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SubjectRe: [PATCH v2] Makefile: support compressed debug info
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.
> +

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