Makefile: support compressed debug info

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

Thanks to:
Nick Clifton helped us to provide the minimal binutils version.
Sedat Dilek found an increase in size of debug .deb package.

Cc: Nick Clifton <nickc@redhat.com>
Suggested-by: David Blaikie <blaikie@google.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -825,6 +825,12 @@ 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
 

lib/Kconfig.debug

@@ -213,6 +213,23 @@ 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 $(as-option,-Wa$(comma)--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 find an increase in
+	  size of their debug .deb packages with this config set, due to the
+	  debug info being compressed with zlib, then the object files being
+	  recompressed with a different compression scheme. But this is still
+	  preferable to setting $KDEB_COMPRESS to "none" which would be even
+	  larger.
+
 config DEBUG_INFO_SPLIT
 	bool "Produce split debuginfo in .dwo files"
 	depends on DEBUG_INFO