Commit 866ced95 authored Jul 30, 2014 by Andi Kleen Committed by Michal Marek Jul 30, 2014

kbuild: Support split debug info v4

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>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

parent 011bf125

.gitignore

+1 −0

Original line number	Diff line number	Diff line
		@@ -34,6 +34,7 @@
		*.gcno
		modules.builtin
		Module.symvers
		*.dwo

		#
		# Top-level generic files

Makefile

+5 −0

Original line number	Diff line number	Diff line
		@@ -684,7 +684,11 @@ 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

		@@ -1372,6 +1376,7 @@ 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.' \

lib/Kconfig.debug

+15 −0

Original line number	Diff line number	Diff line
		@@ -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

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