Re: [PATCH v7 1/2] Kbuild: make DWARF version a choice

[Fangrui Song]

On 2021-02-04, Masahiro Yamada wrote:
> On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > Suggested-by: Fangrui Song <maskray@xxxxxxxxxx>
> > > Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > > Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> >
> > One comment below:
> >
> > Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> >
> > > ---
> > >  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@xxxxxxxxxx/#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