linux-kernel - Re: [PATCH v6 2/2] Kbuild: implement support for DWARF v5

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Date:   Fri, 29 Jan 2021 21:54:58 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-arch <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 v6 2/2] Kbuild: implement support for DWARF v5

On Fri, Jan 29, 2021 at 9:48 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Fri, Jan 29, 2021 at 12:41 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Fri, Jan 29, 2021 at 8:43 PM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 20141cd9319e..bed8b3b180b8 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -832,8 +832,20 @@ endif
> > >
> > >  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > >  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> > >  DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> > >
> > > +# If using clang without the integrated assembler, we need to explicitly tell
> > > +# GAS that we will be feeding it DWARF v5 assembler directives. Kconfig should
> > > +# detect whether the version of GAS supports DWARF v5.
> > > +ifdef CONFIG_CC_IS_CLANG
> > > +ifneq ($(LLVM_IAS),1)
> > > +ifeq ($(dwarf-version-y),5)
> > > +DEBUG_CFLAGS   += -Wa,-gdwarf-5
> >
> > I noticed double "-g -gdwarf-5 -g -gdwarf-5" (a different issue) and
> > that's why I looked again into the top-level Makefile.
>
> That's...unexpected.  I don't see where that could be coming from.
> Can you tell me please what is the precise command line invocation of
> make and which source file you observed this on so that I can
> reproduce?
>

That's everywhere...

$ zstdgrep --color '\-g -gdwarf-5 -g -gdwarf-5'
build-log_5.11.0-rc5-8-amd64-clang12-lto.txt.zst
| wc -l
29529

> > Should this be...?
> >
> > KBUILD_AFLAGS += -Wa,-gdwarf-5
>
> No; under the set of conditions Clang is compiling .c to .S with DWARF
> v5 assembler directives. GAS will choke unless told -gdwarf-5 via
> -Wa,-gdwarf-5 for .c source files, hence it is a C flag, not an A
> flag. A flags are for .S assembler sources, not .c sources.
>

You are right. I mixed again C and A flags.

- Sedat -