linux-kernel - Re: [PATCH] ARM: replace the sole use of a symbol with its definition

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Message-ID: <CAKwvOdk3YG5TFD71E-9vPqssFZW1U3umCR+AWLLp8RZK2zHGsw@mail.gmail.com>
Date:   Mon, 13 Apr 2020 11:29:00 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Jian Cai <caij2003@...il.com>
Cc:     Manoj Gupta <manojgupta@...gle.com>,
        Stefan Agner <stefan@...er.ch>,
        Russell King <linux@...linux.org.uk>,
        Enrico Weigelt <info@...ux.net>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: replace the sole use of a symbol with its definition

On Tue, Apr 7, 2020 at 12:09 PM Jian Cai <caij2003@...il.com> wrote:
>
> ALT_UP_B macro sets symbol up_b_offset via .equ to an expression
> involving another symbol. The macro gets expanded twice when
> arch/arm/kernel/sleep.S is assembled, creating a scenario where
> up_b_offset is set to another expression involving symbols while its
> current value is based on symbols. LLVM integrated assembler does not
> allow such cases, and based on the documentation of binutils, "Values
> that are based on expressions involving other symbols are allowed, but
> some targets may restrict this to only being done once per assembly", so
> it may be better to avoid such cases as it is not clearly stated which
> targets should support or disallow them. The fix in this case is simple,
> as up_b_offset has only one use, so we can replace the use with the
> definition and get rid of up_b_offset.
>
> Signed-off-by: Jian Cai <caij2003@...il.com>

Probably didn't need the extra parens, but it's fine (unless another
reviewer would like a v2).  Maybe Stefan has some thoughts?
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Please add Link tags if these correspond to issues in our link
tracker, they help us track when and where patches land.
Link: https://github.com/ClangBuiltLinux/linux/issues/920

> ---
>  arch/arm/include/asm/assembler.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 99929122dad7..adee13126c62 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -269,10 +269,9 @@
>         .endif                                                  ;\
>         .popsection
>  #define ALT_UP_B(label)                                        \
> -       .equ    up_b_offset, label - 9998b                      ;\
>         .pushsection ".alt.smp.init", "a"                       ;\
>         .long   9998b                                           ;\
> -       W(b)    . + up_b_offset                                 ;\
> +       W(b)    . + (label - 9998b)                                     ;\
>         .popsection
>  #else
>  #define ALT_SMP(instr...)
> --
> 2.26.0.292.g33ef6b2f38-goog
>


-- 
Thanks,
~Nick Desaulniers