linux-kernel - Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

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Date:   Tue, 1 Dec 2020 03:04:50 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jakub Jelinek <jakub@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Fangrui Song <maskray@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

On Wed, Nov 4, 2020 at 9:53 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
> explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
> way that's forward compatible with existing configs, and makes adding
> future versions more straightforward.
>
> Suggested-by: Fangrui Song <maskray@...gle.com>
> Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  Makefile          | 14 ++++++++------
>  lib/Kconfig.debug | 19 +++++++++++++++----
>  2 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 75b1a3dcbf30..e23786a4c1c7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -826,12 +826,14 @@ else
>  DEBUG_CFLAGS   += -g
>  endif
>
> -ifndef LLVM_IAS
> -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> -endif
> -
> -ifdef CONFIG_DEBUG_INFO_DWARF4
> -DEBUG_CFLAGS   += -gdwarf-4
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
> +# Binutils 2.35+ required for -gdwarf-4+ support.
> +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> +DEBUG_CFLAGS   += $(dwarf-aflag)

This changes the behavior.

For the Dwarf-2 case,

Previously, -gdwarf-2 was passed to $(CC),
so the debug info was generated by gcc.

Now, -Wa,-gdwarf-2 is passed to $(CC).
-gdwarf-2 is handled by GNU as.
So, the source info points to /tmp/<hash>.s
instead of the original .c file.



Handling the Dwarf capability is very complicated.

Are you still working for v3?



> +KBUILD_AFLAGS  += $(dwarf-aflag)
>  endif
>
>  ifdef CONFIG_DEBUG_INFO_REDUCED
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 83a860126897..03c494eefabd 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -256,14 +256,25 @@ 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_DWARF2
> +       bool "Generate DWARF v2 debuginfo"
> +       help
> +         Generate DWARF v2 debug info.
> +
>  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.
> -         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"
> --
> 2.29.1.341.ge80a0c044ae-goog
>
> --
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-- 
Best Regards
Masahiro Yamada