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

[Simon Marchi <simark@simark.ca>]

On 4/23/26 12:37 PM, Bratislav Filipovic wrote:
> 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-23)

There are some whitespace issues.  I would suggest setting up pre-commit
and running it:

    $ pre-commit run --all-files
    ...
    check-whitespace.........................................................Failed
    - hook id: check-whitespace
    - exit code: 2

    gdb/dwarf2/read.c:15433: indent with spaces.
    +                      struct symbol *sym)
    gdb/dwarf2/read.c:15440: indent with spaces.
    +                   cu);
    gdb/dwarf2/read.c:15447: indent with spaces.
    +                      &file_cu);


In fact, those warnings are about lines that shouldn't be changed by
this patch.

> @@ -15452,16 +15452,27 @@ new_symbol_file_line (struct die_info *die, struct dwarf2_cu *cu,
>    if (!index_cst.has_value ())
>      return;
>  
> -  if (file_cu->line_header == nullptr)
> +  /* 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.  */

Just a small suggestion to be more precise and less repetitive:

  /* decode_line_header_for_cu is CU-only and asserts on type units.
     For type units, the line header setup runs before processing child
     DIEs, in method dwarf2_cu::setup_type_unit_groups.  So if
     line_header is still nullptr here it means there is no usable line
     table for this unit.  */

With those fixed, the patch LGTM.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

Simon