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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Sat, 18 Jul 2020 09:50:50 +0200
Message-ID: <CAMuHMdU_KfQ-RT_nev5LgN=Vj_P97Fn=nwRoC6ZREFLa3Ysj7w@mail.gmail.com>
Subject: Re: [PATCH] powerpc/boot: Use address-of operator on section symbols
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Geoff Levand <geoff@infradead.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Joel Stanley <joel@jms.id.au>, Arnd Bergmann <arnd@arndb.de>
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Hi Nathan,

On Wed, Jun 24, 2020 at 6:02 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
> arch/powerpc/boot/main.c:107:18: warning: array comparison always
> evaluates to a constant [-Wtautological-compare]
>         if (_initrd_end > _initrd_start) {
>                         ^
> arch/powerpc/boot/main.c:155:20: warning: array comparison always
> evaluates to a constant [-Wtautological-compare]
>         if (_esm_blob_end <= _esm_blob_start)
>                           ^
> 2 warnings generated.
>
> These are not true arrays, they are linker defined symbols, which are
> just addresses.  Using the address of operator silences the warning
> and does not change the resulting assembly with either clang/ld.lld
> or gcc/ld (tested with diff + objdump -Dr).
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/212
> Reported-by: Joel Stanley <joel@jms.id.au>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>  arch/powerpc/boot/main.c | 4 ++--
>  arch/powerpc/boot/ps3.c  | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/boot/main.c b/arch/powerpc/boot/main.c
> index a9d209135975..cae31a6e8f02 100644
> --- a/arch/powerpc/boot/main.c
> +++ b/arch/powerpc/boot/main.c
> @@ -104,7 +104,7 @@ static struct addr_range prep_initrd(struct addr_range vmlinux, void *chosen,
>  {
>         /* If we have an image attached to us, it overrides anything
>          * supplied by the loader. */
> -       if (_initrd_end > _initrd_start) {
> +       if (&_initrd_end > &_initrd_start) {
>

Are you sure that fix is correct?

    extern char _initrd_start[];
    extern char _initrd_end[];
    extern char _esm_blob_start[];
    extern char _esm_blob_end[];

Of course the result of their comparison is a constant, as the addresses
are constant.  If clangs warns about it, perhaps that warning should be moved
to W=1?

But adding "&" is not correct, according to C.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds