linux-kernel - [PATCH] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO

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Message-Id: <20201201032748.486928-1-masahiroy@kernel.org>
Date:   Tue,  1 Dec 2020 12:27:48 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: [PATCH] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

The -gdwarf-4 flag is supported by GCC 4.5+, and also by Clang.

You can see it at https://godbolt.org/z/6ed1oW

  For gcc 4.5.3 pane,    line 37:    .value 0x4
  For clang 10.0.1 pane, line 117:   .short 4

Given Documentation/process/changes.rst stating GCC 4.9 is the minimal
version, this cc-option is unneeded.

Note
----

CONFIG_DEBUG_INFO_DWARF4 controls the DWARF version only for C files.

As you can see in the top Makefile, -gdwarf-4 is only passed to CFLAGS.

  ifdef CONFIG_DEBUG_INFO_DWARF4
  DEBUG_CFLAGS    += -gdwarf-4
  endif

This flag is used when compiling *.c files.

On the other hand, the assembler is always given -gdwarf-2.

  KBUILD_AFLAGS   += -Wa,-gdwarf-2

Hence, the debug info that comes from *.S files is always DWARF v2.
This is simply because GAS supported only -gdwarf-2 for a long time.

Recently, GAS gained the support for --dwarf-[3|4|5] options. [1]
And, also we have Clang integrated assembler. So, the debug info
for *.S files might be improved if we want.

In my understanding, the current code is intentional, not a bug.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31bf18645d98b4d3d7357353be840e320649a67d

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 lib/Kconfig.debug | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index cb8ef4fd0d02..7f80c25497a1 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -258,7 +258,6 @@ config DEBUG_INFO_SPLIT

 config DEBUG_INFO_DWARF4
 	bool "Generate dwarf4 debuginfo"
-	depends on $(cc-option,-gdwarf-4)
 	help
 	  Generate dwarf4 debug info. This requires recent versions
 	  of gcc and gdb. It makes the debug information larger.
-- 
2.27.0