linux-kernel - Re: [PATCH v5 2/3] Kbuild: make DWARF version a choice

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Date:   Thu, 21 Jan 2021 03:35:00 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        linux-kbuild@...r.kernel.org, linux-arch@...r.kernel.org,
        Jakub Jelinek <jakub@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>, Yonghong Song <yhs@...com>,
        Jiri Olsa <jolsa@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v5 2/3] Kbuild: make DWARF version a choice

On Wed, Jan 20, 2021 at 9:40 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Fri, Jan 15, 2021 at 01:06:15PM -0800, Nick Desaulniers 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: Arvind Sankar <nivedita@...m.mit.edu>
> > Suggested-by: Fangrui Song <maskray@...gle.com>
> > Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  Makefile          | 13 ++++++-------
> >  lib/Kconfig.debug | 21 ++++++++++++++++-----
> >  2 files changed, 22 insertions(+), 12 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index d49c3f39ceb4..4eb3bf7ee974 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -826,13 +826,12 @@ else
> >  DEBUG_CFLAGS += -g
> >  endif
> >
> > -ifneq ($(LLVM_IAS),1)
> > -KBUILD_AFLAGS        += -Wa,-gdwarf-2
> > -endif
>
> Aren't you regressing this with this patch? Why is the hunk from 3/3
> that adds
>
> ifdef CONFIG_CC_IS_CLANG
> ifneq ($(LLVM_IAS),1)
>
> not in this patch?
>

Nice catch Nathan.

Just FYI:

The patch...

"kbuild: Remove $(cc-option,-gdwarf-4) dependency from DEBUG_INFO_DWARF4"

... has now entered kbuild-next.
( This patch is included in this patch-series. )

So should we point to this one and add a comment?
Thinking of a next version of this DWARF5 support patch-series.

- Sedat -

[1] https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?h=kbuild&id=3f4d8ce271c7082be75bacbcbd2048aa78ce2b44

> > -ifdef CONFIG_DEBUG_INFO_DWARF4
> > -DEBUG_CFLAGS += -gdwarf-4
> > -endif
> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > +DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
> > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > +dwarf-aflag  := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > +KBUILD_AFLAGS        += $(dwarf-aflag)
> >
> >  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 dd7d8d35b2a5..e80770fac4f0 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -256,13 +256,24 @@ 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 Version 2 debuginfo"
> > +     help
> > +       Generate DWARF v2 debug info.
> > +
> >  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"
> > --
> > 2.30.0.284.gd98b1dd5eaa7-goog
> >