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Re: [RFA] Artifical dwarf2 debug info

On Thu, Jan 02, 2003 at 08:54:33PM +0000, Andrew Cagney wrote:

  fde = get_fde_for_addr (context->ra - 1);

> >+
> >+  if (fde == NULL)
> >+    fde = guess_generic_fde (context->ra - 1);
> >
> >   if (fde == NULL)
> >     return;

Just to be clear. The above is the change that I think is wrong.

Instead of this function `guessing' the source of the FDE, the code needs to be re-structured so that the caller always supplies a pre-created FDE.

That way a dwarf2 cfi frame can call the above function with an FDE built from the object files debug info, while an artifical frame can call it with an artifically created FDE. There is no guessing involved.

Hold a second here; I don't think we're really communicating on what
this change is supposed to do.  Look at where that code is: it's in
frame_state_for.  Its inputs are a CONTEXT and FS (struct frame_state
*).  The first line in your quote is:
  fde = get_fde_for_addr (context->ra - 1);
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).

I just don't understand what you mean by "the caller supplies an FDE";
this is where we locate the FDE.  The caller's got no business knowing
what an FDE is.  This new mechanism is supposed to handled any code
which doesn't have a defined FDE, for which an architecutre-specific
(yes) hook can deduce the appropriate FDE from code inspection.

These are not "artificial frames".  We've got four types of frames that
we've been talking about recently:
[`artificial frame' was a poor choice of name - a real frame that uses artifically created dwarf2 debug info]

  - the magical register frame/inner frame
  - dummy frames
  - sigtramp frames
  - "normal" stack frames caused by compiled code calling other
    compiled code
The latter can be further broken down into:

dwarf2 fde / cfi frames
saved-register frames
artifical fde / cfi frames

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 the event.

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.

> 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?

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.


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