linux-kernel - Re: [PATCH 13/14] ARM: use .arch directives instead of assembler command line flags

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Message-ID: <CAK8P3a2LezhdCczGsgWrrkShvowXMYSSdhfPJqSHwHEBRv4PvQ@mail.gmail.com>
Date:   Tue, 28 Sep 2021 20:32:06 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH 13/14] ARM: use .arch directives instead of assembler
 command line flags

On Tue, Sep 28, 2021 at 7:10 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Sep 28, 2021 at 8:42 AM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > From: Nick Desaulniers <ndesaulniers@...gle.com>
> >
> > Similar to commit a6c30873ee4a ("ARM: 8989/1: use .fpu assembler
> > directives instead of assembler arguments").
> >
> > GCC and GNU binutils support setting the "sub arch" via -march=,
> > -Wa,-march, target function attribute, and .arch assembler directive.
> >
> > Clang's integrated assembler does not support -Wa,-march (and the logic
> > to overrule one when multiple of the above are used), and this can
> > cause annoying warnings such as:
> >
> > clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
> > clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
> > clang: warning: argument unused during compilation: '-march=armv6k' [-Wunused-command-line-argument]
>
> Note, the above lack of support was fixed in clang-13. That said, both
> Clang and GCC defer to -Wa,-march when -march is also present, so
> clang is still correct that -march is ignored.  Thanks for resending;
> this is still helpful for earlier releases of clang that we still
> support.

The -Wunused-command-line-argument warning also caused a build
failure for me when building with 'make W=1', and I think there are
cases where the flags from the command line are contradictory.

Isn't the patch also needed for LTO? All I know is that with this
applied it all builds, but without it I run into link failures. I can dig
more into the specific failures, but overall I think this is the right
thing to do anyway.

       Arnd