[debug-early] equate new DIE with DW_AT_specificationto a previous declaration

[Aldy Hernandez <aldyh@redhat.com>]

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Hi Jason.

I merged mainline into the debug-early branch and I ran into a problem 
compiling a libstdc++ file with -fno-implicit-templates.  The attached 
patch is what I used to solve the problem but I wanted to run it by you, 
to make sure I'm not overlooking something silly, or worse...something 
much more complicated.

The reduced testcase is the following, compiled with 
-fno-implicit-templates -g -O2 -std=gnu++11:

class Object
{
public:
   void Method();
};

void funky()
{
   Object<int> foobar;
   foobar.Method();
}

template<typename SomeType>
void
Object<SomeType>::Method()
{
}

In mainline, we call gen_subprogram_die() twice for 
Object<int>::Method(): once, while generating class members, and once 
while inlining (outlining_inline_function hook).  The debug-early path 
is somewhat different, and we end up calling gen_subprogram_die() three 
times, the last of which ICEs.  What happens is the following...

We call gen_subprogram_die() as usual while generating class members, 
but then we call it again by virtue of it being a reachable function. 
This extra call will follow the DW_AT_specification code path because we 
have a previously cached die:

	  subr_die = new_die (DW_TAG_subprogram, context_die, decl);
	  add_AT_specification (subr_die, old_die);
           add_pubname (decl, subr_die);

The problem is that, for -fno-implicit-templates, the decl is now 
DECL_EXTERNAL, which means we never equate this new "DIE with 
DW_AT_specification" to the DECL.  That is, we never fall through here:

   else if (!DECL_EXTERNAL (decl))
     {
       HOST_WIDE_INT cfa_fb_offset;

       struct function *fun = DECL_STRUCT_FUNCTION (decl);

       if (!old_die || !get_AT (old_die, DW_AT_inline))
	equate_decl_number_to_die (decl, subr_die);

However, when we call gen_subprogram_die() the third time through the 
outlining_inline_function hook (late debug), we again try to add a 
DW_AT_specification to the DIE cached from the first time around, but 
this time we ICE because we're not supposed to have multiple 
DW_AT_specification's pointing to the same DIE (the old original DIE).

My solution is just to call equate_decl_number_to_die() as soon as we 
create the DW_AT_specification marked DIE the second time around.  The 
third time we will just pick up this last cached DIE with 
DW_AT_specification, mark it as DW_AT_inline, and voila, everything 
works.  The dwarf generation is as mainline, and we can build libstdc++ 
with no regressions to the guality testsuite.

Does this sound reasonable, or is this something a lot more complicated?

Thanks.
Aldy

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commit 0a49042b9151e0387efc5f87c32cb24968896ae4
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Tue Mar 17 12:29:27 2015 -0700

    Equate new DIE containing a DW_AT_specification, to the original
    declaration.

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 86815be..c7345d9 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -18809,6 +18809,8 @@ gen_subprogram_die (tree decl, dw_die_ref context_die)
 		add_type_attribute (subr_die, TREE_TYPE (TREE_TYPE (decl)),
 				    TYPE_UNQUALIFIED, context_die);
 	    }
+	  if (early_dwarf_dumping)
+	    equate_decl_number_to_die (decl, subr_die);
 	}
     }
   /* Anything else... create a brand new DIE.  */