[patch/rfc] Add a sentinel frame

[Michal Ludvig]

Andrew Cagney wrote:
> To give this x86-64 thread clear closure.  The internal-error you are 
> seeing from the new frame code is now, officially, "not-a-frame-bug".

Yes, I've already realised so. Thank you for confirmation.

> The underlying problem is caused by a design flaw (one of many) in the 
> original CFI code (on which the x86-64 depends).  It's trying to use the 
> CFI unwinder on a block of code that either: has no CFI information; or 
> has CFI information that isn't relevant to the stack frame being 
> unwound.  Using CFI to unwind such a frame is meaningless.
[...]
> To fix this problem, the x86-64 will need to implement both that and the 
> save_dummy_frame_tos() method.

OK. So, first I need to convert x86-64 target to use all the new 
frame-id stuff I think. And then implement handling of different frame 
types (normal (CFI), dummy, sigtramp, and specifically for x86-64 also 
normal frames without CFI debug info).
Basically all calls to cfi_*() functions from x86-64-*.c files should 
become x86_64_*() functions that call the appropriate cfi_*() functions 
if needed, or a frame-type specific thing otherwise.
Correct?

Michal Ludvig
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