Re: Question on Dwarf2 unwind info and optimized code

[Christophe LYON <christophe dot lyon at st dot com>]

Jim Wilson wrote:
> Christophe LYON wrote:
>
> > I have been look at the Dwarf2 frame info generated by GCC, and how it 
> > works.
> >  From what I can see, only the register saves are recorded, and not 
> > the restores. Why?
>
>
> The frame info is primarily used for C++ EH stack unwinding.  Since you 
> can't throw a C++ exception in an epilogue, epilogue frame info isn't 
> needed for this, and was never implemented for most targets.  Which is a 
> shame.
>
> There is a PR for this, PR 18749, for the x86-64 target.  The lack of 
> epilogue unwind info shows up if you run the libunwind testsuite. 
> Otherwise, it is really hard to find an example where the missing unwind 
> info is a problem.


That's what I thought, but wanted to be sure I was not missing something.

On occasions, I wonder whether it wouldn't make sense to generate
different infos in debug_frame and eh_frame: IIUC, GCC tries to
'compress' the debug frame info by generating few advance_loc
instructions (eg only 1 for the whole prologue), which makes sense in
the C++ EH stack unwinding context, but my cause problems in a debugger.

Thanks

Christophe.