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

[Daniel Jacobowitz <drow at false dot org>]

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.

> 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.

Is its ABI documented?

> 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.

> 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.

-- 
Daniel Jacobowitz
CodeSourcery, LLC