linux-kernel - Re: [PATCH v2 1/2] ARM: drop WASM to work around LLVM issue

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Date:   Mon, 18 Mar 2019 11:09:10 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Stefan Agner <stefan@...er.ch>
Cc:     Russell King <linux@...linux.org.uk>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will.deacon@....com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        julien.thierry@....com, Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/2] ARM: drop WASM to work around LLVM issue

On Sun, Mar 17, 2019 at 4:05 PM Stefan Agner <stefan@...er.ch> wrote:
>
> Currently LLVM's integrated assembler does not recognize .w form
> of the pld instructions (LLVM Bug 40972 [0]):
>
>   ./arch/arm/include/asm/processor.h:133:5: error: invalid instruction
>                           "pldw.w\t%a0 \n"
>                            ^
>   <inline asm>:2:1: note: instantiated into assembly here
>   pldw.w  [r0]
>   ^
>   1 error generated.
>
> The W macro for generating wide instructions when targeting Thumb-2
> is not strictly required for the preload data instructions (pld, pldw)
> since they are only available as wide instructions. The GNU assembler
> works with or without the .w appended when compiling an Thumb-2 kernel.
>
> Drop the macro to work around LLVM Bug 40972 issue.
>
> [0] https://bugs.llvm.org/show_bug.cgi?id=40972
>
> Signed-off-by: Stefan Agner <stefan@...er.ch>

Thanks for the bug report and patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Just curious, there are only 3 other expansion sites of this macro.
Are any of those problematic?  Looks like nop.w, sev.w and b.w?

> ---
> Changes in v2:
> - Reword commit message to reflect the fact that this is a work around
>   for LLVM.
>
>  arch/arm/include/asm/processor.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
> index 57fe73ea0f72..5d06f75ffad4 100644
> --- a/arch/arm/include/asm/processor.h
> +++ b/arch/arm/include/asm/processor.h
> @@ -135,8 +135,8 @@ static inline void prefetchw(const void *ptr)
>         __asm__ __volatile__(
>                 ".arch_extension        mp\n"
>                 __ALT_SMP_ASM(
> -                       WASM(pldw)              "\t%a0",
> -                       WASM(pld)               "\t%a0"
> +                       "pldw\t%a0",
> +                       "pld\t%a0"
>                 )
>                 :: "p" (ptr));
>  }
> --
> 2.21.0
>


-- 
Thanks,
~Nick Desaulniers