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RE: [draft patch] <unavailable> unwinder for btrace [Re: [rfc 3/5] record: make it build again]
- From: "Metzger, Markus T" <markus dot t dot metzger at intel dot com>
- To: Jan Kratochvil <jan dot kratochvil at redhat dot com>, Markus Metzger <markus dot t dot metzger at googlemail dot com>
- Cc: "gdb-patches at sourceware dot org" <gdb-patches at sourceware dot org>
- Date: Thu, 28 Mar 2013 07:44:12 +0000
- Subject: RE: [draft patch] <unavailable> unwinder for btrace [Re: [rfc 3/5] record: make it build again]
- References: <1360337423-27095-1-git-send-email-markus dot t dot metzger at intel dot com> <1360337423-27095-4-git-send-email-markus dot t dot metzger at intel dot com> <20130210221059 dot GC4819 at host2 dot jankratochvil dot net> <A78C989F6D9628469189715575E55B2307B78A04 at IRSMSX102 dot ger dot corp dot intel dot com> <20130211141451 dot GA8962 at host2 dot jankratochvil dot net> <20130211171319 dot GA17524 at host2 dot jankratochvil dot net> <A78C989F6D9628469189715575E55B2307BA9030 at IRSMSX102 dot ger dot corp dot intel dot com> <9B969C1D-95E8-4AD5-BEF0-E269FF8771DF at gmail dot com> <20130328062747 dot GA27157 at host2 dot jankratochvil dot net>
> -----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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