Re: [PATCH v3] Windows x64 SEH unwinder

[Tristan Gingold <gingold@adacore.com>]

On Aug 22, 2013, at 5:26 PM, Pedro Alves <palves@redhat.com> wrote:

> On 08/22/2013 10:33 AM, Tristan Gingold wrote:
>> 
>> after discussion with Roland Schwingel, I have found that the patch
>> doesn't handle well dwarf3 DW_OP_call_frame_cfa, because the SEH
>> unwinder is before the dwarf2 one.
> 
> Can you clarify this a little better for the archives?

Sure.

> So that mean that for binaries built before that gcc fix,
> the SEH unwinder won't kick in at all, right?

No.
If dwarf2 info are presents, they will be used to unwind the
frames.  If they aren't, unwinding will probably fail.

>  Then,
> how come this fixes Roland's age old issue, and improves
> unwinding for him?

For the part compiled with gcc, the patch shouldn't change
anything.  But for function compiled by MS compilers (those
in dll), gdb will now unwind their frame by using the SEH
unwinder,

> In the previous versions, there was talk about needing
> finer ordering of the unwinders in order to support both
> old and new binaries.  What changed?  Why is this okay
> now?

Unwinding was possible when compiled with -g.  It is still
possible.
Version 1 and 2 of the patch failed, because the dwarf
unwinder was never used (always masked by the SEH unwinder).

It is still possible that gdb with support for SEH unwinder
fails where gdb without for binaries produced by old gcc for
the cases where the prologue analyzer using heuristic was
correct.  But debugging with this unwinder isn't reliable.

Hopes it clarifies :-)

Tristan.