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

Manfred
2018-02-07T16:19:56Z

On 2/7/2018 4:15 PM, Jonathan Wakely wrote:
> On 7 February 2018 at 15:07, Manfred <mx2927@gmail.com> wrote:
>>
>>
>> On 02/07/2018 02:44 PM, Simon Marchi wrote:
>>>
[...]
>>>
>>> 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.
>
> What do you mean by "such templates"? There have been several
> different examples in the thread, which should be handled differently.

From Roman 2/3/2018
#include <iostream>
struct base {
virtual void print() = 0;
};

template< auto IVAL>
struct foo : base {
decltype(IVAL) x = -IVAL;
void print() override { std::cout << x << std::endl; };
};

From Simon 2/4/2018
base * fi = new foo<10>();
base * fu = new foo<10u>();

You are right that the original thread was started by Roman with:

struct base { virtual ~base(){} };

template< int IVAL, unsigned UVAL, unsigned long long ULLVAL>
struct derived : base {
int x = IVAL + + UVAL + ULLVAL;
};

>
>> 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,
>
> Not really :-)
Sorry for that :-)

>
> You're basically just saying "GCC and GDB should do the right thing"
> which is a statement of the obvious.
Besides the obvious, the main point was:
"IMHO I think that there should be a one-to-one relationship between the
compiled code output and debug info"
and:
"If I understand it right, currently the issue is that gcc emits two
types with the same debug identifier."

Which was an attempt to help by making obvious what I understood was
going wrong.

>
>
>> [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?
>
> As I said above, it's not clear what you're referring to.
I had in mind foo<10> and foo<10u>

After your remark, I realize I should have left out "older GCC" because
'auto' does not apply to it - older GCC dealt with the initial example:
template< int IVAL, unsigned UVAL, unsigned long long ULLVAL>

>
>> [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.
>
> That's just a GCC 7 bug in the handling of auto template parameters,
> see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79092
> It's not really relevant here, and is already fixed on trunk.
>
Thanks for pointing this out.
If I understand it correctly, the solution of the bug is that foo<10>
and foo<10u> result in two different classes
(according to your comment #1 in the bug, which by the way I am not sure
how it plays with the wording of the standard, but that's beyond gdb
compatibility)

Has -g type identification been differentiated too?

Getting back to auto/non-auto template arguments, I understand they are
different subjects, still, though, type identification in debug info
applies to both.

I am just a C++ gcc and gdb user, so I am missing their internals.
I think I'll keep quiet if this leads to unintentional noise.