Re: [PATCH] Fix macro info lookup for binaries containing DWARFv5 line table

[Simon Marchi via Gdb-patches <gdb-patches@sourceware.org>]

> One thing I noticed that changed is that GCC is now emitting `dir0` entry in line table as per DWARF5. However this still seems
> Incomplete to me:
> $llvm-dwarfdump --debug-line a.out
> ```
> include_directories[  1] = "/usr/include" // index still starting from `1` even at "-gdwarf-5"
> file_names[  1]:
>            name: "macro.c"
>       dir_index: 0               // -> dir 0 mentioned but dir 0 is not present in list of directories, i.e index still starting from 1
>        mod_time: 0x00000000
>          length: 0x00000000
> file_names[  2]:
>            name: "stdc-predef.h"
>       dir_index: 1
>        mod_time: 0x00000000
>          length: 0x00000000
> ```

It's not clear how you produced this output (which compiler and which
command).  What's the version of that section?  Because it looks like a
DWARF 4 or before section.  GCC 10 produces a DWARF 3 .debug_line
section, even when using -gdwarf-5.

In DWARF 4 and before, a dir_index of 0 referred to the compilation
directory, even if there wasn't an explicit dir[0] value.  See section
6.2.4, bullet 12, of DWARF4.pdf.  So you would look up the DW_AT_comp
attribute of the compilation unit, append "macro.c", and that would give
you the location of that file.

On my side, I see this with gcc 11 (from the Ubuntu 21.04 package):

    root@bc7daa9a3ffc:/# gcc-11 --version
    gcc-11 (Ubuntu 11.1.0-1ubuntu1~21.04) 11.1.0
    root@bc7daa9a3ffc:/# cat test.c
    #include <stdio.h>

    #define FOO

    int main() {
    }
    root@bc7daa9a3ffc:/# gcc-11 -gdwarf-5 -g3 test.c
    root@bc7daa9a3ffc:/# llvm-dwarfdump-12 --debug-line a.out
    ...
    include_directories[  0] = "/usr/include"
    include_directories[  1] = "/usr/include"
    include_directories[  2] = "/usr/include/x86_64-linux-gnu/bits"
    include_directories[  3] = "/usr/include/x86_64-linux-gnu/sys"
    include_directories[  4] = "/usr/include/x86_64-linux-gnu/gnu"
    include_directories[  5] = "/usr/lib/gcc/x86_64-linux-gnu/11/include"
    include_directories[  6] = "/usr/include/x86_64-linux-gnu/bits/types"
    file_names[  0]:
	       name: "test.c"
	  dir_index: 0
    file_names[  1]:
	       name: "test.c"
	  dir_index: 0
    file_names[  2]:
	       name: "stdc-predef.h"
	  dir_index: 1
    file_names[  3]:
	       name: "stdio.h"
	  dir_index: 1

So from what I see, GCC 11 generates a dir[0] entry and a file[0] entry.
However, the dir[0] value does not look good.  It makes it appear
as if my source file was at /usr/include/test.c, which it wasn't (it was
at /test.c).

Simon