[PATCH] D82975: [DebugInfo] Allow GNU macro extension to be emitted

[David Blaikie via Phabricator via llvm-commits]

dblaikie added a comment.

In D82975#2134347 <https://reviews.llvm.org/D82975#2134347>, @probinson wrote:

> In D82975#2134132 <https://reviews.llvm.org/D82975#2134132>, @dblaikie wrote:
>
> > In D82975#2128353 <https://reviews.llvm.org/D82975#2128353>, @dstenb wrote:
> >
> > > In D82975#2127201 <https://reviews.llvm.org/D82975#2127201>, @SouraVX wrote:
> > >
> > > > I think if it's about compatibility(analogous behavior with GCC), existing infra is Okay/Fine(Since same encodings are used). We just need to emit the `.debug_macro` section with `version` 4 and teach the `llvm-dwarfdump` to parse it correctly.
> > >
> > >
> > > One difference though is that the GNU extension does not have anything like the strx entries that LLVM currently emits: https://github.com/gcc-mirror/gcc/blob/master/include/dwarf2.h#L425, so I assume we still need code to emit the strp entries when targeting DWARF 4?
> >
> >
> > Likely - but might want to check what GCC does - maybe it uses some kind of strx encoding that's not documented, etc.
>
>
> My recollection is that .debug_macro was invented independently of the strx forms so the prototype probably wouldn't have used them.  Easy enough to check whether GCC's `-fdebug-macro` with v4 is emitting a .debug_str_offsets section.
>
> LLVM wouldn't be using strx forms from .debug_info for v4, and would have no other reason to emit .debug_str_offsets, so I wouldn't want LLVM to use them in a v4 compatibility mode .debug_macro section either.


GCC certainly seems to produce some kind of debug_macro.dwo section (& binutils dwp supports it in the index, if I recall correctly) using some form llvm-dwarfdump currently doesn't understand:

  $ g++-tot -g3 main.cpp -c -gsplit-dwarf && llvm-objdump -h main.dwo | grep " \.debug"
    1 .debug_info.dwo        0000003c 0000000000000000 
    2 .debug_abbrev.dwo      0000003e 0000000000000000 
    3 .debug_macro.dwo       0000001e 0000000000000000 
    4 .debug_macro.dwo       00000364 0000000000000000 
    5 .debug_macro.dwo       00000013 0000000000000000 
    6 .debug_line.dwo        00000048 0000000000000000 
    7 .debug_str_offsets.dwo 000002d5 0000000000000000 
    8 .debug_str.dwo         00000e05 0000000000000000 
  $ llvm-dwarfdump-tot main.dwo -debug-macro
  main.dwo:       file format elf64-x86-64
  
  .debug_macro.dwo contents:
  0x00000000:
   - lineno: 19 macro: 
  DW_MACINFO_invalid

I mean, I don't have strong feelings about supporting macro debug info in general, but if someone feels strongly about debug_macro GNU extension DWARFv4 support, there's certainly some GCC behavior that one could use to model the Split DWARF support for that off.


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