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

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Message-ID: <20201201093253.GJ2672@gate.crashing.org>
Date:   Tue, 1 Dec 2020 03:32:53 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Fāng-ruì Sòng <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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>,
        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 Tue, Dec 01, 2020 at 12:38:16PM +0900, Masahiro Yamada wrote:
> > We can bump -Wa,-gdwarf-2 to -Wa,-gdwarf-3 since GNU actually emits
> > DWARF v3 DW_AT_ranges (see
> > https://sourceware.org/bugzilla/show_bug.cgi?id=26850 )
> > This can avoid the `warning: DWARF2 only supports one section per
> > compilation unit` warning for Clang.

That warning should be "there can be only one section with executable
code per translation unit", or similar.

> I am not a DWARF spec expert.

Neither am I.

> Please teach me.
> 
> In my understanding, "DWARF2 only supports one section ..."
> is warned only when building .S files with LLVM_IAS=1

.S files are simply run through the C preprocessor first, and then given
to the assembler.  The only difference there should be wrt debug info is
you could have some macros that expand to assembler debug statements.

> If this is due to the limitation of DWARF v2, why is it OK to
> build .c files with LLVM_IAS?

The compiler can of course make sure not to use certain constructs in
its generated assembler code, while the assembler will have to swallow
whatever the user wrote.


Segher