[PATCH] Fix for the complaint observed when symbol reading due to unsupported .debug_names form.

[nitachra <Nitika.Achra@amd.com>]

Thanks Tom for the review. Incorporated the review comment.
Thanks Tom de Vries for adding the test case. This test fails before the patch 
and pass afterward.

---
Following complaint is observed with the executable compiled with -gdwarf-5
and -gpubnames flags- "During symbol reading: Unsupported .debug_names form
DW_FORM_ref4". This is the form corresponding to DW_IDX_die_offset attribute.
This patch fixes this complaint. Tested with clang 10.0.0. Test case used-

int main()
{
int sum,a,b;
sum = a + b;
return sum;
}

clang -gdwarf-5 -gpubnames test.c -o test.out

gdb -q test.out -ex "set complaints 1" -ex "start"

Reading symbols from test.out...
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 [in module
/home/nitika/workspace/test.out] Temporary breakpoint 1 at 0x400484
Starting program: /home/nitika/workspace/test.out
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 [in module
/home/nitika/workspace/test.out]
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 [in module
/home/nitika/workspace/test.out]
During symbol reading: Unsupported .debug_names form DW_FORM_ref4 [in module
/home/nitika/workspace/test.out]

gdb/dwarf2/ChangeLog:

*read.c (dw2_debug_names_iterator::next): Handle DW_FORM_ref*
 and DW_IDX_die_offset. If there is no compilation unit attribute in the index
 entry, then there is a single CU. Return the CU at O index of compilation
 unit vector.
---
 gdb/dwarf2/read.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 1813085d0d..08a3bed1a9 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -5352,6 +5352,18 @@ dw2_debug_names_iterator::next ()
 	  ull = read_unsigned_leb128 (abfd, m_addr, &bytes_read);
 	  m_addr += bytes_read;
 	  break;
+	case DW_FORM_ref4:
+	  ull = read_4_bytes (abfd, m_addr);
+	  m_addr += 4;
+	  break;
+	case DW_FORM_ref8:
+	  ull = read_8_bytes (abfd, m_addr);
+	  m_addr += 8;
+	  break;
+	case DW_FORM_ref_sig8:
+	  ull = read_8_bytes (abfd, m_addr);
+	  m_addr += 8;
+	  break;
 	default:
 	  complaint (_("Unsupported .debug_names form %s [in module %s]"),
 		     dwarf_form_name (attr.form),
@@ -5384,6 +5396,12 @@ dw2_debug_names_iterator::next ()
 	    }
 	  per_cu = &dwarf2_per_objfile->get_tu (ull)->per_cu;
 	  break;
+	case DW_IDX_die_offset:
+	  /* In a per_cu index, index entries without CU attribute implicitly
+	     refer to the single CU.  */
+	  if (per_cu == NULL)
+	    per_cu = dwarf2_per_objfile->get_cu (0);
+	  break;
 	case DW_IDX_GNU_internal:
 	  if (!m_map.augmentation_is_gdb)
 	    break;
-- 
2.17.1