[PATCH] dwarf2: avoid decoding .debug_line for type units

[Bratislav Filipovic <bfilipov@amd.com>]

Running gdb.dwarf2/pr13961.exp with clang fails with an internal error:

  gdb/dwarf2/read.c: internal-error: decode_line_header_for_cu:
  Assertion `!cu->per_cu->is_debug_types ()' failed.

pr13961 is a legacy .S test that references a .debug_line label from
both a CU and a TU.  The assembly includes an empty .debug_line section
declaration:

  .section .debug_line,"",%progbits
.Ldebug_line0:

With gcc this results in a dummy (valid, but content-less) .debug_line
header being emitted, so GDB can build a line header and resolve file
indices.  With clang the .debug_line section can be missing/empty,
leaving the line header unset.

During symbol creation, new_symbol may then try to lazily decode the
CU-only line header while processing a type unit, which triggers the
assertion above.

Fix this by only decoding the line header for non-type units.  If no
line header is available, emit a complaint and continue without setting
the symtab rather than attempting CU-only line decoding from a TU.

Tested: gdb.dwarf2/pr13961.exp (CC_FOR_TARGET=clang)
---
 gdb/dwarf2/read.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 8b87d58d..2e897e65 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -15484,23 +15484,32 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	  if (index_cst.has_value ())
 	    {
 	      file_name_index file_index = (file_name_index) *index_cst;
-	      struct file_entry *fe;
 
-	      if (file_cu->line_header == nullptr)
+	      /* Resolve DW_AT_decl_file/DW_AT_call_file.  These attributes are
+		 indices into the file-name table, which lives in the unit's
+		 decoded .debug_line header.
+		 decode_line_header_for_cu is CU-only and asserts on debug_types.
+		 For type units, the TU setup code runs before processing child
+		 DIEs, so if line_header is still nullptr here it means there is no
+		 usable line table for this unit.  */
+	      if (file_cu->line_header == nullptr
+		  && !file_cu->per_cu->is_debug_types ())
 		{
 		  file_and_directory fnd (nullptr, nullptr);
 		  decode_line_header_for_cu (file_cu->dies, file_cu, fnd);
 		}
 
 	      if (file_cu->line_header != nullptr)
-		fe = file_cu->line_header->file_name_at (file_index);
-	      else
-		fe = NULL;
-
-	      if (fe == NULL)
-		complaint (_("file index out of range"));
+		{
+		  if (file_entry *fe
+		      = file_cu->line_header->file_name_at (file_index);
+		      fe != nullptr)
+		    sym->set_symtab (fe->symtab (*file_cu));
+		  else
+		    complaint (_("file index out of range"));
+		}
 	      else
-		sym->set_symtab (fe->symtab (*file_cu));
+		complaint (_("missing .debug_line information to resolve file index"));
 	    }
 	}
 
-- 
2.43.0