https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53927
Pierre-Marie de Rodat <derodat at adacore dot com> changed:
What |Removed |Added
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CC| |derodat at adacore dot com
--- Comment #13 from Pierre-Marie de Rodat <derodat at adacore dot com> ---
(In reply to Tom Tromey from comment #8)
> > Yes, but you can do something useful even with this value of
> > DW_AT_static_link, albeit not exactly what DWARF means.
>
> Regardless, I think GCC should emit correct DWARF.
I gave it a try: see the attached patch.
Jason suggested to change DW_AT_frame_base in order to make it equal to the
address of the FRAME object. I was not sure:
1) how to do it: location descriptions for all local variables would need to
be updated;
2) whether it's safe to do this: what if optimizers move/duplicate this FRAME
object in the stack frame or do similar "disturbing" things? I'm not familiar
enough with optimization passes to estimate if it's likely: feedback welcome.
:-)
I thought: why not make DW_AT_static_link compute the parent frame base address
from the current static link argument? Well, when generating DW_AT_static_link
for a nested subprogram, we do not know yet the offset between the FRAME object
and the frame base address. This is because nested subprograms reach the
back-end before their parent. Besides, see point 2: are we only sure that such
a constant offset exists?
So instead, I patched tree-nested.c in order to append a field at the end of
the FRAME object to hold the frame base address (computed with the DWARF_CFA
builtin). Then, dwarf2out.c just has to emit a location description for nested
subprograms' DW_AT_static_link that fetches this using the static link
argument. It's consuming a little stack space, I'm not sure if it's a problem.
Once again: feedback welcome!
Besides, this does not work yet with Fortran since for this front-end, the
DWARF_CFA builtin is not registered. None of builtins.def are registered by the
way: how could I register only this one without duplicating code? Is it a good
idea anyway?
For the record, I bootstrapped and reg-tested this patch on x86_64-linux and
preliminary manual testing with a patched GDB[1] and Ada reproducers shows that
this approach is working.
Thoughts?
[1] This patch teaches GDB how to use DW_AT_static_link in order to find the
frame corresponding to the lexically enclosing scope. I think I will try to
submit it to GDB soon.
