[PATCH v2] Makefile: support compressed debug info

[Nick Desaulniers <ndesaulniers@google.com>]

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

Suggested-by: David Blaikie <blakie@google.com>
Suggested-by: Fangrui Song <maskray@google.com>
Suggested-by: Nick Clifton <nickc@redhat.com>
Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V1 -> V2:
* rebase on linux-next.
* Add assembler flags as per Fangrui.
* Add note about KDEB_COMPRESS+scripts/package/builddeb
  as per Sedat and Masahiro.
* Add note about bintutils version requirements as per Nick C.
* Add note about measured increased build time and max RSS.

 Makefile          |  6 ++++++
 lib/Kconfig.debug | 15 +++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/Makefile b/Makefile
index 71687bfe1cd9..be8835296754 100644
--- 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)
+	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.
+
 config DEBUG_INFO_SPLIT
 	bool "Produce split debuginfo in .dwo files"
 	depends on DEBUG_INFO
-- 
2.26.2.761.g0e0b3e54be-goog