RE: [draft patch] unwinder for btrace [Re: [rfc 3/5] record: make it build again]

Metzger, Markus T
2013-03-27T15:23:17Z

> -----Original Message-----
> From: Jan Kratochvil [mailto:jan.kratochvil@redhat.com]
> Sent: Monday, February 11, 2013 6:13 PM

[...]

> gdb/
> 2013-02-11 Jan Kratochvil <jan.kratochvil@redhat.com>
>
> New record unwinder reporting <unavailable>.
> * dwarf2-frame.c (dwarf2_frame_cfa): Move UNWIND_UNAVAILABLE check
> earlier.
> * frame-unwind.c: Include target.h.
> (frame_unwind_try_unwinder): New function with code from ...
> (frame_unwind_find_by_frame): ... here. New variable
> unwinder_from_target, call also target_get_unwinder and
> frame_unwind_try_unwinder for it.
> * frame.c (get_frame_unwind_stop_reason): Unconditionally call
> get_prev_frame_1.
> * record.c: Include frame-unwind.h.
> (record_frame_unwind_stop_reason, record_frame_this_id)
> (record_frame_prev_register, record_frame_sniffer, record_frame_unwind):
> New.
> (init_record_ops, init_record_core_ops): Install it.
> * target.c (target_get_unwinder): New.
> * target.h (struct target_ops): New field to_get_unwinder.
> (target_get_unwinder): New declaration.

I've been experimenting with this a bit. It looks like there will always be
a sentinel frame at the very bottom that is reading the registers directly
from the inferior. I can only hook in at the second frame.

In order to fake the back trace for btrace replay, I would also need to
replace the sentinel frame, since otherwise, the first frame will always
point to the current location.

Am I doing something wrong?

If not, would you please point me to some code that I would need to
touch to replace the sentinel frame? Would I maybe provide another
target method to allow targets to overwrite the sentinel frame?

On a related but different topic, I added a btrace frame type and
prologue cache. The cache holds a pointer to some btrace data
structure that is used to compute the fake back trace. In order to
unwind a btrace frame, I would need to access the next frame's
location in this btrace data structure.

The easiest would be to check for the next frame's type and then
access it's cache - which doesn't work since struct frame_info is
opaque. I ended up encoding the pointer into the special_addr
of a btrace frame's frame_id - which is somewhat ugly. Any better
idea?

Also what's the lifetime of a frame_info and frame_id object?
When the branch trace is cleared, any pointers to it will become
stale.

Thanks,
Markus.
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