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    SubjectRe: [PATCH v7 1/2] Kbuild: make DWARF version a choice
    On 2021-02-04, Masahiro Yamada wrote:
    >On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@kernel.org> wrote:
    >>
    >> On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote:
    >> > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is
    >> > the default. Does so in a way that's forward compatible with existing
    >> > configs, and makes adding future versions more straightforward.
    >> >
    >> > GCC since ~4.8 has defaulted to this DWARF version implicitly.
    >> >
    >> > Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
    >> > Suggested-by: Fangrui Song <maskray@google.com>
    >> > Suggested-by: Nathan Chancellor <nathan@kernel.org>
    >> > Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
    >> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    >>
    >> One comment below:
    >>
    >> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
    >>
    >> > ---
    >> > Makefile | 5 ++---
    >> > lib/Kconfig.debug | 16 +++++++++++-----
    >> > 2 files changed, 13 insertions(+), 8 deletions(-)
    >> >
    >> > diff --git a/Makefile b/Makefile
    >> > index 95ab9856f357..d2b4980807e0 100644
    >> > --- a/Makefile
    >> > +++ b/Makefile
    >> > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1)
    >> > KBUILD_AFLAGS += -Wa,-gdwarf-2
    >>
    >> It is probably worth a comment somewhere that assembly files will still
    >> have DWARF v2.
    >
    >I agree.
    >Please noting the reason will be helpful.
    >
    >Could you summarize Jakub's comment in short?
    >https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667
    >
    >
    >
    >
    >
    >
    >One more question.
    >
    >
    >Can we remove -g option like follows?
    >
    >
    > ifdef CONFIG_DEBUG_INFO_SPLIT
    > DEBUG_CFLAGS += -gsplit-dwarf
    >-else
    >-DEBUG_CFLAGS += -g
    > endif

    GCC 11/Clang 12 -gsplit-dwarf no longer imply -g2
    (https://reviews.llvm.org/D80391). May be worth checking whether
    -gsplit-dwarf is used without a debug info enabling option.

    >
    >
    >
    >
    >In the current mainline code,
    >-g is the only debug option
    >if CONFIG_DEBUG_INFO_DWARF4 is disabled.
    >
    >
    >The GCC manual says:
    >https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options
    >
    >
    >-g
    >
    > Produce debugging information in the operating system’s
    > native format (stabs, COFF, XCOFF, or DWARF).
    > GDB can work with this debugging information.
    >
    >
    >Of course, we expect the -g option will produce
    >the debug info in the DWARF format.
    >
    >
    >
    >
    >
    >With this patch set applied, it is very explicit.
    >
    >Only the format type, but also the version.
    >
    >The compiler will be given either
    >-gdwarf-4 or -gdwarf-5,
    >making the -g option redundant, I think.

    -gdwarf-N does imply -g2 but personally I'd not suggest remove it if it
    already exists. The non-orthogonality is the reason Clang has
    -fdebug-default-version (https://reviews.llvm.org/D69822).

    >
    >
    >
    >
    >
    >
    >
    >
    >>
    >> > endif
    >> >
    >> > -ifdef CONFIG_DEBUG_INFO_DWARF4
    >> > -DEBUG_CFLAGS += -gdwarf-4
    >> > -endif
    >> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
    >> > +DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
    >> >
    >> > ifdef CONFIG_DEBUG_INFO_REDUCED
    >> > DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
    >> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
    >> > index e906ea906cb7..94c1a7ed6306 100644
    >> > --- a/lib/Kconfig.debug
    >> > +++ b/lib/Kconfig.debug
    >> > @@ -256,13 +256,19 @@ config DEBUG_INFO_SPLIT
    >> > to know about the .dwo files and include them.
    >> > Incompatible with older versions of ccache.
    >> >
    >> > +choice
    >> > + prompt "DWARF version"
    >> > + help
    >> > + Which version of DWARF debug info to emit.
    >> > +
    >> > config DEBUG_INFO_DWARF4
    >> > - bool "Generate dwarf4 debuginfo"
    >> > + bool "Generate DWARF Version 4 debuginfo"
    >> > help
    >> > - Generate dwarf4 debug info. This requires recent versions
    >> > - of gcc and gdb. It makes the debug information larger.
    >> > - But it significantly improves the success of resolving
    >> > - variables in gdb on optimized code.
    >> > + Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
    >> > + It makes the debug information larger, but it significantly
    >> > + improves the success of resolving variables in gdb on optimized code.
    >> > +
    >> > +endchoice # "DWARF version"
    >> >
    >> > config DEBUG_INFO_BTF
    >> > bool "Generate BTF typeinfo"
    >> > --
    >> > 2.30.0.365.g02bc693789-goog
    >> >
    >
    >
    >
    >--
    >Best Regards
    >Masahiro Yamada

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