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Date:   Wed, 20 May 2020 16:22:01 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Fangrui Song <maskray@google.com>,
        Nick Clifton <nickc@redhat.com>,
        David Blaikie <blakie@google.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Changbin Du <changbin.du@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2] Makefile: support compressed debug info
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On Wed, 20 May 2020 12:36:36 -0700 Nick Desaulniers <ndesaulniers@google.com> 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.
> 
> All of the debug sections will have a `C` flag set.
> $ readelf -S <object file>
> 
> $ bloaty vmlinux.gcc75.compressed.dwarf4 -- \
>     vmlinux.gcc75.uncompressed.dwarf4
> 
>     FILE SIZE        VM SIZE
>  --------------  --------------
>   +0.0%     +18  [ = ]       0    [Unmapped]
>  -73.3%  -114Ki  [ = ]       0    .debug_aranges
>  -76.2% -2.01Mi  [ = ]       0    .debug_frame
>  -73.6% -2.89Mi  [ = ]       0    .debug_str
>  -80.7% -4.66Mi  [ = ]       0    .debug_abbrev
>  -82.9% -4.88Mi  [ = ]       0    .debug_ranges
>  -70.5% -9.04Mi  [ = ]       0    .debug_line
>  -79.3% -10.9Mi  [ = ]       0    .debug_loc
>  -39.5% -88.6Mi  [ = ]       0    .debug_info
>  -18.2%  -123Mi  [ = ]       0    TOTAL
> 
> $ bloaty vmlinux.clang11.compressed.dwarf4 -- \
>     vmlinux.clang11.uncompressed.dwarf4
> 
>     FILE SIZE        VM SIZE
>  --------------  --------------
>   +0.0%     +23  [ = ]       0    [Unmapped]
>  -65.6%    -871  [ = ]       0    .debug_aranges
>  -77.4% -1.84Mi  [ = ]       0    .debug_frame
>  -82.9% -2.33Mi  [ = ]       0    .debug_abbrev
>  -73.1% -2.43Mi  [ = ]       0    .debug_str
>  -84.8% -3.07Mi  [ = ]       0    .debug_ranges
>  -65.9% -8.62Mi  [ = ]       0    .debug_line
>  -86.2% -40.0Mi  [ = ]       0    .debug_loc
>  -42.0% -64.1Mi  [ = ]       0    .debug_info
>  -22.1%  -122Mi  [ = ]       0    TOTAL
> 
> For x86_64 defconfig + LLVM=1 (before):
> Elapsed (wall clock) time (h:mm:ss or m:ss): 3:22.03
> Maximum resident set size (kbytes): 43856
> 
> For x86_64 defconfig + LLVM=1 (after):
> Elapsed (wall clock) time (h:mm:ss or m:ss): 3:32.52
> Maximum resident set size (kbytes): 1566776

I'm not sure who we're expecting to merge this, but I like shiny things ;)

> --- a/Makefile
> +++ b/Makefile
> @@ -822,6 +822,12 @@ DEBUG_CFLAGS	+= $(call cc-option, -femit-struct-debug-baseonly) \
>  		   $(call cc-option,-fno-var-tracking)
>  endif
>  
> +ifdef CONFIG_DEBUG_INFO_COMPRESSED
> +DEBUG_CFLAGS	+= -gz=zlib
> +KBUILD_AFLAGS	+= -Wa,--compress-debug-sections=zlib
> +KBUILD_LDFLAGS	+= --compress-debug-sections=zlib
> +endif
> +
>  KBUILD_CFLAGS += $(DEBUG_CFLAGS)
>  export DEBUG_CFLAGS
>  
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index b8f023e054b9..5a423cbfaea4 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -225,6 +225,21 @@ config DEBUG_INFO_REDUCED
>  	  DEBUG_INFO build and compile times are reduced too.
>  	  Only works with newer gcc versions.
>  
> +config DEBUG_INFO_COMPRESSED
> +	bool "Compressed debugging information"
> +	depends on DEBUG_INFO
> +	depends on $(cc-option,-gz=zlib)
> +	depends on $(as-option,-Wa,--compress-debug-sections=zlib)
> +	depends on $(ld-option,--compress-debug-sections=zlib)

huh, I didn't know kbuild/kconfig could do this.  Does it all work as
expected when cross-compiling?

> +	help
> +	  Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
> +	  5.0+, binutils 2.26+, and zlib.
> +
> +	  Users of dpkg-deb via scripts/package/builddeb may
> +	  wish to set the $KDEB_COMPRESS env var to "none" to avoid recompressing
> +	  the debug info again with a different compression scheme, which can
> +	  result in larger binaries.
> +