Re: [PATCH 1/2] kbuild: Support split debug info v4

[Sam Ravnborg <sam@ravnborg.org>]

On Wed, Jul 30, 2014 at 08:50:18PM +0200, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> This is an alternative approach to lower the overhead of debug info
> (as we discussed a few days ago)
> 
> gcc 4.7+ and newer binutils have a new "split debug info" debug info
> model where the debug info is only written once into central ".dwo" files.
> 
> This avoids having to copy it around multiple times, from the object
> files to the final executable. It lowers the disk space
> requirements. In addition it defaults to compressed debug data.
> 
> More details here: http://gcc.gnu.org/wiki/DebugFission
> 
> This patch adds a new option to enable it. It has to be an option,
> because it'll undoubtedly break everyone's debuginfo packaging scheme.
> gdb/objdump/etc. all still work, if you have new enough versions.
> 
> I don't see big compile wins (maybe a second or two faster or so), but the
> object dirs with debuginfo get significantly smaller. My standard kernel
> config (slightly bigger than defconfig) shrinks from 2.9G disk space
> to 1.1G objdir (with non reduced debuginfo). I presume if you are IO limited
> the compile time difference will be larger.
> 
> Only problem I've seen so far is that it doesn't play well with older
> versions of ccache (apparently fixed, see
> https://bugzilla.samba.org/show_bug.cgi?id=10005)
> 
> v2: various fixes from Dirk Gouders. Improve commit message slightly.
> v3: Fix clean rules and improve Kconfig slightly
> v4: Fix merge error in last version (Sam Ravnborg)
>     Clarify description that it mainly helps disk size.
> Cc: Dirk Gouders <dirk@gouders.net>
> Cc: mmarek@suse.cz
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
> ---
>  .gitignore        |  1 +
>  Makefile          |  5 +++++
>  lib/Kconfig.debug | 15 +++++++++++++++
>  3 files changed, 21 insertions(+)
> 
> diff --git a/.gitignore b/.gitignore
> index f4c0b09..e213b27 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -34,6 +34,7 @@
>  *.gcno
>  modules.builtin
>  Module.symvers
> +*.dwo
>  
>  #
>  # Top-level generic files
> diff --git a/Makefile b/Makefile
> index f3c543d..6a717cd 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -689,7 +689,11 @@ endif
>  endif
>  
>  ifdef CONFIG_DEBUG_INFO
> +ifdef CONFIG_DEBUG_INFO_SPLIT
> +KBUILD_CFLAGS   += $(call cc-option, -gsplit-dwarf, -g)
> +else
>  KBUILD_CFLAGS	+= -g
> +endif
>  KBUILD_AFLAGS	+= -Wa,-gdwarf-2
>  endif
>  
> @@ -1377,6 +1381,7 @@ clean: $(clean-dirs)
>  	@find $(if $(KBUILD_EXTMOD), $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
>  		\( -name '*.[oas]' -o -name '*.ko' -o -name '.*.cmd' \
>  		-o -name '*.ko.*' \
> +		-o -name '*.dwo'  \
>  		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \
>  		-o -name '*.symtypes' -o -name 'modules.order' \
>  		-o -name modules.builtin -o -name '.tmp_*.o.*' \
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 7a638aa..0360468 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -143,6 +143,21 @@ config DEBUG_INFO_REDUCED
>  	  DEBUG_INFO build and compile times are reduced too.
>  	  Only works with newer gcc versions.
>  
> +config DEBUG_INFO_SPLIT
> +	bool "Produce split debuginfo in .dwo files"
> +	depends on DEBUG_INFO
> +	help
> +	  Generate debug info into separate .dwo files. This significantly
> +	  reduces the build directory size for builds with DEBUG_INFO,
> +	  because it stores the information only once on disk in .dwo
> +	  files instead of multiple times in object files and executables.
> +	  In addition the debug information is also compressed.
> +
> +	  Requires recent gcc (4.7+) and recent gdb/binutils.
> +	  Any tool that packages or reads debug information would need
> +	  to know about the .dwo files and include them.
> +	  Incompatible with older versions of ccache.
> +
>  config ENABLE_WARN_DEPRECATED
>  	bool "Enable __deprecated logic"
>  	default y
> -- 
> 1.8.5.2
> 
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