Re: [PATCH] Makefile: support compressed debug info

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, May 12, 2020 at 1:01 PM Fangrui Song <maskray@google.com> wrote:
>
> >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

This assumes you knew to look at the binutils-2_26 tag, which is
putting the cart before the horse. ;)

I guess:
$ git log gas/as.c
/compress-debug-sections
commit 19a7fe52ae3d ("Make default compression gABI compliant")
looks related
$ git describe --contains "19a7fe52ae3d" | sed 's/~.*//'
users/hjl/linux/release/2.25.51.0.4
so it landed in 2.25.51.0.4.

+ Nick, H.J.
I'm unfamiliar with the git tag conventions of binutils.  Does a patch
that landed in 2.25.51.0.4 mean it shipped in the official 2.25
release, or 2.26 release?  Specifically, commit 19a7fe52ae3d.

> --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:/

The kernel uses the compiler as the driver for out of line assembly,
as they are all preprocessed first.  Most out of line assembly in the
kernel uses the C preprocessor to #include headers that share #defines
of common constants shared between C and asm.  #ifdef __ASSEMBLY__ is
used frequently in these headers.  But for the linker, the linker
itself is invoked as the driver, though there are a few
inconsistencies we've cleaned up or still have to.

>
> >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@google.com>

Thanks, will add that to v2.

>
> >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:)

-- 
Thanks,
~Nick Desaulniers