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

Metzger, Markus T
2013-03-28T07:44:49Z

> -----Original Message-----
> From: Jan Kratochvil [mailto:jan.kratochvil@redhat.com]
> Sent: Thursday, March 28, 2013 7:28 AM

> > > 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?
>
> special_addr is really not right.
>
> dwarf2-frame-tailcall.c uses for a similar problem 'htab_t cache_htab' which
> is indexed by 'struct frame_info *' which you can iterate in any direction so
> you even do not need a new cache entry for every 'struct frame_info *'.

Thanks, I'll try that. Although, in the end, it's really the same as if we made
other frames prologue cache visible.

> > > 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.
>
> There is frame_unwind->dealloc_cache, any reinit_frame_cache() call inside GDB
> will clear the prologue cache which is very common.
>
> I see now btrace_thread_info->btrace may change more often - such as during
> each "info record" command. So call reinit_frame_cache() in the cases btrace
> cache may get rebuilt.

The data structure should only change when there is new trace, which requires
the target to continue. "info record" should, like any other record-btrace command,
fetch the new trace once and then operate on the cached trace data.

Is there a guarantee that frame_info and frame_id objects are destroyed
when the target resumes? Or could I trigger their destruction from within
btrace_clear?

Thanks,
Markus.

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