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Re: [RFA] Artifical dwarf2 debug info
- From: Andrew Cagney <ac131313 at redhat dot com>
- To: Daniel Jacobowitz <drow at mvista dot com>
- Cc: Michal Ludvig <mludvig at suse dot cz>,GDB Patches <gdb-patches at sources dot redhat dot com>
- Date: Fri, 03 Jan 2003 00:28:09 +0000
- Subject: Re: [RFA] Artifical dwarf2 debug info
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The logic that makes the decision to use dwarf2 cfi, the dwarf2 like
unwind code, or this artificial info, or even the old style saved-regs.
That logic lives in get_prev_frame().
Right. That gets removed. Instead that info gets passed to the CFI
code as a `parameter' (perhaphs explicitly, or perhaps implicitly as
part of a member of the CONTEXT object).
This I don't get at all. Why? What advantage is there? Whose
business besides the CFI reader's is it whether or not there is an FDE?
Why should the dwarf2 unwind et.al. code assume that the info it needs
is comming from the dwarf2 reader? I thought the point of the change
was that the dwarf2 cfi reader from all the code that uses that
information - hence allowing us to fake up dwarf2 info.
A saved-registers frame uses the existing code (frame->saved_regs) to
locate a saved register. It doesn't read dwarf2 debug info and then use
that to do CFI.
The latter can be further broken down into:
dwarf2 fde / cfi frames
artifical fde / cfi frames
I still don't see the point of this distinction. What differentiates a
saved-register frame from a dwarf2 fde / cfi frame, anyway? There are
saved registers; we figure out where they are via CFI.
CFI is just one of a number of ways to unwind a normal frame.
The `struct frame_info' creation logic lives in frame.c - see
get_prev_frame(). It hasn't moved but is being expanded. Only problem
is with the x86-64. That code is trying to locally handle cases that
are better handled by get_prev_frame(). Again, note all those
pc_in_call_dummy() et.al. tests?
>Maybe there will be others, but notice that all the above are
>conceptually different kinds of things. These "artifical" frames are
>just normal frames, where we synthesize the debug information because
>we didn't have any. It's a mechanism to coalesce things like prologue
>readers. It is absolutely not a new type of frame.
>That's why I think this code is in exactly the right place, right now.
>Are you saying that the CFI code should just be returning, saying "no
>idea, go away, don't talk to me", and leaving this be?
Why was the dwarf2cfi code even called? Since there is no dwarf2 cfi
that code path should not have been reached. Per my comment below, this
would have happend because the caller (or something up the stack) failed
to check for an edge condition. That change is patching things up after
Instead, during `struct frame_info' creation, if there isn't any dwarf2
info, and the architecture really wants to use the dwarf2cfi logic, it
should create an `artifical fde / cfi frame' that first fakes up the FDE
info and then supplies that to the dwarf2cfi logic. Similarly, a dwarf2
cfi frame can first read the fde and then call the relevant code.
It sounds to me as if you want to move the call to the frame creation
logic somewhere else and that's it. The place you want to move it
doesn't exist yet, as far as I can tell. Am I right?
> That's all well
> and good but that way we end up duplicating the whole of the CFI
> reader. A good long term direction, with appropriate code factoring,
> but it's hardly practical.
How does this result in the duplication of the CFI reader?
The entire CFA program execution and unwind logic would be exactly
identical to that of the "artificial frame" case.
And hence wouldn't / shouldn't be duplicated.
>>This is part of a long standing problem - it predates dwarf2cf by many
>>years. Instead of using recursion, people modify debug/target dependent
>>frame code so that it attempts to directly handle all cases. Cf all the
>>PC_IN_CALL_DUMMY(frame->next), PC_IN_SIGTRAMP(frame->next) and other
>>tests scattered through out the -tdep.c code; and the calls to
>>get_next_frame() in dwarf2cfi.c.
>The one call to get_next_frame, which parallels init_frame_pc_default.
Right. And init_frame_pc_default() is, again, typical of the problem.
It shouldn't need to refer to frame->next.
I can't claim to understand how that would work.
Separate functions would handle separate cases.