Re: [PATCH dwarves 0/2] dwarf_loader: improve cus__merging_cu()

[Bill Wendling <morbo@google.com>]

On Thu, Apr 1, 2021 at 12:35 PM Bill Wendling <morbo@google.com> wrote:
>
> On Wed, Mar 31, 2021 at 7:58 PM Yonghong Song <yhs@fb.com> wrote:
> >
> > Function cus__merging_cu() is introduced in Commit 39227909db3c
> > ("dwarf_loader: Permit merging all DWARF CU's for clang LTO built
> > binary") to test whether cross-cu references may happen.
> > The original implementation anticipates compilation flags
> > in dwarf, but later some concerns about binary size surfaced
> > and the decision is to scan .debug_abbrev as a faster way
> > to check cross-cu references. Also putting a note in vmlinux
> > to indicate whether lto is enabled for built or not can
> > provide a much faster way.
> >
> > This patch set implemented this two approaches, first
> > checking the note (in Patch #2), if not found, then
> > check .debug_abbrev (in Patch #1).
> >
> > Yonghong Song (2):
> >   dwarf_loader: check .debug_abbrev for cross-cu references
> >   dwarf_loader: check .notes section for lto build info
> >
> >  dwarf_loader.c | 76 ++++++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 55 insertions(+), 21 deletions(-)
> >
> With this series of patches, the compilation passes for me with
> ThinLTO. You may add this if you like:
>
> Tested-by: Bill Wendling <morbo@google.com>

I did notice these warnings following the "pahole -J .tmp_vmlinux.btf"
command. I don't know the severity of them, but it might be good to
investigate.

$ ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
  BTFIDS  vmlinux
WARN: multiple IDs found for 'inode': 355, 8746 - using 355
WARN: multiple IDs found for 'file': 588, 8779 - using 588
WARN: multiple IDs found for 'path': 411, 8780 - using 411
WARN: multiple IDs found for 'seq_file': 1414, 8836 - using 1414
WARN: multiple IDs found for 'vm_area_struct': 538, 8873 - using 538
WARN: multiple IDs found for 'task_struct': 28, 8880 - using 28
WARN: multiple IDs found for 'inode': 355, 9484 - using 355
WARN: multiple IDs found for 'file': 588, 9517 - using 588
WARN: multiple IDs found for 'path': 411, 9518 - using 411
WARN: multiple IDs found for 'seq_file': 1414, 9578 - using 1414
WARN: multiple IDs found for 'vm_area_struct': 538, 9615 - using 538
WARN: multiple IDs found for 'task_struct': 28, 9622 - using 28
WARN: multiple IDs found for 'seq_file': 1414, 12223 - using 1414
WARN: multiple IDs found for 'file': 588, 12237 - using 588
WARN: multiple IDs found for 'path': 411, 12238 - using 411
...

-bw