Re: [RFA/commit+doco 2/2] Windows x64 SEH unwinder.

[Tristan Gingold <gingold@adacore.com>]

On Jan 9, 2013, at 5:05 PM, Eli Zaretskii wrote:

>> From: Joel Brobecker <brobecker@adacore.com>
>> Cc: Joel Brobecker <brobecker@adacore.com>
>> Date: Wed,  9 Jan 2013 14:53:01 +0400
>> 
>> This is the main part of the patch series, which actually
>> adds the unwinder.
> 
> Thanks!
> 
>> One element worth mentioning, perhaps, is the fact that we prepend
>> the unwinder, and the sniffer is the default_frame_sniffer which
>> always returns 1 (absence of SEH info is normal and means it is
>> a leaf function).  This effectively means that all other unwinders
>> effectively get shunted. And in particular, I do not think that
>> the DWARF unwinder will kick in even if DWARF unwind info is found.
>> 
>> The problem is that we want to be ahead of the default amd64-tdep
>> unwinders, which is kind of a last-resort unwinder doing a good
>> job under limited situations only. We'd like to be behind the DWARF
>> unwinder if we could, but I don't think there is a way to ensure
>> that yet.
>> 
>> In practice, this shouldn't be a problem, since this unwinder
>> has been very reliable for us so far.  But it does assume that
>> the compiler is recent enough to generate native SEH data which,
>> for GCC, means GCC 4.7 (I have been told). On the other hand,
>> it looks like the first GCC release to support x64-windows was
>> GCC 4.6.
>> 
>> I don't really see a real way of supporting both old and new versions
>> of GCC, unless we have a way of more finely ordering the unwinders.
>> Worse case scenario, we stop supporting code generated by GCC 4.6.
>> Or an alternative option is to provide a setting to disable this
>> unwinder.
> 
> I'm confused: is this change only for 64-bit Windows programs?

Yes.

>  If
> not, what GCC versions will this support for debugging 32-bit
> executables?
> 
>> diff --git a/gdb/NEWS b/gdb/NEWS
>> index 3451505..d981ac5 100644
>> --- a/gdb/NEWS
>> +++ b/gdb/NEWS
>> @@ -134,6 +134,8 @@ show print type typedefs
>>   feature to be enabled.  For more information, see:
>>       http://fedoraproject.org/wiki/Features/MiniDebugInfo
>> 
>> +* GDB can now use Windows x64 unwinding data.
>> +
> 
> This is OK to go in, but it seems to imply that only 64-bit
> executables are affected.  What would it take to do the same on 32-bit
> Windows?

There is no such thing as unwind info for SEH on windows 32-bit.

Tristan.