[PATCH] Makefile: support compressed debug info

[Nick Desaulniers]

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

Suggested-by: David Blaikie <blakie@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <nick.desaulniers@xxxxxxxxx>
---
 Makefile          | 5 +++++
 lib/Kconfig.debug | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/Makefile b/Makefile
index 981eb902384b..313a054e5dc6 100644
--- a/Makefile
+++ b/Makefile
@@ -825,6 +825,11 @@ ifdef CONFIG_DEBUG_INFO_REDUCED
 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_LDFLAGS	+= --compress-debug-sections=zlib
+endif
 endif
 
 KBUILD_CFLAGS += $(DEBUG_CFLAGS)
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index f6f9a039f736..1f4a47ba6c1b 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -213,6 +213,15 @@ 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 $(ld-option,--compress-debug-sections=zlib)
+	help
+	  Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
+	  5.0+.
+
 config DEBUG_INFO_SPLIT
 	bool "Produce split debuginfo in .dwo files"
 	depends on DEBUG_INFO
-- 
2.17.1