/binutils-gdb/gdb/main.c:356
 #34 0x0000aaaaaaef1a04 in captured_command_loop () at ../../../repos/binutils-gdb/gdb/main.c:416
 #35 0x0000aaaaaaef3338 in captured_main (data=0xfffffffff1f0) at ../../../repos/binutils-gdb/gdb/main.c:1254
 #36 0x0000aaaaaaef33a0 in gdb_main (args=0xfffffffff1f0) at ../../../repos/binutils-gdb/gdb/main.c:1269
 #37 0x0000aaaaaab6e0dc in main (argc=6, argv=0xfffffffff348) at ../../../repos/binutils-gdb/gdb/gdb.c:32

The following patch addresses this by using a function that unwinds the PC
from the next (inline) frame directly as opposed to creating a lazy value
that is bound to the next frame's ID (still not computed).

gdb/ChangeLog:

2020-04-23  Luis Machado  <luis.machado@linaro.org>

	* dwarf2/frame-tailcall.c (dwarf2_tailcall_sniffer_first): Use
	get_frame_register instead of gdbarch_unwind_pc.