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

Roman Popov
2018-03-01T20:18:18Z

Is there any progress on this problem?

I'm not familiar with G++ , but I have little experience with LLVM. I can
try make LLVM emitting mangled names to DW_AT_name, instead of demangled
ones.
This way GDB can match DW_AT_name against RTTI. And for display it can
call abi::__cxa_demangle(name, NULL, NULL, &status), from #include
<cxxabi.h>.

Will it work?

Thanks, Roman

2018-02-08 7:05 GMT-08:00 Richard Biener <richard.guenther@gmail.com>:

> On Mon, Feb 5, 2018 at 6:06 AM, Simon Marchi <simon.marchi@polymtl.ca>
> wrote:
> > Hi Martin,
> >
> > Thanks for the reply.
> >
> > On 2018-02-04 02:17 PM, Martin Sebor wrote:
> >> Printing the suffix is unhelpful because it leads to unnecessary
> >> differences in diagnostics (even in non-template contexts). For
> >> templates with non-type template parameters there is no difference
> >> between, say A<1>, A<1U>, A<(unsigned) 1>, or even A<Green> when
> >> Green is an enumerator that evaluates to 1, so including the suffix
> >> serves no useful purpose.
> >
> > This is the part I don't understand. In Roman's example, spelling
> > foo<10> and foo<10u> resulted in two different instantiations of the
> > template, with different code. So that means it can make a difference,
> > can't it?
> >
> >> In the GCC test suite, it would tend to
> >> cause failures due to differences between the underlying type of
> >> common typedefs like size_t and ptrdiff_t. Avoiding these
> >> unnecessary differences was the main motivation for the change.
> >> Not necessarily just in the GCC test suite but in all setups that
> >> process GCC messages.
> >
> > Ok, I understand.
> >
> >> I didn't consider the use of auto as a template parameter but
> >> I don't think it changes anything. There, just like in other
> >> contexts, what's important is the deduced types and the values
> >> of constants, not the minute details of how they are spelled.
> >
> > Well, it seems like using decltype on a template constant value is
> > a way to make the type of constants important, in addition to their
> > value. I know the standard seems to say otherwise (what Manfred
> > quoted), but the reality seems different. I'm not a language expert
> > so I can't tell if this is a deficiency in the language or not.
> >
> >> That said, it wasn't my intention to make things difficult for
> >> the debugger.
> >
> > I hope so :).
> >
> >> But changing GCC back to include the suffix,
> >> even just in the debug info, isn't a solution. There are other
> >> compilers besides GCC that don't emit the suffixes, and there
> >> even are some that prepend a cast to the number, so if GDB is
> >> to be usable with all these kinds of producers it needs to be
> >> able to handle all of these forms.
> >
> > As I said earlier, there are probably ways to make GDB cope with it.
> > The only solution I saw (I'd like to hear about other ones) was to make
> > GDB ignore the template part in DW_AT_name and re-build it from the
> > DW_TAG_template_* DIEs in the format it expects. It can already do
> > that somewhat, because, as you said, some compilers don't emit
> > the template part in DW_AT_name.
> >
> > Doing so would cause major slowdowns in symbol reading, I've tried it
> > for the sake of experimentation/discussion. I have a patch available
> > on the "users/simark/template-suffix" branch in the binutils-gdb
> > repo [1]. It works for Roman's example, but running the GDB testsuite
> > shows that, of course, the devil is in the details.
> >
> > Consider something like this:
> >
> > template <int *P>
> > struct foo { virtual ~foo() {} };
> >
> > int n;
> >
> > int main ()
> > {
> > foo<&n> f;
> > }
> >
> >
> > The demangled name that GDB will be looking up is "foo<&n>". The
> > debug info about the template parameter only contains the resulting
> > address of n (the value of &n):
> >
> > <2><bf>: Abbrev Number: 11 (DW_TAG_template_value_param)
> > <c0> DW_AT_name : P
> > <c2> DW_AT_type : <0x1ac>
> > <c6> DW_AT_location : 10 byte block: 3 34 10 60 0 0 0 0 0 9f
> (DW_OP_addr: 601034; DW_OP_stack_value)
> >
> > I don't see how GDB could reconstruct the "&n" in the template, so
> > that's where my idea falls short.
>
> For other reasons I've always wanted sth like
>
> DW_OP_addr; DW_OP_name: n; DW_OP_stack_value
>
> thus put symbolical expressions in locations and have the consumer look
> them up
> (in context obviously). That way gdb can also choose to print foo<n>
> instead of
> foo<1> or foo<<optimized out>>.
>
> Of course that needs DWARF extensions.
>
> Richard.
>
> > Simon
> >
> > [1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;
> a=shortlog;h=refs/heads/users/simark/template-suffix
>