[Bug debug/98796] Incorrect .debug_line emitted for DWARF5

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98796

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:b485fa167ef35c8facbd7c21cb86fd1abc77efcf

commit r11-6868-gb485fa167ef35c8facbd7c21cb86fd1abc77efcf
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 22 22:37:36 2021 +0100

    dwarf2out: Always emit required 0 entries for DWARF 5 in *.debug_line
[PR98796]

    When GCC is emitting .debug_line or .gnu.debuglto_.debug_line section by
    itself (happens either with too old or non-GNU assembler, with
    -gno-as-loc-support or with -flto) on empty translation units, it violates
    the DWARF 5 requirements.
    The standard says:
    "The first entry is the current directory of the compilation."
    and a few lines later:
    "The first entry in the sequence is the primary source file whose file name
    exactly matches that given in the DW_AT_name attribute in the compilation
    unit debugging information entry."
    GCC emits 4 zeros (directory entry format count, directories count,
    filename entry format count and filename count), which would be ok if the
    spec said The first entry may be rather than is.

    I had a brief look at whether I could just fall through into the rest of
the
    function, but there are too many assumptions that there is at least one
    normal file that it can't be done that way easily.

    So this patch instead extends the early out code to emit the required
    minimum, which is 15 bytes more than we used to emit before.

    2021-01-22  Jakub Jelinek  <jakub@redhat.com>

            PR debug/98796
            * dwarf2out.c (output_file_names): For -gdwarf-5, if there are no
            filenames to emit, still emit the required 0 index directory and
            filename entries that match DW_AT_comp_dir and DW_AT_name of the
            compilation unit.