Re: [debug-early] C++ clones and limbo DIEs

[Aldy Hernandez <aldyh at redhat dot com>]

Phew... ok, I'm a little stuck here with the interaction between 
dwarf2out and LTO, and I'm hoping y'all can shed some light.  Please 
bear with me through the verbosity, it gets clearer (I hope) near the end.

On 01/16/2015 12:45 PM, Jason Merrill wrote:
> On 01/16/2015 12:50 PM, Aldy Hernandez wrote:
> > > Can you remove the first flush and just do it in the second place?
> >
> > If I only flush the limbo list in the second place, that's basically
> > what mainline does, albeit abstracted into a function.  I thought the
> > whole point was to get rid of the limbo list, or at least keep it from
> > being a structure that has to go through LTO streaming.
>
> It would expect it to be before free_lang_data and LTO streaming.

The reason this wouldn't make a difference is because, as it stands, 
dwarf for the clones are not generated until final.c:

  if (!DECL_IGNORED_P (current_function_decl))
    debug_hooks->function_decl (current_function_decl);

which happens after free_lang_data.

However, we can generate early dwarf for the clones right from the 
parser, and things _mostly_ work:

diff --git a/gcc/cp/optimize.c b/gcc/cp/optimize.c
index 62e32d2..5539244 100644
--- a/gcc/cp/optimize.c
+++ b/gcc/cp/optimize.c
@@ -539,6 +539,11 @@ maybe_clone_body (tree fn)
       /* Start processing the function.  */
       start_preparsed_function (clone, NULL_TREE, SF_PRE_PARSED);

+      /* Generate early dwarf for the clone now that we have a body
+	 for it.  */
+	debug_hooks->early_global_decl (clone);
+
       /* Tell cgraph if both ctors or both dtors are known to have
 	 the same body.  */
       if (can_alias

And I say _mostly_ work, because now local statics will have the 
DECL_ABSTRACT_P bit set, making LTO think that things live in another 
partition, and are no longer trivially needed.  For instance:

struct TYPE { TYPE (int);  } ;

TYPE::TYPE (int some_argument)
{
  static int static_p = 5;
}

With the above patch, early dwarf generation will call gen_decl_die, 
which eventually does:

      /* If we're emitting a clone, emit info for the abstract 
instance.  */
      if (origin || DECL_ORIGIN (decl) != decl)
	dwarf2out_abstract_function (origin
				     ? DECL_ORIGIN (origin)
				     : DECL_ABSTRACT_ORIGIN (decl));

Where decl and DECL_ABSTRACT_ORIGIN(decl) are:
(gdb) p decl
$87 = <function_decl 0x7ffff030f0d8 __base_ctor >
(gdb) p decl.decl_common.abstract_origin
$88 = <function_decl 0x7ffff02fbe58 TYPE>

Now dwarf2out_abstract_function() will set DECL_ABSTRACT_P for all the 
children of the abstract origin:

  was_abstract = DECL_ABSTRACT_P (decl);
  set_decl_abstract_flags (decl, 1);
  dwarf2out_decl (decl);
  if (! was_abstract)
    set_decl_abstract_flags (decl, 0);

Unfortunately, this sets DECL_ABSTRACT_P for the "static_p" above, and 
refuses to unset it after the call to dwarf2out_decl.

Through some miraculous gymnastics, LTO streams out symbols without the 
"analyzed" bit set if things are not "in partition", which happens 
because symtab_node::get_partitioning_class() returns SYMBOL_EXTERNAL 
(ahem, not in partition) when the DECL_ABSTRACT_P bit is set.

So... dwarf2out_abstract_function() sets the DECL_ABSTRACT_P bit, and 
LTO thinks that local statics in a clone are in another partition, which 
causes the analyzed bit to be dropped on the floor, which causes 
symbol_table::remove_unreferenced_decls() to no longer think that local 
statics are trivially needed.  And if the local static is no longer 
needed, varpool will not output its definition in the assembly file.

I am very uncomfortable with a call to dwarf2out_abstract_function() 
changing the behavior of the LTO streamer (and the optimizers), but it 
looks like twiddling the DECL_ABSTRACT_P flag (and refusing to reset it) 
is by design.

I am all ears on solutions on either the LTO or the dwarf side.

Thanks.
Aldy