Re: [PATCH] Makefile: support compressed debug info

[Fangrui Song]

On 2020-05-12, Nick Desaulniers wrote:
> On Mon, May 11, 2020 at 10:54 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> > > >On Mon, May 4, 2020 at 5:13 AM Nick Desaulniers
> > > ><nick.desaulniers@xxxxxxxxx> wrote:
> > > >>
> > > >> As debug information gets larger and larger, it helps significantly save
> > > >> the size of vmlinux images to compress the information in the debug
> > > >> information sections. Note: this debug info is typically split off from
> > > >> the final compressed kernel image, which is why vmlinux is what's used
> > > >> in conjunction with GDB. Minimizing the debug info size should have no
> > > >> impact on boot times, or final compressed kernel image size.
> > > >>
> > Nick,
> >
> > I am OK with this patch.
> >
> > Fangrui provided the minimal requirement for
> > --compress-debug-sections=zlib
> >
> >
> > Is it worth recording in the help text?
> > Do you want to send v2?
>
> Yes I'd like to record that information.  I can also record Sedat's
> Tested-by tag.  Thank you for testing Sedat.
>
> I don't know what "linux-image-dbg file" are, or why they would be
> bigger.  The size of the debug info is the primary concern with this
> config.  It sounds like however that file is created might be
> problematic.
>
> Fangrui, I wasn't able to easily find what version of binutils first
> added support.  Can you please teach me how to fish?

I actually downloaded https://ftp.gnu.org/gnu/binutils/ archives and
located the sources... I think an easier way is:

% cd binutils-gdb
% git show binutils-2_26:./gas/as.c | grep compress-debug-sections
--compress-debug-sections[={none|zlib|zlib-gnu|zlib-gabi}]\n\
...

GNU as 2.25 only supports --compress-debug-sections which means "zlib-gnu" in
newer versions.

Similarly, for GNU ld:

% git show binutils-2_26:./ld/lexsup.c | grep compress-debug-sections
  --compress-debug-sections=[none|zlib|zlib-gnu|zlib-gabi]\n\

(I have spent a lot of time investigating GNU ld's behavior :)

> Another question I had for Fangrui is, if the linker can compress
> these sections, shouldn't we just have the linker do it, not the the
> compiler and assembler?  IIUC the debug info can contain relocations,
> so the linker would have to decompress these, perform relocations,
> then recompress these?  I guess having the compiler and assembler
> compress the debug info as well would minimize the size of the .o
> files on disk.

The linker will decompress debug info unconditionally. Because
input .debug_info sections need to be concatenated to form the output
.debug_info . Whether the output .debug_info is compressed is controlled
by the linker option --compress-debug-sections=zlib, which is not
affected by the compression state of object files.

Both GNU as and GNU ld name the option --compress-debug-sections=zlib.
In a compiler driver context, an unfamiliar user may find
-Wa,--compress-debug-sections=zlib -Wl,--compress-debug-sections=zlib
confusing:/

> Otherwise I should add this flag to the assembler invocation, too, in
> v2.  Thoughts?

Compressing object files along with the linked output should be fine. It
can save disk space. (It'd be great if you paste the comparison
with and w/o object files compressed)

Feel free to add:

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> I have a patch series that enables dwarf5 support in the kernel that
> I'm working up to.  I wanted to send this first.  Both roughly reduce
> the debug info size by 20% each, though I haven't measured them
> together, yet.  Requires ToT binutils because there have been many
> fixes from reports of mine recently.

This will be awesome! I also heard that enabling DWARF v5 for our object
files can easily make debug info size smaller by 20%. Glad that the
kernel can benefit it as well:)