Re: about how to add support to new c++ compiler in GDB

["Wu Zhou" <woodzltc at cn dot ibm dot com>]

> On Tue, Apr 26, 2005 at 10:53:03AM +0800, Wu Zhou wrote:
> > Hello Daniel and all,
> >
> > I am recently running GDB testsuite against IBM's xlc compiler, and
found
> > some problems with c++ testcases, especially while handling
virtual-table
> > related stuff. After some analysis, I found there are some difference
> > between the debuginfo output between xlc++ and g++. The main difference
I
> > noted now is that the class in xlc++ didn't depend on
DW_TAG_containing_type
> > to find its virutal function table. Instead, every virtual class in
xlc++
> > had its own "__vfp", which act the same role as "_vptr.CLASSNAME" in
g++.
> > This will make gdb crash (SEGV) on quite a few c++ testcase.
>
> I have some patches for this, originally developed for the ARM RVCT
> compiler.  I haven't had the time to clean them up and post them.
> They will probably be helpful to you.
>
> If I haven't posted them by the end of this week, please, remind me.

Thank you, Daniel! I am very glad to try your patch.

>
> > I am now considering add some code in GDB to fix this. After doing some
> > search in this mail-list, I found there are some discussion on similar
> > problem before, such as
http://sourceware.org/ml/gdb/2004-12/msg00077.html.
> > So I think maybe some of you might have experience or thought or idea on
> > this already. So I write this mail to ask for your expert advice:
> >
> > 1. To support a new c++ compiler, what work need to bo done? I am now
> > thinking of is a new source file(maybe xlc-abi.c) to register its own
cp-abi
> > operation, and some tweak in dwarf2read.c to let GDB understand xlc
specific
> > debuginfo. Any else places need my special attention? Please advice.
>
> Do you need an xlc-abi.c?  What ABI does this compiler follow?  If it
> doesn't follow the Itanium C++ ABI or one of the older GNU ABIs, then
> considerably more work may be required.

I am not very sure. They(the developer of xlc) said that they also follow
GNU V3 C++ ABI. But there are quite a few difference between the debuginfo
generated by g++ and xlc++. An new file named xlc-abi.c is what I could
thought of at this time. It could verify my idea, and at the same time not
impact the debugging of g++ code. If the changes is not big, maybe we could
integrate them back into gnu-v3-abi.c at last.

>
> Is its ABI documented?
>

I am not very sure. I need to ask the compiler developer.

> > 2. while coding this support, what design consideration I need to take
into
> > account? Is there any document on this? GDB internal didn't mention
this.
> > Maybe there are some in somebody's mind, would you like to share with
me?
> > Thanks a lot. :-)
>
> I'd be glad to answer more specific questions, but I've not got any
> answers for this general one.
>
> Do you have arangements for copyright assignment?  I don't know how
> exactly that works in IBM, but I suspect it's complicated.

My manager are arranging this for me. Do I need to assign any document
myself? If yes, I am very glad to do so.

>
> > 3. To verify that my code does work ok and don't bring any other
> > side-effect, what I need to take into account? Running the testsuite
against
> > the modified code might be a help. Any specific area need some more
> > attention?
>
> Testing with already supported C++ compilers is very important.  It's
> easy to fix one ABI and break another.

Thanks. I will keep this in mind.

>
> -- 
> Daniel Jacobowitz
> CodeSourcery, LLC