[PATCH v3] Makefile.debug: support for -gz=zstd

[Nick Desaulniers <ndesaulniers@google.com>]

Make DEBUG_INFO_COMPRESSED a choice; DEBUG_INFO_COMPRESSED_NONE is the
default, DEBUG_INFO_COMPRESSED_ZLIB uses zlib,
DEBUG_INFO_COMPRESSED_ZSTD uses zstd.

This renames the existing KConfig option DEBUG_INFO_COMPRESSED to
DEBUG_INFO_COMPRESSED_ZLIB so users upgrading may need to reset the new
Kconfigs.

Some quick N=1 measurements with du, /usr/bin/time -v, and bloaty:

clang-16, x86_64 defconfig plus
CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_COMPRESSED_NONE=y:
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:55.43
488M vmlinux
27.6%   136Mi   0.0%       0    .debug_info
 6.1%  30.2Mi   0.0%       0    .debug_str_offsets
 3.5%  17.2Mi   0.0%       0    .debug_line
 3.3%  16.3Mi   0.0%       0    .debug_loclists
 0.9%  4.62Mi   0.0%       0    .debug_str

clang-16, x86_64 defconfig plus
CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_COMPRESSED_ZLIB=y:
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:00.35
385M vmlinux
21.8%  85.4Mi   0.0%       0    .debug_info
 2.1%  8.26Mi   0.0%       0    .debug_str_offsets
 2.1%  8.24Mi   0.0%       0    .debug_loclists
 1.9%  7.48Mi   0.0%       0    .debug_line
 0.5%  1.94Mi   0.0%       0    .debug_str

clang-16, x86_64 defconfig plus
CONFIG_DEBUG_INFO=y CONFIG_DEBUG_INFO_COMPRESSED_ZSTD=y:
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:59.69
373M vmlinux
21.4%  81.4Mi   0.0%       0    .debug_info
 2.3%  8.85Mi   0.0%       0    .debug_loclists
 1.5%  5.71Mi   0.0%       0    .debug_line
 0.5%  1.95Mi   0.0%       0    .debug_str_offsets
 0.4%  1.62Mi   0.0%       0    .debug_str

That's only a 3.11% overall binary size savings over zlib, but at no
performance regression.

Link: https://maskray.me/blog/2022-09-09-zstd-compressed-debug-sections
Link: https://maskray.me/blog/2022-01-23-compressed-debug-sections
Suggested-by: Sedat Dilek (DHL Supply Chain) <sedat.dilek@dhl.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v2 -> v3:
* Fix scripts/Makefile.debug as per Sedat.
* Update commit message as per Nicolas.

Changes v1 -> v2:
* Remove `depends on DEBUG_KERNEL` as per Nathan.
* Rename Kconfigs as per Sedat and Masahiro.
* Add note about renamed Kconfigs to commit message.
* Add more help text to DEBUG_INFO_COMPRESSED_ZSTD.


 lib/Kconfig.debug      | 29 +++++++++++++++++++++++++++--
 scripts/Makefile.debug |  6 +++++-
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 29280072dc0e..7c28a8fba02e 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -312,8 +312,21 @@ 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"
+choice
+	prompt "Compressed Debug information"
+	help
+	  Compress the resulting debug info. Results in smaller debug info sections,
+	  but requires that consumers are able to decompress the results.
+
+	  If unsure, choose DEBUG_INFO_COMPRESSED_NONE.
+
+config DEBUG_INFO_COMPRESSED_NONE
+	bool "Don't compress debug information"
+	help
+	  Don't compress debug info sections.
+
+config DEBUG_INFO_COMPRESSED_ZLIB
+	bool "Compress debugging information with zlib"
 	depends on $(cc-option,-gz=zlib)
 	depends on $(ld-option,--compress-debug-sections=zlib)
 	help
@@ -327,6 +340,18 @@ config DEBUG_INFO_COMPRESSED
 	  preferable to setting $KDEB_COMPRESS to "none" which would be even
 	  larger.
 
+config DEBUG_INFO_COMPRESSED_ZSTD
+	bool "Compress debugging information with zstd"
+	depends on $(cc-option,-gz=zstd)
+	depends on $(ld-option,--compress-debug-sections=zstd)
+	help
+	  Compress the debug information using zstd.  This may provide better
+	  compression than zlib, for about the same time costs, but requires newer
+	  toolchain support.  Requires GCC 13.0+ or Clang 16.0+, binutils 2.40+, and
+	  zstd.
+
+endchoice # "Compressed Debug information"
+
 config DEBUG_INFO_SPLIT
 	bool "Produce split debuginfo in .dwo files"
 	depends on $(cc-option,-gsplit-dwarf)
diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
index 332c486f705f..059ff38fe0cb 100644
--- a/scripts/Makefile.debug
+++ b/scripts/Makefile.debug
@@ -27,10 +27,14 @@ else
 DEBUG_RUSTFLAGS	+= -Cdebuginfo=2
 endif
 
-ifdef CONFIG_DEBUG_INFO_COMPRESSED
+ifdef CONFIG_DEBUG_INFO_COMPRESSED_ZLIB
 DEBUG_CFLAGS	+= -gz=zlib
 KBUILD_AFLAGS	+= -gz=zlib
 KBUILD_LDFLAGS	+= --compress-debug-sections=zlib
+else ifdef CONFIG_DEBUG_INFO_COMPRESSED_ZSTD
+DEBUG_CFLAGS	+= -gz=zstd
+KBUILD_AFLAGS	+= -gz=zstd
+KBUILD_LDFLAGS	+= --compress-debug-sections=zstd
 endif
 
 KBUILD_CFLAGS	+= $(DEBUG_CFLAGS)

base-commit: 1767a722a708f1fa3b9af39eb091d79101f8c086
-- 
2.38.1.431.g37b22c650d-goog