[PATCH] Allow dwarf2 debug info for function at address 0

[Frederic RISS <frederic dot riss at st dot com>]

Hi,

This is a follow-up to the "Problems with startup code symbols (Copious
warnings)" gdb@ thread found here:
http://sources.redhat.com/ml/gdb/2006-06/msg00051.html

The attached patch replaces the HAS_RELOC checks in dwarf2read.c with a
per-objfile lookup for a section loaded at 0. With this modification,
non-relocatable files that have a code at address 0 should be handled
better (no discarded debug information). 
As Daniel pointed out in the referenced thread, this solution isn't
perfect because you can have valid code at 0 and at the same time
'orphaned' debug info that seemingly references the 0 address. However
in the long run tools should correctly handle things like .gnu.linkonce,
and with correct debug info this patch looks like the right thing. (BTW,
was my testing defficient, or do recent GNU toolchains handle linkonce
sections and associated debug info correctly?)

I regression tested this on native i686-pc-linux-gnu.

Steven, by re-reading the thread that initiated this proposal I'm not
sure that this will really change something for you because the subject
had a bit diverged from your particular problem. Nevertheless could you
still give it a try to see if it helps with your warning issue?

:ADDPATCH Dwarf2:

(Nice side-effect of the patch tracker: It reminded me that I needed to
actually attach the patch :-) )

Cheers,
Fred

2006-07-21  Frederic Riss  <frederic.riss@st.com>

	* dwarf2read.c (struct dwarf2_per_objfile): Add has_section_at_zero 
	field.
	(dwarf2_has_info): Initialize dwarf2_per_objfile->has_section_at_zero.
	(dwarf2_get_pc_bounds): Use dwarf2_per_objfile->has_section_at_zero 
	instead of HAS_RELOC test.
	(read_partial_die): Ditto.


Index: dwarf2read.c
===================================================================
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.200
diff -u -p -r1.200 dwarf2read.c
--- dwarf2read.c	12 Jul 2006 21:14:57 -0000	1.200
+++ dwarf2read.c	20 Jul 2006 16:24:42 -0000
@@ -180,6 +180,10 @@ struct dwarf2_per_objfile
   /* A chain of compilation units that are currently read in, so that
      they can be freed later.  */
   struct dwarf2_per_cu_data *read_in_chain;
+
+  /* A flag indicating wether this objfile has a section loaded at a
+     VMA of 0.  */
+  int has_section_at_zero;
 };
 
 static struct dwarf2_per_objfile *dwarf2_per_objfile;
@@ -1083,6 +1087,7 @@ int
 dwarf2_has_info (struct objfile *objfile)
 {
   struct dwarf2_per_objfile *data;
+  asection *lower_sect = NULL;
 
   /* Initialize per-objfile state.  */
   data = obstack_alloc (&objfile->objfile_obstack, sizeof (*data));
@@ -1090,6 +1095,10 @@ dwarf2_has_info (struct objfile *objfile
   set_objfile_data (objfile, dwarf2_objfile_data_key, data);
   dwarf2_per_objfile = data;
 
+  bfd_map_over_sections (objfile->obfd, find_lowest_section, &lower_sect);
+  if (lower_sect && bfd_section_vma (objfile->obfd, lower_sect) == 0)
+    data->has_section_at_zero = 1;
+
   dwarf_info_section = 0;
   dwarf_abbrev_section = 0;
   dwarf_line_section = 0;
@@ -1099,7 +1108,7 @@ dwarf2_has_info (struct objfile *objfile
   dwarf_eh_frame_section = 0;
   dwarf_ranges_section = 0;
   dwarf_loc_section = 0;
-  
+
   bfd_map_over_sections (objfile->obfd, dwarf2_locate_sections, NULL);
   return (dwarf_info_section != NULL && dwarf_abbrev_section != NULL);
 }
@@ -3177,7 +3186,7 @@ dwarf2_get_pc_bounds (struct die_info *d
      labels are not in the output, so the relocs get a value of 0.
      If this is a discarded function, mark the pc bounds as invalid,
      so that GDB will ignore it.  */
-  if (low == 0 && (bfd_get_file_flags (obfd) & HAS_RELOC) == 0)
+  if (low == 0 && !dwarf2_per_objfile->has_section_at_zero)
     return 0;
 
   *lowpc = low;
@@ -5505,7 +5514,7 @@ read_partial_die (struct partial_die_inf
   if (has_low_pc_attr && has_high_pc_attr
       && part_die->lowpc < part_die->highpc
       && (part_die->lowpc != 0
-	  || (bfd_get_file_flags (abfd) & HAS_RELOC)))
+	  || dwarf2_per_objfile->has_section_at_zero))
     part_die->has_pc_info = 1;
   return info_ptr;
 }