gdb 8.x - g++ 7.x compatibility

Wed Feb 7 15:07:00 GMT 2018

On 02/07/2018 02:44 PM, Simon Marchi wrote:
> On 2018-02-07 02:21, Daniel Berlin wrote:
>> As the person who, eons ago, wrote a bunch of the the GDB code for 
>> this C++
>> ABI support, and as someone who helped with DWARF support in both GDB and
>> GCC, let me try to propose a useful path forward (in the hopes that 
>> someone
>> will say "that's horrible, do it this <clearly better way> instead")
>> Here are the constraints i believe we are working with.
>> 1. GDB should work with multiple DWARF producers and multiple C++ 
>> compilers
>> implementing the C++ ABI
>> 2. There is no canonical demangled format for the C++ ABI
>> 3. There is no canoncial target demangler you can say everyone should use
>> (and even if there was, you don't want to avoid debugging working because
>> someone chose not to)
>> 4. You don't want to slow down GDB if you can avoid it
>> 5. Despite them all implementation the same ABI, it's still possible to
>> distinguish the producers by the producer/compiler in the dwarf info.
>> Given all that:
>> GDB has ABI hooks that tell it what to do for various C++ ABIs. This 
>> is how
>> it knows to call the right demangler for gcc v3's abi vs gcc v2's abi. 
>> and
>> handle various differences between them.
>> See gdb/cp-abi.h
>> The IMHO, obvious thing to do here is: Handle the resulting demangler
>> differences with 1 or more new C++ ABI hooks.
>> Or, introduce C++ debuginfo producer hooks that the C++ ABI hooks use if
>> folks want it to be separate.
>> Once the producer is detected, fill in the hooks with a set of functions
>> that does the right thing.
>> I imagine this would also clean up a bundle of hacks in various parts of
>> gdb trying to handle these differences anyway (which is where a lot of 
>> the
>> multiple symbol lookups/etc that are often slow come from.
>> If we just detected and said "this is gcc 6, it behaves like this", we
>> wouldn't need to do that)
>> In case you are worried, you will discover this is how a bunch of 
>> stuff is
>> done and already contains a ball of hacks.
>> Using hooks would be, IMHO, a significant improvement.
> Hi Daniel,
> Thanks for chiming in.
> This addresses the issue of how to do good software design in GDB to 
> support different producers cleanly, but I think we have some issues 
> even before that, like how to support g++ 7.3 and up.  I'll try to 
> summarize the issue quickly.  It's now possible to end up with two 
> templated classes with the same name that differ only by the signedness 
> of their non-type template parameter.  One is Foo<int N> and the other 
> is Foo<unsigned int N> (the 10 is unsigned).  Until 7.3, g++ would 
> generate names like Foo<10> for the former and names like Foo<10u> for 
> the later (in the DW_AT_name attribute of the classes' DIEs).  Since 
> 7.3, it produces Foo<10> for both.
> When GDB wants to know the run time type of an object, it fetches the 
> pointer to its vtable, does a symbol lookup to get the linkage name and 
> demangles it, which gives a string like "vtable for Foo<10>" or "vtable 
> for Foo<10u>".  It strips the "vtable for " and uses the remainder to do 
> a type lookup.  Since g++ 7.3, you can see that doing a type lookup for 
> Foo<10> may find the wrong type, and doing a lookup for Foo<10u> won't 
> find anything.
> So the problem here is how to uniquely identify those two classes when 
> we are doing this run-time type finding operation (and probably in other 
> cases too).
> Simon

Hi all,

In the perspective of "type identity", the way I see it the issue has a 
few parts:

1) How GCC compiles such templates
2) How GCC emits debugging information via -g
3) How such information is interpreted (and merged with the compiled 
code) by GDB

Regarding 1) and 2), IMHO I think that there should be a one-to-one 
relationship between the compiled code output and debug info:

This means that if GCC compiles such templates into two different 
classes[1], it should generate two different type identifiers.
Conversely, if it compiles the templates into the same class, then a 
single identifier should be emitted for the single class compiled.
(This goes besides the point of what the standard dictates[2])

If I understand it right, currently the issue is that gcc emits two 
types with the same debug identifier.

Regarding 3), I think that after 1) and 2) are set up, GDB should be 
able to find the correct type definition (using the most appropriate 
design choice).

Hope this helps,

[1] According to the findings of Simon, this appears to be the case with 
clang, older GCC, and current GCC master. Do I understand this right?

[2] About handling both templates instantiation as a single class, I 
think that if GCC wants to emit a single class, then its argument type 
instantiation should be well-definined,i.e. independent of the order of 
declaration - see the findings from Simon earlier in this thread where 
you could get the program output either -10 or 4294967286 depending on 
which declaration would come first.

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