linux-kernel - Re: [RESEND,PATCH] ARM: fix __div64_32() error when compiling with clang

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Date:   Mon, 23 Nov 2020 11:16:02 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Antony Yu <swpenim@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [RESEND,PATCH] ARM: fix __div64_32() error when compiling with
 clang

On Mon, Nov 23, 2020 at 03:36:32PM +0800, Antony Yu wrote:
> __do_div64 clobbers the input register r0 in little endian system.
> According to the inline assembly document, if an input operand is
> modified, it should be tied to a output operand. This patch can
> prevent compilers from reusing r0 register after asm statements.
> 
> Signed-off-by: Antony Yu <swpenim@...il.com>
> ---
>  arch/arm/include/asm/div64.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> index 898e9c78a7e7..809efc51e90f 100644
> --- a/arch/arm/include/asm/div64.h
> +++ b/arch/arm/include/asm/div64.h
> @@ -39,9 +39,10 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
>  	asm(	__asmeq("%0", __xh)
>  		__asmeq("%1", "r2")
>  		__asmeq("%2", "r0")
> -		__asmeq("%3", "r4")
> +		__asmeq("%3", "r0")
> +		__asmeq("%4", "r4")
>  		"bl	__do_div64"
> -		: "=r" (__rem), "=r" (__res)
> +		: "=r" (__rem), "=r" (__res), "=r" (__n)
>  		: "r" (__n), "r" (__base)
>  		: "ip", "lr", "cc");
>  	*n = __res;
> -- 
> 2.23.0
> 

I am not sure that I am qualified to review this (my assembly knowledge
is not the best) but your commit title mentions an error when compiling
with clang. What is the exact error, what configuration generates it,
and what version of clang? We have done fairly decent testing for
32-bit ARM, I would like to know what we are missing.

Cheers,
Nathan