[Bug debug/48229] DW_TAG_type_unit has no DW_AT_producer

["jan.kratochvil at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48229

--- Comment #5 from Jan Kratochvil <jan.kratochvil at redhat dot com> 2011-03-28 09:12:20 UTC ---
Created attachment 23788
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23788
GDB patch attempt with ptype regressions.

(In reply to comment #4)
> Still I believe DW_TAG_type_unit should have DW_AT_producer as it is a
> standalone entity - but currently it is not.  I guess I have to implement your
> described althorithm into GDB instead.

It cannot work as GDB often looks up the type without any referrer from
DW_TAG_compile_unit, such as during the `ptype' GDB command.  A draft patch
thus has many regressions such as:
-PASS: gdb.base/nofield.exp: ptype struct not_empty
+FAIL: gdb.base/nofield.exp: ptype struct not_empty (GDB internal error)

This means GDB will have to start full read (like -readnow) of CUs till it
finds some CU referencing the specific type to find its DW_AT_producer.  During
scan of .debug_info for GDB partial symbols GDB currently skips over subtrees
of DIEs which reference the type signature for performance reasons, it no
longer can.

With .gdb_index there is no GDB partial symbols scan but .gdb_index also
indexes types by their name and there is no referrer ever seen during the
`ptype' GDB command.

So it means performance regression for non-.gdb_index case and a possible new
extension/version of .gdb_index to handle it.