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

["Filipovic, Bratislav" <Bratislav.Filipovic@amd.com>]

[AMD Official Use Only - AMD Internal Distribution Only]

PING
Can someone take a look at this?

Regards Bratislav

-----Original Message-----
From: Filipovic, Bratislav <Bratislav.Filipovic@amd.com>
Sent: Wednesday, March 18, 2026 2:36 PM
To: gdb-patches@sourceware.org
Cc: simon.marchi@efficios.com; Filipovic, Bratislav <Bratislav.Filipovic@amd.com>
Subject: [PATCH] dwarf2: avoid decoding .debug_line for type units

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