linux-kernel - Re: [PATCH] ARM: mtd-xip: work around clang/llvm bug

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Date:   Mon, 8 Jul 2019 13:54:53 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King <linux@...linux.org.uk>,
        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: mtd-xip: work around clang/llvm bug

On Mon, Jul 8, 2019 at 1:31 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> llvm gets confused by inline asm with .rep directives, which
> can lead to miscalculating the number of instructions inside it,
> and in turn lead to an overflow for relative address calculation:
>
> /tmp/cfi_cmdset_0002-539a47.s: Assembler messages:
> /tmp/cfi_cmdset_0002-539a47.s:11288: Error: bad immediate value for offset (4100)
> /tmp/cfi_cmdset_0002-539a47.s:11289: Error: bad immediate value for offset (4100)
>
> This might be fixed in future clang versions, but is not hard
> to work around by just replacing the .rep with a series of
> eight unrolled nop instructions.

Seems like this is fixable on the Clang side as well. For now, thanks
for the patch.
Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Link: https://bugs.llvm.org/show_bug.cgi?id=42539
> https://godbolt.org/z/DSM2Jy

^ prefix with Link: on both lines?
also, linking to clang trunk will become stale once the issue is
fixed.  When possible, link to a release version of clang that's not
prone to change over time.

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/arm/include/asm/mtd-xip.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/include/asm/mtd-xip.h b/arch/arm/include/asm/mtd-xip.h
> index dfcef0152e3d..5ad0325604e4 100644
> --- a/arch/arm/include/asm/mtd-xip.h
> +++ b/arch/arm/include/asm/mtd-xip.h
> @@ -15,6 +15,8 @@
>  #include <mach/mtd-xip.h>
>
>  /* fill instruction prefetch */
> -#define xip_iprefetch()        do { asm volatile (".rep 8; nop; .endr"); } while (0)
> +#define xip_iprefetch()        do {                                            \
> +        asm volatile ("nop; nop; nop; nop; nop; nop; nop; nop;");      \
> +} while (0)                                                            \
-- 
Thanks,
~Nick Desaulniers