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

[Simon Marchi <simon.marchi@polymtl.ca>]

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.

Simon

[1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/template-suffix